No humidity does not have any effect on fuel economy. However Temperature does have a large affect on gas mileage. Thanks!
is the ratio of the partial pressure of water vapor in an air-water mixture to the saturated vapor pressure of water at a prescribed temperature. The relative humidity of air depends on temperature and the pressure of the system of interest.
The relative humidity
of an air-water mixture is defined as the ratio of the partial pressure of water vapor (H2
in the mixture to the saturated vapor pressure of water
at a given temperature.
Relative humidity is normally expressed as a percentage and is calculated by using the following equation:
Climate control refers to the control of temperature and relative humidity for human comfort, health, and safety; for the technical requirements of machines and processes; and in buildings, vehicles, and other enclosed spaces.
Humans are sensitive to humid air because the human body uses evaporative cooling (perspiration) as the primary mechanism to rid itself of waste heat. The rate at which perspiration evaporates on the skin under humid conditions is lower than under arid conditions. Because humans perceive a low rate of heat transfer
from the body the same as a higher air temperature the body experiences greater distress of waste heat burden at a lower temperature with high RH than at a higher temperature at lower RH.
If, for example, the air temperature is 24 °C (75 °F) and the relative humidity is zero percent then the air temperature feels like 21 °C (69 °F). If the relative humidity is 100 percent at the same air temperature then it feels like 27 °C (80 °F). If the air is 24 °C (75 °F) and contains saturated water vapor, in other words, then the human body cools itself at the same rate as it would if it were 27 °C (80 °F) and dry. The heat index and the humidex are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body.
Humans can be comfortable within a wide range of humidities depending on the temperature — from thirty to seventy percent - but ideally between fifty and sixty percent.
For climate control in buildings using HVAC systems, the key is to maintain the relative humidity at a comfortable range—low enough to be comfortable but high enough to avoid problems associated with very dry air.
When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hung on a line or rack dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink, causing the paint that covers these surfaces to fracture.
When the temperature is high and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100 percent, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration.
Certain production and technical processes and treatments in factories, laboratories, hospitals, and other facilities require specific relative humidity levels to be maintained using humidifiers, dehumidifiers and associated control systems.
The basic principles for buildings, above, also apply to vehicles. In addition, there may be safety considerations. For instance, high humidity inside a vehicle can lead to problems of condensation, such as misting of windshields and shorting of electrical components.
In sealed vehicles and pressure vessels such as pressurized airliners, submersibles and spacecraft, these considerations may be critical to safety, and complex environmental control systems including equipment to maintain pressure are needed. Airliners operate with low internal relative humidity, often under 10%, especially on long flights. The low humidity is a consequence of drawing in the very cold air with a low absolute humidity, which is found at airliner cruising altitudes. Subsequent warming of this air lowers its relative humidity. This causes discomfort such as sore eyes, dry skin, and drying out of mucosa, but humidifiers are not employed to raise it to comfortable mid-range levels because the volume of water required to be carried on board can be a significant weight penalty. As airliners descend from colder altitudes into warmer air (perhaps even flying through clouds a few thousand feet above the ground), the ambient relative humidity can increase dramatically. Some of this moist air is usually drawn into the pressurized aircraft cabin and into other non-pressurized areas of the aircraft and condenses on the cold aircraft skin. Liquid water can usually be seen running along the aircraft skin, both on the inside and outside of the cabin. Because of the drastic changes in relative humidity inside the vehicle, components must be qualified to operate in those environments. The recommended environmental qualifications for most commercial aircraft components is listed in RTCA DO-160.
Cold humid air can provoke the formation of ice, which is a danger to aircraft as it affects the wing profile and increases weight. Carbureted engines have a further danger of ice forming inside the carburetor. Aviation weather reports (METARs) therefore include an indication of relative humidity, usually in the form of the dew point.
In addition, air with higher humidity is less dense, providing lower lift and lower propeller or turbine efficiency (this is somewhat offset by decreased drag). Pilots must take humidity into account when calculating takeoff distances because high humidity will require longer runways and will decrease the climb gradient.
The humidity of an air-water vapor mixture is determined through the use of psychrometric charts if both the dry bulb temperature (T
) and the wet bulb temperature (Tw
) of the mixture are known. These quantities are readily estimated by using a sling psychrometer.
There are several empirical correlations that can be used to estimate the saturated vapor pressure of water vapor as a function of temperature. The Antoine equation is among the least complex of these formulas, having only three parameters (A, B, and C). Other correlations, such as those presented by Goff-Gratch and Magnus Tetens approximation, are more complicated but yield better accuracy]
[. The correlation presented by Buck is commonly encountered in the literature and provides a reasonable balance between complexity and accuracy]
is the dry bulb temperature expressed in degrees Celsius (°C
is the absolute pressure expressed in hectopascals (hPa
is the saturated vapor pressure expressed in hectopascals (hPa
Buck has reported that the maximum relative error is less than 0.20% between -20°C and +50°C when this particular form of the generalized formula is used to estimate the saturated vapor pressure of water.
The notion of air holding water vapor is sometimes used to describe the concept of relative humidity. This, however, is a misconception. Air is a mixture of gases (nitrogen, oxygen, argon, water vapor, and other gases) and as such the constituents of the mixture simply act as a transporter of water vapor but are not a holder of it.
is defined in the physical properties of water and thus is unrelated to the notion of air holding water
. An air-less volume can, in fact, contain water vapor and therefore the humidity of this volume can be readily determined.
The relative humidity of an air-water system is dependent not only on the temperature but also on the absolute pressure of the system of interest. This dependence is demonstrated by considering the air-water system shown below. The system is closed (i.e., no matter enters or leaves the system).
If the system at State A is isobarically heated (heating with no change in system pressure) then the relative humidity of the system decreases because the saturated vapor pressure of water increases with increasing temperature. This is shown in State B.
If the system at State A is isothermally compressed (compressed with no change in system temperature) then the relative humidity of the system increases because the partial pressure of water in the system increases with the volume reduction. This is shown in State C. Above 202.64 kPa, the RH would exceed 100% and water may begin to condense.
If the pressure of State A was changed by simply adding more dry air, without changing the volume, the relative humidity would not change.
Therefore a change in relative humidity can be explained by a change in system temperature, a change in the volume of the system, or change in both of these system properties.
The enhancement factor
is defined as the ratio of the saturated vapor pressure of water in moist air
to the saturated vapor pressure of pure water.
The enhancement factor is equal to unity for ideal gas systems. However, in real systems the interaction effects between gas molecules result in a small increase of the saturation vapor pressure of water in air relative to saturated vapor pressure of pure water vapor. Therefore, the enhancement factor is normally slightly greater than unity for real systems.
The enhancement factor is commonly used to correct the saturated vapor pressure of water vapor when empirical relationships, such as those developed by Wexler, Goff, and Gratch, are used to estimate the properties of psychrometric systems.
Buck has reported that, at sea level, the vapor pressure of water in saturated moist air amounts to an increase of approximately 0.5% over the saturated vapor pressure of pure water.
The term relative humidity is reserved for systems of water vapor in air. The term relative saturation
is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.
A gas in this context is referred to as saturated when the vapor pressure of water in the air is at the equilibrium vapor pressure for water vapor at the temperature of the gas and water vapor mixture; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of dew or fog forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of very minute droplets of liquid, primarily held aloft by isostatic motion (in other words, the droplets fall through the air at terminal velocity, but as they are very small, this terminal velocity is very small too, so it doesn't look to us like they are falling, and they seem to be held aloft).
The statement that relative humidity (RH%) can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the Wilson cloud chamber, which uses, in nuclear physics experiments, an extremely brief state of "supersaturation" to accomplish its function.
For a given dewpoint and its corresponding absolute humidity, the relative humidity will change inversely, albeit nonlinearly, with the temperature. This is because the partial pressure of water increases with temperature – the operative principle behind everything from hair dryers to dehumidifiers.
Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry skin, itchy eyes, and persistence of static electric charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies and household plants.)
Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint, and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.
A useful rule of thumb is that the maximum absolute humidity doubles for every 20 °F or 10 °C increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F or 10 °C increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 68 °F or 20 °C and 50% relative humidity will become saturated if cooled to 50°F or 10 °C, its dewpoint, and 41 °F or 5 °C air at 80% relative humidity warmed to 68 °F or 20 °C will have a relative humidity of only 29% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort-controlled environments (ASHRAE Standard 55 - see thermal comfort).
Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air will tend to rise by natural convection. This is a mechanism behind thunderstorms and other weather phenomena. Relative humidity is often mentioned in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, it also increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin as the relative humidity rises. This effect is calculated as the heat index or humidex.
A device used to measure humidity is called a hygrometer; one used to regulate it is called a humidistat, or sometimes hygrostat. (These are analogous to a thermometer and thermostat for temperature, respectively.)
A humid subtropical climate (Köppen climate classification Cfa or Cwa) is a zone of subtropical climate characterized by hot, humid summers and generally mild to cool winters. Under the Köppen climate definition, this category of climate type covers a broad range of attributes, especially in terms of winter temperatures. The term "subtropical" may be a misnomer for locations along the cooler ranges.
The Köppen definition of this climate is for the coldest month's mean temperature to be between and , and the warmest month to be above . Some climatologists prefer to use as the lower bound for the coldest month's mean temperature. It is either accompanied with a dry winter (Köppen: w) — or has no distinguished dry season (Köppen: f).
Significant amounts of precipitation occur in all seasons in most areas, and though in regions bordering on semi-arid climates (usually at the western margins), irregular droughts can be common and catastrophic to agriculture. Winter rainfall (and sometimes snowfall) is associated with large storms that the westerlies steer from west to east. Most summer rainfall occurs during thunderstorms and an occasional tropical storm, hurricane or cyclone.
Humid subtropical climates normally lies on the southeast side of all continents, generally between latitudes 25° and 40° north and tend to be located at coastal or near coastal locations. However in some cases the climate extends well inland, most notably in China and the United States.
In Africa, the humid subtropical climates are found in two separate areas on the southern hemisphere of the continent. The Cwa climate is found in over a large portion of the interior of the Middle and Eastern African regions. This area includes; central Angola, northeastern Zimbabwe, the Niassa, Manica and Tete provinces of Mozambique, the southern Congo provinces, southwest Tanzania, and the majority of Malawi, and Zambia. Some lower portions of the Ethiopian Highlands also have this climate. The climate is also found in the narrow coastal sections of southern and eastern South Africa, primarily in KwaZulu-Natal and the Eastern Cape provinces. South Africa's version of this climate features heavy oceanic influences resulting in generally milder temperatures. This is particularly evident in its winters when temperatures do not drop as low as in many other regions within the humid subtropical category.
Locations in Asia with a humid subtropical climate differ from those in other continents in that they often have marked seasonal differences in precipitation, if not very dry winters.
In East Asia, this type climate is found the southeastern quarter of mainland China, the northern half of Taiwan, northern Vietnam, narrow areas along the coast of South Korea, and Japan (Kyushu, Shikoku, and most of Honshu). Cities on the equatorward boundary of this zone include Hong Kong, Hanoi and Taipei while Qingdao is on the northern boundary.
The influence of the strong Siberian anticyclone in East Asia, brings colder winter temperatures southward, pushing the 0 °C isotherm as far south as the valleys of the Yellow and Wei, roughly latitude 34° N. At Hainan Island and in Taiwan, the climate transitions from subtropical into fully tropical. In most of this region, there is extremely limited precipitation during the winter, owing to the powerful anticyclonic winds from Siberia. Only in inland areas below the Yangtze River and coastal areas between approximately the Huai River and the beginning of the coast of Guangdong is there sufficient winter rainfall to produce a Cfa climate; even in these areas, rainfall and streamflow show a highly pronounced summer peak quite unlike other regions of this climate type. The only area where winter precipitation equals or even exceeds the summer rain is on the "San-in" (Sea of Japan), or western, coast of Japan, which during winter is on the windward side of the westerlies. The winter precipitation in these regions is usually produced by low-pressure systems off the east coast that develop in the onshore flow from the Siberian high. Summer rainfall comes from the East Asian Monsoon and from frequent typhoons. Annual rainfall is generally over 1,000 millimetres (39 in), and in areas below the Himalayas can be much higher still.
Humid subtropical climates can also be found in South Asia, primarily along the Ganges river. However, the humid subtropical climates exhibited here differ markedly from humid subtropical climates in East Asia (and for that matter a good portion of the globe). Winters here are typically mild, dry and relatively short. They also tend to be foggy. Summers tend to be long and very hot, starting from mid-April and peaking in May and early June with high temperatures often exceeding 40°C. They also tend to be extremely dry, complete with dust storms, traits usually associated with arid or semiarid climates. During this period many native trees defoliate to save water. This is followed by the cooler monsoons, where the region experiences heavy rains on almost a daily basis. Average high temperatures decreases during the monsoon season but the humidity increases. This results in hot and humid conditions, similar to summers in humid subtropical climates. Cities such as New Delhi, Lucknow, Kanpur and Patna exhibit this atypical version of the climate in India. In Pakistan, the twin cities of Rawalpindi/Islamabad also feature this weather pattern, but with wetter and relatively cooler winters.
In South Asia, humid subtropical climates generally border on continental climates as altitude increases, or on winter-rainfall climates in Pakistan. Further east, in highland areas with lengthier monsoons such as Nepal, seasonal temperature variation is lower than in the lowlands.
Although humid subtropical climates in Asia are mostly confined to the southeastern quarter of the continent, there are areas on the Caspian Sea and Black Sea with humid subtropical climates that are unusually warm for their high latitudes and also unusual for this climate type, that snowfall in winter is relatively common, but is usually of a short duration.
In Southwestern Asia, the climate is prevalent in the Gilan of Iran, in parts of the Caucasus, in Azerbaijan and in Georgia wedged between the Caspian Sea and Black seas. The climate is also present in small areas of the southern Russian Federation and coastal (Black Sea) Turkey. In the narrow Caspian coastal strip of Iran (Gilan and Mazandaran) a humid subtropical climate prevails . Annual rainfall ranges from around 740 mm (29 inches) at Sari to over 2,000 mm (78 inches) at Bandar-e Anzali, and is heavy throughout the year, with a maximum in October or November when Bandar-e Anzali can average 400 millimetres (16 inches). Temperatures are generally moderate in comparison with other parts of Southwestern Asia. In Rasht, the average maximum in July is around 28 °C (82 °F) but with near-saturation humidity, whilst in January it is around 9 °C (48 °F). The heavy, evenly distributed rainfall extends north into the Caspian coastal strip of Azerbaijan up to its northern border but this climate in Azerbaijan is, however, a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case. During winter, the coastal areas can receive snowfall, but is usually of a short duration. Annual rainfall in Lankaran in the southeast averages up to 1,800 mm (70 inches) and is heavy throughout the year; and annual rainfall is generally over 1,000 mm (40 inches) in the foothills of the Caucasus in the northeast, as altitude increases and the humid subtropical climate changes to the oceanic climate
Western Georgia in the Kolkheti Lowland and the north coast of Turkey, have a climate similar to that of Gilan and Mazandaran in Iran and very similar to that of southeastern and northern Azerbaijan. Temperatures range from 22 °C in summer to 5 °C in winter and rainfall is even heavier than in Caspian Iran, up to 2,300 millimetres per year in Hopa (Turkey) and up to 2,718 millimetres per year in Batumi (Georgia) falling throughout the year. This climate in northern Turkey and western Georgia is, again, a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case. And again, during winter, the coastal areas can receive snowfall, but is usually of a short duration.
In North America, humid subtropical climates are almost exclusively the domain of the American South, including the following states: the eastern half of Texas, Oklahoma, Louisiana, Arkansas, Alabama, Mississippi, North Carolina, South Carolina, Tennessee, Georgia, Kentucky, most of Florida and Virginia and sections of West Virginia. The climate in many of these states is subject to extremes. The humid subtropical climate can also be found in the Mid-Atlantic, primarily Maryland, Delaware, Washington, D.C., southeastern Pennsylvania, southern New Jersey and far southern New York, specifically New York City and Long Island. It can also be found in the Midwest, primarily in the central and southern portions of Kansas and Missouri, and the southern portions of Illinois, Indiana and Ohio. The Mid-Atlantic and Midwestern areas included in this climate typically see snowfall during the winter, with occasional heavy storms. On the other extreme end, most of Florida only occasionally see the extremes that are more commonplace in the rest of the American South. In southern Florida, the climate is either a Tropical savanna climate (wet/dry) or Tropical monsoon climate climate. The archetypal humid subtropical climate is best exemplified by the American Deep South, because the summers are long and almost tropical, and temperatures reach freezing only a few times in the winter with rare snowfall, usually three inches or less. Summers in this zone are hot and humid, with daily averages above with average daily maxima above .
In Mexico, there are small areas of Cfa and Cwa climates. The climate can be found small areas scattered around the northeastern part of the country, in proximity to the Gulf of Mexico. Other areas where the climate can be found is in the high elevations of Trans-Mexican Volcanic Belt and Sierra Madre Oriental. Despite being located at higher elevations, these locations have summers that are too warm to qualify as a subtropical highland climate. Guadalajara’s climate is a major example of this.
Outside of isolated sections of Mexico, the southernmost limits of this climate in North America lie just north of South Florida and around southern coastal Texas. Cities at the southernmost limits of this climate, such as Orlando and Tampa and along the Texas coast from Brownsville to Corpus Christi generally feature warm weather year round and minimal temperature differences between seasons. These cities fall just short of having a true tropical climate. In contrast, cities at the northernmost limits of the humid subtropical region, such as New York City, Philadelphia, and Louisville, Kentucky feature winters that are barely warm enough to qualify as a humid subtropical climate. These cities generally experience much greater seasonal variation, featuring hot, humid summers and chillier winters. Areas farther north than this, inland, or at a higher elevation, fall into the humid continental climate category with harsher winters.
Snowfall varies greatly in this climate zone. In locations at the southern limits of this zone and areas around the Gulf Coast, cities such as Orlando, Tampa, Houston, and New Orleans rarely see snowfall, which occurs, at most, a few times per generation. In Southern cities farther north or inland, such as Birmingham, Atlanta, Memphis, Little Rock, Nashville, Dallas, Norfolk, Charlotte, and Raleigh, snow typically falls once or twice a season and is usually three inches or less. Ice storms are not unusual at these locations. However for the majority of the winter here, temperatures remain above or well above freezing, with slight plant growth. In the northern limits of this climate zone, cities such as Philadelphia and New York City experience snow every winter, sometimes accumulating heavily although it melts more quickly than in regions to the north.
Precipitation is plentiful in the humid subtropical climate zone in North America. Although most areas tend to have precipitation spread evenly throughout the year, a somewhat monsoon-like pattern is seen in parts of the Southeast (in locales such as Augusta, Georgia and Columbia, South Carolina), which experience dry winters (by humid subtropical standards) and warm springs, followed immediately by a long, hot, rainy and humid summer. In addition, areas in Texas that are slightly inland from the Gulf of Mexico, such as Austin and San Antonio that border the semi-arid climate zone, generally see a peak of precipitation in the spring, and a deep, drought-like nadir in mid-summer.
Humid subtropical climates are found in a sizeable portion of South America. The climate extends over a few states of southern Brazil, including Paraná (state), into sections of Paraguay, all of Uruguay, and the Río de la Plata region in Argentina. Major cities such as São Paulo, Buenos Aires, Porto Alegre and Montevideo have a humid subtropical climate, generally in the form of hot humid summers and mild to cool winters. These areas, which include the Pampas, generally feature a Cfa climate categorization.
The Cwa climate occurs in parts of tropical highlands of São Paulo state, Minas Gerais and near the Andean highland in northwestern Argentina. These highland areas feature summer temperatures that are warm enough to fall outside the subtropical highland climate category.
The humid subtropical climate dominates half of eastern Australia.
This zone contains the only regions where soils are not acutely deficient in phosphorus, as well as the heaviest rainfall south of the Tropic of Capricorn, making it prime agricultural country, centred on towns such as Coffs Harbour, Grafton, Kempsey, Port Macquarie, Tamworth, and Moree.][
There is variation in climate within this zone. Annual rainfall on the coast can reach as high as 2,000 mm (80 inches) in favourable locations and is generally above 1,000 mm (40 inches).][ However, because most of the heaviest two- and three-day rainfalls in the world occur in this coastal zone as a result of east coast lows forming to the north of a large high pressure system, there can be great variation in rainfall from year to year. At Lismore in the centre of this zone][, the annual rainfall can range from less than 550 mm (22 inches) in 1915 to more than 2,780 mm (110 inches) in 1950. There is usually a distinct summer rainfall maximum that becomes more pronounced moving northwards: in Brisbane the wettest month (February) receives five times the rainfall of the driest (September). Temperatures are very warm to hot but not excessive: the average maximum in February is usually around 29 °C (84 °F) and in July around 21 °C (70 °F).][ Frosts are extremely rare except at higher elevations, but temperatures over 35˚C (95˚F) are not common on the coast.][
North of the Cfa climate zone there is a zone centred upon Rockhampton and extending up to the Atherton Tableland of Köppen Cwa climate.][ This has a very pronounced dry winter with often negligible rainfall between June and October, and winter temperatures generally only slightly below 18°C, above which one would have a tropical savanna, or Aw, climate.][
Humid subtropical climates are located in relatively small sections of Europe. The Toulouse region of France, and in places along the Adriatic and Black Sea coasts which are too wet for inclusion in the Mediterranean climate schema, inland from these areas there are isolated pockets where the climate is borderline subtropical but these zones are usually classed as oceanic or humid continental. Average summer temperatures in areas of Europe with this climate are generally not as hot as most other subtropical zones around the world, but the growing season can be adequately long.
Some areas of Europe, such as parts of the northeastern interior of the Iberian Peninsula, southern France Garonne Valley and Rhone Valley, Adriatic northern Italy, parts of Epirus in Greece around the area of Ioannina, parts of coastal northern Croatia, and coastal Slovenia fall into this classification. Along the Black Sea coast of Bulgaria, Romania, Sochi, Russia and southernmost Ukraine have summers too warm (>22°C in the warmest month) to qualify as oceanic, no freezing month, and enough summer precipitation and sometimes humid conditions to preclude their classification as Mediterranean but rather border on or are sometimes defined as Humid continental climates. All these areas are subject to occasional, in some cases repeated snowfalls and freezes during winter. In the Azores, some islands have this climate, with very mild and rainy winters (> 13°C) and no snowfall, hot summers (> 22 or 23°C) but with no dry season during the warmest period, which means that they can be classified neither as oceanic, nor as Mediterranean, but only as humid subtropical climate, as with Corvo Island.
In many other climate classification systems outside of the Köppen, most of these locations would not be included the humid subtropical grouping. The higher precipitation and high humidity of summers is not present nearly to the degree that it is in subtropical regions of North America and Asia, making its distinction in Europe all the more difficult.
The Köppen climate classification is one of the most widely used climate classification systems. It was first published by Russian German climatologist Wladimir Köppen in 1884, with several later modifications by Köppen himself, notably in 1918 and 1936. Later, German climatologist Rudolf Geiger collaborated with Köppen on changes to the classification system, which is thus sometimes referred to as the Köppen–Geiger climate classification system. The system is based on the concept that native vegetation is the best expression of climate. Thus, climate zone boundaries have been selected with vegetation distribution in mind. It combines average annual and monthly temperatures and precipitation, and the seasonality of precipitation.:200–1
The Köppen climate classification scheme divides climates into five main groups, each having several types and subtypes. Each particular climate type is represented by a 2 to 4 letter symbol.
Tropical climates are characterized by constant high temperature (at sea level and low elevations) — all twelve months of the year have average temperatures of or higher. They are subdivided as follows:
These climates are characterized by the fact that actual precipitation is less than a threshold value set equal to the potential evapotranspiration.:212 The threshold value (in millimeters) is determined as follows:
These climates have an average temperature above in their warmest months (April to September in northern hemisphere), and a coldest month average between −3 and 18 °C (27 and 64 °F).
Some climatologists, particularly in the United States, however, prefer to observe rather than in the coldest month as the boundary between this group and the colder Group D (Humid Continental).This is also done to prevent certain mild headland locations on the upper East Coast of the USA and Japan from fitting into the C group.
When the boundary between C (Mild Temperate/mesothermal climates) and D (Cold winter/microthermal climates) is increased to 32 F (not the 27 F suggested by Köppen), this creates a smaller C zone located further southward. In the USA, areas from the NYC metropolitan area (NYC/New Jersey/southern Connecticut) southward, as well as the lower Ohio Valley, lower Midwest, and southern Plains are located in the mild C group...while locations to the north of these regions (Northern Plains, Great Lakes, Midwest, upper Ohio Valley and upper East Coast (Boston northward), are located in the cooler D group. Using 32 F also pushes parts of the northeast and northcentral Asia (northern Japan, northern China, and northern Korea) into the colder D/microthermal group (aka known as humid continental).
These climates have an average temperature above in their warmest months, and a coldest month average below −3 °C (or 0 °C in some versions, as noted previously). These usually occur in the interiors of continents and on their upper east coasts, normally north of 40° North latitude. In the Southern Hemisphere, Group D climates are extremely rare due to the smaller land masses in the middle latitudes and the almost complete absence of land at 40°–60° South latitude, existing only in some highland locations.
Group D climates are subdivided as follows:
These climates are characterized by average temperatures below in all twelve months of the year:
Some climatologists have argued that Köppen's system could be improved upon. One of the most frequently-raised objections concerns the temperate Group C category, regarded by many as overbroad. Using the 0°C isotherm, New Orleans, LA and London would both fall into this climate scheme, despite dramatic differences between these 2 locations. In Applied Climatology (first edition published in 1966), John F. Griffiths proposed a new subtropical zone, encompassing those areas with a coldest month of between 6 and 18 °C (43 and 64 °F), effectively subdividing Group C into two nearly equal parts (his scheme assigns the letter B to the new zone, and identifies dry climates with an additional letter immediately following the temperature-based letter).
Another point of contention involves the dry B climates; the argument here is that their separation by Köppen into only two thermal subsets is inadequate. Those who hold this view (including Griffiths) have suggested that the dry climates be placed on the same temperature continuum as other climates, with the thermal letter being followed by an additional capital letter — S for steppe or W (or D) for desert — as applicable (Griffiths also advances an alternate formula for use as an aridity threshold: R = 160 + 9T, with R equalling the threshold, in millimeters of mean annual precipitation, and T denoting the mean annual temperature in degrees Celsius).
A third idea is to create a maritime polar or EM zone within Group E to separate relatively mild marine locations (such as the Falkland Islands, and the outer Aleutian Islands) from the colder, continental tundra climates. Specific proposals vary; some advocate setting a coldest-month parameter, such as , while others support assigning the new designation to areas with an average annual temperature of above 0 °C.
The accuracy of the 10 °C warmest-month line as the start of the polar climates has also been questioned; Otto Nordenskiöld, for example, devised an alternate formula: W = 9 − 0.1 C, with W representing the average temperature of the warmest month and C that of the coldest month, both in degrees Celsius (for instance, if the coldest month averaged −20 °C, a warmest-month average of 11 °C or higher would be necessary to prevent the climate from being polar). This boundary does appear to more closely follow the tree line, or the latitude poleward of which trees cannot grow, than the 10 °C warmest-month isotherm; the former tends to run poleward of the latter near the western margins of the continents, but at a lower latitude in the landmass interiors, the two lines crossing at or near the east coasts of both Asia and North America.
The Trewartha climate classification scheme (1966 and 1980 update) is a modified version of the Köppen system, and was an answer to some of the deficiencies of the 1899 Köppen system. The newer Trewartha theme attempts to redefine the middle latitudes in such a way as to be closer to vegetational zoning and genetic climate systems. This change was seen as most effective in Asia and North America, where many areas fell into a single zone (the C climate group). Under the standard Köppen system in the USA for example, western Washington and Oregon are classed into the same climate as southern California, even though the two regions have strikingly different weather and vegetation. The Köppen system also classes Midwest into the same climate as the Gulf Coast.
Trewartha's modifications sought to reclass the middle latitudes into zones; 1) Subtropical - 8 or more months have a mean temperature of 50 F/10 C or higher. 2) Temperate - 4 to 7 months have a mean temperature of 10 C or higher. 3) Boreal (or subarctic) - 1 to 3 months have a mean temperature of 10 C or higher. This change from the older Köppen system was thought to reflect a more true or "real world" reflection of the global climate.
Based on recent data sets from the Climatic Research Unit (CRU) of the University of East Anglia and the Global Precipitation Climatology Centre (GPCC) at the German Weather Service, a new digital Köppen–Geiger world map on climate classification for the second half of the 20th century has been compiled.
All maps use the ≥0 °C definition for temperate climates and the 18 °C annual mean temperature threshold to distinguish between hot and cold dry climates.
Köppen map of Africa
Köppen map of the Americas
Köppen map of Asia
Köppen map of Australia/Oceania
Köppen map of Brazil
Köppen map of Europe
Köppen map of North America
Köppen map of South Asia
Köppen map of Russia
Köppen map of South America
Köppen map of the Middle East
A pusher trailer is a device attached to the rear of a vehicle or bike that provides force to assist the vehicle.
Electric pusher trailers use energy stored in a battery, typically of lithium ion or sealed lead acid chemistry to provide power. Two wheel and one wheel designs are common.
Gas][ powered pusher trailers typically employ a two or four stroke internal combustion engine to provide power.
Pusher trailers are gasoline, diesel or electric fueled trailers with a traditional internal combustion engine (petroleum engines) and transmission which can be hitched up to battery electric vehicles and run from the cockpit to give the vehicle increased range.
The trailer provides ground traction through the wheels to push the trailer forward, and by default, the electric vehicle as well. In this way, a trip beyond the normal range of the EV can be undertaken without stopping for recharging.
Some types of articulated bus have the engine (and propulsion) in the rear section.
is the amount of water vapor in the air. Water vapor is the gas phase of water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table or humidex, used during summer weather.
There are three main measurements of humidity: absolute, relative and specific. Absolute humidity
is the water content of air. Relative humidity
, expressed as a percent, measures the current absolute humidity relative
to the maximum for that temperature. Specific humidity
is a ratio of the water vapor content of the mixture to the total air content on a mass basis.
Absolute humidity is an amount of water vapor, usually discussed per unit volume. The mass of water vapor,
, per unit volume of total air and water vapor mixture,
, can be expressed as follows:
Absolute humidity in air ranges from zero to roughly 30 grams per cubic meter when the air is saturated at 30 °C. (See also Climate/Humidity table)
The absolute humidity changes as air temperature or pressure changes. This is very inconvenient for chemical engineering calculations, e.g. for clothes dryers, where temperature can vary considerably. As a result, absolute humidity is generally defined in chemical engineering as mass of water vapor per unit mass of dry air, also known as the mass mixing ratio
(see below), which is much more rigorous for heat and mass balance calculations. Mass of water per unit volume as in the equation above would then be defined as volumetric humidity
. Because of the potential confusion, British Standard BS 1339 (revised 2002) suggests avoiding the term "absolute humidity". Units should always be carefully checked. Most humidity charts are given in g/kg or kg/kg, but any mass units may be used.
The field concerned with the study of physical and thermodynamic properties of gas–vapor mixtures is named psychrometrics.
is the ratio of the partial pressure of water vapor in the air–water mixture to the saturated vapor pressure of water at those conditions. The relative humidity of air is a function of both its water content and temperature.
Relative humidity is normally expressed as a percentage and is calculated by using the following formula. It is defined as the ratio of the partial pressure of water vapor (H2
in the mixture to the saturated vapor pressure of water
at a prescribed temperature.
Relative humidity is an important metric used in weather forecasts and reports, as it is an indicator of the likelihood of precipitation, dew, or fog. In hot summer weather, a rise in relative humidity increases the apparent temperature to humans (and other animals) by hindering the evaporation of perspiration from the skin. For example, according to the Heat Index, a relative humidity of 75% at 80.0°F (26.7°C) would feel like 83.6°F ±1.3 °F (28.7°C ±0.7 °C) at ~44% relative humidity.
is the ratio of water vapor to dry air in a particular mass, and is sometimes referred to as humidity ratio. Specific humidity is expressed as a ratio of mass of water vapor,
, per unit mass of dry air
. This quantity is also known as the water vapor "mixing ratio".
That ratio is defined as:
Specific humidity can be expressed in other ways including:
Using this definition of specific humidity, the relative humidity can be expressed as
However, specific humidity is also defined as the ratio of water vapor to the total mass of the system (dry air plus water vapor). For example, the ASHRAE 2009 Handbook, Ch1,1.2, (9a) defines specific humidity as "the ratio of the mass of water vapor to total mass of the moist air sample".
There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is a humidity-triggered switch, often used to control a dehumidifier.
Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future weather forecasts.
While humidity itself is a climate variable, it also interacts strongly with other climate variables. The humidity is affected by winds and by rainfall. At the same time, humidity affects the energy budget and thereby influences temperatures in two major ways. First, water vapor in the atmosphere contains "latent" energy. During transpiration or evaporation, this latent heat is removed from surface liquid, cooling the earth's surface. This is the biggest non-radiative cooling effect at the surface. It compensates for roughly 70% of the average net radiative warming at the surface. Second, water vapor is the most important of all greenhouse gases. Water vapor, like a green lens that allows green light to pass through it but absorbs red light, is a "selective absorber". Along with other greenhouse gases, water vapor is transparent to most solar energy, as you can literally see. But it absorbs the infrared energy emitted (radiated) upward by the earth's surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect. It raises the surface temperature substantially above its theoretical radiative equilibrium temperature with the sun, and water vapor is the cause of more of this warming than any other greenhouse gas.
The most humid cities on earth are generally located closer to the equator, near coastal regions. Cities in South and Southeast Asia are among the most humid. Kuala Lumpur and Singapore have very high humidity all year round because of their proximity to water bodies and the equator and oftentimes overcast weather. Some places experience extreme humidity during their rainy seasons combined with warmth giving the feel of a lukewarm sauna, such as Kolkata, Chennai and Cochin in India, Lahore in Pakistan, the cities of Manila in the Philippines, Mogadishu in Somalia and Bangkok in Thailand. Darwin, Australia experiences an extremely humid wet season from December to April. Shanghai and Hong Kong in China also have an extreme humid period in their summer months. During the South-west and North-east Monsoon seasons (respectively, late May to September and November to March), expect heavy rains and a relatively high humidity post-rainfall. Outside the monsoon seasons, humidity is high (in comparison to countries North of the Equator), but completely sunny days abound. In cooler places such as Northern Tasmania, Australia, high humidity is experienced all year due to the ocean between mainland Australia and Tasmania. In the summer the hot dry air is absorbed by this ocean and the temperature rarely climbs above .
In the United States the most humid cities, strictly in terms of relative humidity, are Forks and Olympia, Washington. This fact may come as a surprise to many, as the climate in this region rarely exhibits the discomfort usually associated with high humidity. This is because high dew points play a more significant role than relative humidity in discomfort, and so the air in these western cities usually does not feel "humid" as a result. In general, dew points are much lower in the Western U.S. than those in the Eastern U.S.
The highest dew points in the US are found in coastal Florida and Texas. When comparing Key West and Houston, two of the most humid cities from those states, coastal Florida seems to have the higher dew points on average. However, Houston lacks the coastal breeze present in Key West, and, as a much larger city, it suffers from the urban heat island effect. A dew point of was recorded in Moorhead Minnesota on July 19, 2011, with a heat index of 133.5, although dew points over are rare there. The US city with the lowest annual humidity is Las Vegas, Nevada, averaging 39% for a high and 21% as a low.
Humidity depends on water vaporization and condensation, which, in turn, mainly depends on temperature. Therefore, when applying more pressure to a gas saturated with water, all components will initially decrease in volume approximately according to the ideal gas law. However, some of the water will condense until returning to almost the same humidity as before, giving the resulting total volume deviating from what the ideal gas law predicted. Conversely, decreasing temperature would also make some water condense, again making the final volume deviate from predicted by the ideal gas law. Therefore, gas volume may alternatively be expressed as the dry volume, excluding the humidity content. This fraction more accurately follows the ideal gas law. On the contrary the saturated volume is the volume a gas mixture would have if humidity was added to it until saturation (or 100% relative humidity).
Humid air is less dense than dry air because a molecule of water (M ≈ 18 u ) is less massive than either a molecule of nitrogen (M ≈ 28) or a molecule of oxygen (M ≈ 32). About 78% of the molecules in dry air are nitrogen (N2
). Another 21% of the molecules in dry air are oxygen (O2
). The final 1% of dry air is a mixture of other gases.
For any gas, at a given temperature and pressure, the number of molecules present in a particular volume is constant – see ideal gas law. So when water molecules (vapor) are introduced into that volume of dry air, the number of air molecules in the volume must decrease by the same number, if the temperature and pressure remain constant. (The addition of water molecules, or any other molecules, to a gas, without removal of an equal number of other molecules, will necessarily require a change in temperature, pressure, or total volume; that is, a change in at least
one of these three parameters. If temperature and pressure remain constant, the volume increases, and the dry air molecules that were displaced will initially move out into the additional volume, after which the mixture will eventually become uniform through diffusion.) Hence the mass per unit volume of the gas—its density—decreases. Isaac Newton discovered this phenomenon and wrote about it in his book Opticks
Humidity is one of the fundamental abiotic factors that defines any habitat, and is a determinant of which animals and plants can thrive in a given environment.
The human body dissipates heat through perspiration and its evaporation. Heat convection to the surrounding air, and thermal radiation are the primary modes of heat transport from the body. Under conditions of high humidity, the rate of evaporation of sweat from the skin decreases. Also, if the atmosphere is as warm as or warmer than the skin during times of high humidity, blood brought to the body surface cannot dissipate heat by conduction to the air, and a condition called hyperpyrexia results. With so much blood going to the external surface of the body, relatively less goes to the active muscles, the brain, and other internal organs. Physical strength declines, and fatigue occurs sooner than it would otherwise. Alertness and mental capacity also may be affected, resulting in heat stroke
Humans are sensitive to humid air because the human body uses evaporative cooling as the primary mechanism to regulate temperature. Under humid conditions, the rate
at which perspiration evaporates on the skin is lower than it would be under arid conditions. Because humans perceive the rate of heat transfer from the body rather than temperature itself, we feel warmer when the relative humidity is high than when it is low.
Some people experience difficulty breathing in high humidity environments. Some cases may possibly be related to respiratory conditions such as asthma, while others may be the product of anxiety. Sufferers will often hyperventilate in response, causing sensations of numbness, faintness, and loss of concentration, among others.
Air conditioning reduces discomfort in the summer not only by reducing temperature, but also by reducing humidity. In winter, heating cold outdoor air can decrease relative humidity levels indoor to below 30%, leading to discomfort such as dry skin and excessive thirst.
Many electronic devices have humidity specifications, for example, 5% to 95%. At the top end of the range, moisture may increase the conductivity of permeable insulators leading to malfunction. Too low humidity may make materials brittle. A particular danger to electronic items, regardless of the stated operating humidity range, is condensation. When an electronic item is moved from a cold place (e.g. garage, car, shed, an air conditioned space in the tropics) to a warm humid place (house, outside tropics), condensation may coat circuit boards and other insulators, leading to short circuit inside the equipment. Such short circuits may cause substantial permanent damage if the equipment is powered on before the condensation has evaporated. A similar condensation effect can often be observed when a person wearing glasses comes in from the cold (i.e. the glasses become foggy). It is advisable to allow electronic equipment to acclimatise for several hours, after being brought in from the cold, before powering on. Some electronic devices can detect such a change and indicate, when plugged in and usually with a small droplet symbol, that they cannot be used until the risk from condensation has passed. In situations where time is critical, increasing air flow through the device's internals when, such as removing the side panel from a PC case and directing a fan to blow into the case will reduce significantly the time needed to acclimatise to the new environment.
On the opposite, very low humidity level favors the build-up of static electricity, which may result in spontaneous shutdown of computers when discharges occur. Apart from spurious erratic function, electrostatic discharges can cause dielectric breakdown in solid state devices, resulting in irreversible damage. Data centers often monitor relative humidity levels for these reasons.
Traditional building designs typically had weak insulation, and it allowed air moisture to flow freely between the interior and exterior. The energy-efficient, heavily-sealed architecture introduced in the 20th century also sealed off the movement of moisture, and this has resulted in a secondary problem of condensation forming in and around walls, which encourages the development of mold and mildew. Additionally, buildings with foundations not properly sealed will allow water to flow through the walls due to capillary action of pores found in masonry products. Solutions for energy-efficient buildings that avoid condensation are a current topic of architecture.
The indirect land use change impacts of biofuels, also known as ILUC, relates to the unintended consequence of releasing more carbon emissions due to land-use changes around the world induced by the expansion of croplands for ethanol or biodiesel production in response to the increased global demand for biofuels.
As farmers worldwide respond to higher crop prices in order to maintain the global food supply-and-demand balance, pristine lands are cleared to replace the food crops that were diverted elsewhere to biofuels production. Because natural lands, such as rainforests and grasslands, store carbon in their soil and biomass as plants grow each year, clearance of wilderness for new farms translates to a net increase in greenhouse gas emissions. Due to this change in the carbon stock of the soil and the biomass, indirect land use change has consequences in the GHG balance of a biofuel.
Other authors have also argued that indirect land use changes produce other significant social and environmental impacts, affecting biodiversity, water quality, food prices and supply, land tenure, worker migration, and community and cultural stability.
The estimates of carbon intensity for a given biofuel depend on the assumptions regarding several variables. As of 2008, multiple full life cycle studies had found that corn ethanol, cellulosic ethanol and Brazilian sugarcane ethanol produce lower greenhouse gas emissions than gasoline. None of these studies, however, considered the effects of indirect land-use changes, and though land use impacts were acknowledged, estimation was considered too complex and difficult to model. A controversial paper published in February 2008 in Sciencexpress by a team led by Searchinger from Princeton University concluded that such effects offset the (positive) direct effects of both corn and cellulosic ethanol and that Brazilian sugarcane performed better, but still resulted in a small carbon debt.
After the Searchinger team paper, estimation of carbon emissions from ILUC, together with the food vs. fuel debate, became one of the most contentious issues relating to biofuels, debated in the popular media, scientific journals, op-eds and public letters from the scientific community, and the ethanol industry, both American and Brazilian. This controversy intensified in April 2009 when the California Air Resources Board (CARB) set rules that included ILUC impacts to establish the California Low-Carbon Fuel Standard that entered into force in 2011.
In May 2009 U.S. Environmental Protection Agency (EPA) released a notice of proposed rulemaking for implementation of the 2007 modification of the Renewable Fuel Standard (RFS). EPA's proposed regulations also included ILUC, causing additional controversy among ethanol producers. EPA's February 3, 2010 final rule incorporated ILUC based on modelling that was significantly improved over the initial estimates.
The UK Renewable Transport Fuel Obligation program requires the Renewable Fuels Agency (RFA) to report potential indirect impacts of biofuel production, including indirect land use change or changes to food and other commodity prices. A July 2008 RFA study, known as the Gallager Review, found several risks and uncertainties, and that the "quantification of GHG emissions from indirect land-use change requires subjective assumptions and contains considerable uncertainty", and required further examination to properly incorporate indirect effects into calculation methodologies. A similarly cautious approach was followed by the European Union. In December 2008 the European Parliament adopted more stringent sustainability criteria for biofuels and directed the European Commission to develop a methodology to factor in GHG emissions from indirect land use change.
Before 2008, several full life cycle ("Well to Wheels" or WTW) studies had found that corn ethanol reduced transport-related greenhouse gas emissions. In 2007 a University of California, Berkeleyteam led by Farrel evaluated six previous studies, concluding that corn ethanol reduced GHG emissions by only 13 percent. However, 20 to 30 percent reduction for corn ethanol, and 85 to 85 percent for cellulosic ethanol, both figures estimated by Wang from Argonne National Laboratory, are more commonly cited. Wang reviewed 22 studies conducted between 1979 and 2005, and ran simulations with Argonne's GREET model. These studies accounted for direct land use changes. Several studies of Brazilian sugarcane ethanol showed that sugarcane as feedstock reduces GHG by 86 to 90 percent given no significant land use change. Estimates of carbon intensity depend on crop productivity, agricultural practices, power sources for ethanol distilleries and the energy efficiency of the distillery. None of these studies considered ILUC, due to estimation difficulties. Preliminary estimates by Delucchi from the University of California, Davis, suggested that carbon released by new lands converted to agricultural use was a large percentage of life-cycle emissions.
In 2008 Timothy Searchinger, a lawyer from Environmental Defense Fund, concluded that ILUC affects the life cycle assessment and that instead of saving, both corn and cellulosic ethanol increased carbon emissions as compared to gasoline by 93 and 50 percent respectively. Ethanol from Brazilian sugarcane performed better, recovering initial carbon emissions in 4 years, while U.S. corn ethanol required 167 years and cellulosic ethanol required a 52 years payback period. The study limited the analysis a 30 year period, assuming that land conversion emits 25 percent of the carbon stored in soils and all carbon in plants cleared for cultivation. Brazil, China, and India were considered among the overseas locations where land use change would occur as a result of diverting U.S. corn cropland, and it was assumed that new cropland in each of these regions correspond to different types of forest, savanna or grassland based on the historical proportion of each converted to cultivation in these countries during the 1990s.
Fargione and his team published a separate paper in the same issue of Sciencexpress claiming that clearing lands to produce biofuel feedstock created a carbon deficit. This deficit applies to both direct and indirect land use changes. The study examined six conversion scenarios: Brazilian Amazon to soybean biodiesel, Brazilian Cerrado to soybean biodiesel, Brazilian Cerrado to sugarcane ethanol, Indonesian or Malaysian lowland tropical rainforest to palm biodiesel, Indonesian or Malaysian peatland tropical rainforest to palm biodiesel, and U.S. Central grassland to corn ethanol. The carbon debt was defined as the amount of CO2 released during the first 50 years of this process of land conversion. For the two most common ethanol feedstocks, the study found that sugarcane ethanol produced on natural cerrado lands would take about 17 years to repay its carbon debt, while corn ethanol produced on U.S. central grasslands would result in a repayment time of about 93 years. The worst-case scenario is converting Indonesian or Malaysian tropical peatland rainforest to palm biodiesel production, which would require about 420 years to repay.
The Searchinger and Fargione studies created controversy in both the popular media and in scientific journals. Robert Zubrin observed that Searchinger's "indirect analysis" approach is pseudo-scientific and can be used to "prove anything".
Wang and Haq from Argonne National Laboratory claiming: the assumptions were outdated; they ignored the potential of increased efficiency; and no evidence showed that "U.S. corn ethanol production has so far caused indirect land use in other countries." They concluded that Searchinger demonstrated that ILUC "is much more difficult to model than direct land use changes". In his response Searchinger rebutted each technical objection and asserted that "... any calculation that ignores these emissions, however challenging it is to predict them with certainty, is too incomplete to provide a basis for policy decisions."
Another criticism, by Kline and Dale from Oak Ridge National Laboratory, held that Searchinger et al. and Fargione et al. "... do not provide adequate support for their claim that bioufuels cause high emissions due to land-use change", as their conclusions depends on a misleading because more comprehensive field research found that these land use changes "... are driven by interactions among cultural, technological, biophysical, economic, and demographic forces within a spational and temporal contest rather than by a single crop market". Fargione et al. responded in part that although many factors contributed to land clearing, this "observation does not diminish the fact that biofuels also contribute to land clearing if they are produced on existing cropland or on newly cleared lands". Searching disagreed with all of Kline and Dale arguments.
The U.S. biofuel industry also reacted, claiming that the "Searchinger study is clearly a 'worst case scenario' analysis ..." and that this study "relies on a long series of highly subjective assumptions ..." Searchinger rebutted each claim, concluding that NFA's criticisms were invalid. He noted that even if some of his assumptions are high estimates, the study also made many conservative assumptions.
In February 2010 Lapola estimated that planned expansion of Brazilian sugarcane and soybean biofuel plantations through 2020 would replace rangeland, with small direct land-use impact on carbon emissions. However, the expansion of the rangeland frontier into Amazonian forests, driven by cattle ranching would indirectly offset the savings. "Sugarcane ethanol and soybean biodiesel each contribute to nearly half of the projected indirect deforestation of 121,970 km2 by 2020, creating a carbon debt that would take about 250 years to be repaid...."
The research also found that oil palm would cause the least land-use changes and associated carbon debt. The analysis also modeled livestock density increases and found that "a higher increase of 0.13 head per hectare in the average livestock density throughout the country could avoid the indirect land-use changes caused by biofuels (even with soybean as the biodiesel feedstock), while still fulfilling all food and bioenergy demands." The authors conclude that intensification of cattle ranching and concentration on oil palm are required to achieve effective carbon savings, recommending closer collaboration between the biofuel and cattle-ranching sectors.
The main Brazilian ethanol industry organization (UNICA) commented that such studies missed the continuing intensification of cattle production already underway.
An study by Arima et al. published in May 2011 used spatial regression modeling to provide the first statistical assessment of ILUC for the Brazilian Amazon due to soy production. Previously, the indirect impacts of soy crops were only anecdotal or analyzed through demand models at a global scale, while the study took a regional approach. The analysis showed a strong signal linking the expansion of soybean fields in settled agricultural areas at the southern and eastern rims of the Amazon basin to pasture encroachments for cattle production on the forest frontier. The results demonstrate the need to include ILUC in measuring the carbon footprint of soy crops, whether produced for biofuels or other end-uses.
The Arima study is based on 761 municipalities located in the Legal Amazon of Brazil, and found that between 2003 and 2008, soybean areas expanded by 39,100 km2 in the basin's agricultural areas, mainly in Mato Grosso. The model showed that a 10% (3,910 km2) reduction of soy in old pasture areas would have led to a reduction in deforestation of up to 40% (26,039 km2) in heavily forested municipalities of the Brazilian Amazon. The analysis showed that the displacement of cattle production due to agricultural expansion drives land use change in municipalities located hundreds of kilometers away, and that the Amazonian ILUC is not only measurable but its impact is significant.
On April 23, 2009, California Air Resources Board (CARB) approved the specific rules and carbon intensity reference values for the California Low-Carbon Fuel Standard (LCFS) that take effect January 1, 2011. CARB's rulemaking included ILUC. For some biofuels, CARB identified land use changes as a significant source of additional GHG emissions. It established one standard for gasoline and alternative fuels, and a second for diesel fuel and its replacements.
The public consultation process before the ruling, and the ruling itself were controversial, yielding 229 comments. ILUC was one of the most contentious issues. On June 24, 2008, 27 scientists and researchers submitted a letter saying, "As researchers and scientists in the field of biomass to biofuel conversion, we are convinced that there simply is not enough hard empirical data to base any sound policy regulation in regards to the indirect impacts of renewable biofuels production. The field is relative new, especially when compared to the vast knowledge base present in fossil fuel production, and the limited analyses are driven by assumptions that sometimes lack robust empirical validation." The New Fuels Alliance, representing more than two-dozen biofuel companies, researchers and investors, questioned the Board intention to include indirect land use change effects into account, wrote "While it is likely true that zero is not the right number for the indirect effects of any product in the real world, enforcing indirect effects in a piecemeal way could have very serious consequences for the LCFS.... The argument that zero is not the right number does not justify enforcing a different wrong number, or penalizing one fuel for one category of indirect effects while giving another fuel pathway a free pass."
On the other side, more than 170 scientists and economists urged that CARB, "include indirect land use change in the lifecycle analyses of heat-trapping emissions from biofuels and other transportation fuels. This policy will encourage development of sustainable, low-carbon fuels that avoid conflict with food and minimize harmful environmental impacts.... There are uncertainties inherent in estimating the magnitude of indirect land use emissions from biofuels, but assigning a value of zero is clearly not supported by the science."
Industry representatives complained that the final rule overstated the environmental effects of corn ethanol, and also criticized the inclusion of ILUC as an unfair penalty to domestic corn ethanol because deforestation in the developing world was being tied to U.S. ethanol production. The 2011 limit for LCFS means that Mid-west corn ethanol failed, unless current carbon intensity was reduced. Oil industry representatives complained that the standard left oil refiners with few options, such as Brazilian sugarcane ethanol, with its accompanying tariff. CARB officials and environmentalists counter that time and economic incentives will allow produces to adapt.
UNICA welcomed the ruling, while urging CARB to better reflect Brazilian practices, lowering their estimates of Brazilian emissions.
The only Board member who voted against the ruling explained that he had a "hard time accepting the fact that we're going to ignore the comments of 125 scientists", referring to the letter submitted by a group of scientists questioning the ILUC penalty. "They said the model was not good enough ... to use at this time as a component part of such an historic new standard." CARB advanced the expected date for an expert working group to report on ILUC with refined estimates from January 2012 to January 2011.
On December 2009 the Renewable Fuels Association (RFA) and Growth Energy, two U.S. ethanol lobbying groups, filed a lawsuit challenging LCFS' constitutionality. The two organizations argued that LCFS violated both the Supremacy Clause and the Commerce Clause, jeopardizing the nationwide ethanol market.
The Energy Independence and Security Act of 2007 (EISA) established new renewable fuel categories and eligibility requirements, setting mandatory lifecycle emissions limits. EISA explicitly mandated EPA to include "direct emissions and significant indirect emissions such as significant emissions from land use changes."
EISA required a 20% reduction in lifecycle GHG emissions for any fuel produced at facilities that commenced construction after December 19, 2007 to be classified as a "renewable fuel"; a 50% reduction for fuels to be classified as "biomass-based diesel" or "advanced biofuel", and a 60% reduction to be classified as "cellulosic biofuel". EISA provided limited flexibility to adjust these thresholds downward by up to 10 percent, and EPA proposed this adjustment for the advanced biofuels category. Existing plants were grandfathered in.
On May 5, 2009, EPA released a notice of proposed rulemaking for implementation of the 2007 modification of the Renewable Fuel Standard, known as RFS2. The draft of the regulations was released for public comment during a 60-day period, a public hearing was held on 9 June 2009, and also a workshop was conducted on 10–11 June 2009.
EPA's draft analysis stated that ILUC can produce significant near-term GHG emissions due to land conversion, but that biofuels can pay these back over subsequent years. EPA highlighted two scenarios, varying the time horizon and the discount rate for valuing emissions. The first assumed a 30 year time period uses a 0 percent discount rate (valuing emissions equally regardless of timing). The second scenario used a 100 year time period and a 2% discount rate.
On the same day that EPA published its notice of proposed rulemaking, President Obama signed a Presidential Directive seeking to advance biofuels research and commercialization. The Directive established the Biofuels Interagency Working Group, to develop policy ideas for increasing investment in next-generation fuels and for reducing their environmental footprint.
The inclusion of ILUC in the proposed ruling provoked complaints from ethanol and biodiesel producers. Several environmental organizations welcomed the inclusion of ILUC but criticized the consideration of a 100 year payback scenario, arguing that it underestimated land conversion effects. American corn growers, biodiesel producers, ethanol producers and Brazilian sugarcane ethanol producers complained about EPA's methodology, while the oil industry requested an implementation delay.
On June 26, 2009, the House of Representatives approved the American Clean Energy and Security Act 219 to 212, mandating EPA to exclude ILUC for a 5-year period, vis a vis RFS2. During this period, more research is to be conducted to develop more reliable models and methodologies for estimating ILUC, and Congress will review this issue before allowing EPA to rule on this matter. The bill failed in the U.S. Senate.
On February 3, 2010, EPA issued its final RFS2 rule for 2010 and beyond. The rule incorporated direct and significant indirect emissions including ILUC. EPA incorporated comments and data from new studies. Using a 30 year time horizon and a 0% discount rate, EPA concluded that multiple biofuels would meet this standard.
EPA's analysis accepted both ethanol produced from corn starch and biobutanol from corn starch as "renewable fuels". Ethanol produced from sugarcane became an "advanced fuel". Both diesel produced from algal oils and biodiesel from soy oil and diesel from waste oils, fats, and greases fell in the "biomass-based diesel" category. Cellulosic ethanol and cellulosic diesel met the "cellulosic biofuel" standard.
The table summarizes the mean GHG emissions estimated by EPA modelling and the range of variations considering that the main source of uncertainty in the life cycle analysis is the GHG emissions related to international land use change.
UNICA welcomed the ruling, in particular, for the more precise lifecycle emissions estimate and hoped that classification the advanced biofuel designation would help eliminate the tariff.
The U.S. Renewable Fuels Association (RFA) also welcomed the ruling, as ethanol producers "require stable federal policy that provides them the market assurances they need to commercialize new technologies", restating their ILUC objection.
RFA also complained that corn-based ethanol scored only a 21% reduction, noting that without ILUC, corn ethanol achieves a 52% GHG reduction. RFA also objected that Brazilian sugarcane ethanol "benefited disproportionally" because EPA's revisions lowered the initially equal ILUC estimates by half for corn and 93% for sugarcane.
Several Midwestern lawmakers commented that they continued to oppose EPA's consideration of the "dicey science" of indirect land use that "punishes domestic fuels". House Agriculture Chairman Collin Peterson said, "... to think that we can credibly measure the impact of international indirect land use is completely unrealistic, and I will continue to push for legislation that prevents unreliable methods and unfair standards from burdening the biofuels industry."
EPA Administrator Lisa P. Jackson commented that the agency "did not back down from considering land use in its final rules, but the agency took new information into account that led to a more favorable calculation for ethanol". She cited new science and better data on crop yield and productivity, more information on co-products that could be produced from advanced biofuels and expanded land-use data for 160 countries, instead of the 40 considered in the proposed rule.
As of 2010, European Union and United Kingdom regulators had recognized the need to take ILUC into account, but had not determined the most appropriate methodology.
The UK Renewable Transport Fuel Obligation (RTFO) program requires fuel suppliers to report direct impacts, and asked the Renewable Fuels Agency (RFA) to report potential indirect impacts, including ILUC and commodity price changes. The RFA's July 2008 "Gallager Review", mentioned several risks regarding biofuels and required feedstock production to avoid agricultural land that would otherwise be used for food production, despite concluding that "quantification of GHG emissions from indirect land-use change requires subjective assumptions and contains considerable uncertainty". Some environmental groups argued that emissions from ILUC were not being taken into account and could be creating more emissions.
On December 17, 2008, the European Parliament approved the Renewable Energy Sources Directive (COM(2008)19) and amendments to the Fuel Quality Directive (Directive 2009/30), which included sustainability criteria for biofuels and mandated consideration of ILUC. The Directive established a 10% biofuel target. A separate Fuel Quality Directive set the EU's Low Carbon Fuel Standard, requiring a 6% reduction in GHG intensity of EU transport fuels by 2020. The legislation ordered the European Commission to develop a methodology to factor in GHG emissions from ILUC by December 31, 2010, based on the best available scientific evidence.
In the meantime, the European Parliament defined lands that were ineligible for producing biofuel feedstocks for the purpose of the Directives. This category included wetlands and continuously forested areas with canopy cover of more than 30 percent or cover between 10 and 30 percent given evidence that its existing carbon stock was low enough to justify conversion.
The Commission subsequently published terms of reference for three ILUC modeling exercises: one using a General Equilibrium model; one using a Partial Equilibrium model and one comparing other global modeling exercises. It also consulted on a limited range of high-level options for addressing ILUC to which 17 countries and 59 organizations responded. The United Nations Special Rapporteur on the Right to Food and several environmental organizations complained that the 2008 safeguards were inadequate. UNICA called for regulators to establish an empirical and "globally accepted methodology" to consider ILUC, with the participation of researchers and scientists from biofuel crop-producing countries.
In 2010 some NGOs accused the European Commission of lacking transparency given its reluctance to release documents relating to the ILUC work. In March 2010 the Partial and General Equilibrium Modelling results were made available, with the disclaimer that the EC had not adopted the views contained in the materials. These indicate that a 1.25% increase in EU biofuel consumption would require around 5,000,000 hectares (12,000,000 acres) of land globally.
The scenarios for varied from 5.6-8.6% of road transport fuels. The study found that ILUC effects offset part of the emission benefits, and that above the 5.6% threshold, ILUC emissions increase rapidly increase. For the expected scenario of 5.6% by 2020, the study estimated that biodiesel production increases would be mostly domestic, while bioethanol production would take place mainly in Brazil, regardless of EU duties. The analysis concluded that eliminating trade barriers would further reduce emissions, because the EU would import more from Brazil. Under this scenario, "direct emission savings from biofuels are estimated at 18 Mt , additional emissions from ILUC at 5.3 Mt (mostly in Brazil), resulting in a global net balance of nearly 13 Mt savings in a 20 years horizon. The study also found that ILUC emissions were much greater for biodiesel from vegetable oil and estimated that in 2020 even at the 5.6% level were over half the greenhouse gas emissions from diesel.
As part of the announcement, the Commission stated that it would publish a report on ILUC by the end of 2010.
On June 10, 2010, the EC announced its decision to set up certification schemes for biofuels, including imports as part of the Renewable Energy Directive. The Commission encouraged E.U. nations, industry and NGOs to set up voluntary certification schemes. EC figures for 2007 showed that 26% of biodiesel and 31% of bioethanol used in the E.U. was imported, mainly from Brazil and the United States.
UNICA welcomed the EU efforts to "engage independent experts in its assessments" but requested that improvements because "... the report currently contains a certain number of inaccuracies, so once these are corrected, we anticipate even higher benefits resulting from the use of Brazilian sugarcane ethanol." UNICA highlighted the fact that the report assumed land expansion that "does not take into consideration the agro-ecological zoning for sugarcane in Brazil, which prevents cane from expanding into any type of native vegetation."
Critics said the 10% figure was reduced to 5.6% of transport fuels partly by exaggerating the contribution of electric vehicles (EV) in 2020, as the study assumed EVs would represent 20% of new car sales, two and six times the car industry's own estimate. They also claimed the study "exaggerates to around 45 percent the contribution of bioethanol—the greenest of all biofuels—and consequently downplays the worst impacts of biodiesel."
Environmental groups found that the measures "are too weak to halt a dramatic increase in deforestation". According to Greenpeace, "indirect land-use change impacts of biofuel production still are not properly addressed", which for them was the most dangerous problem of biofuels
Industry representatives welcomed the certification system and some dismissed concerns regarding the lack of land use criteria. UNICA and other industry groups wanted the gaps in the rules filled to provide a clear operating framework.
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