Question:

# Does humidity affect gas mileage?

## No humidity does not have any effect on fuel economy. However Temperature does have a large affect on gas mileage. Thanks!

Relative humidity 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 $\left(\phi\right)$ of an air-water mixture is defined as the ratio of the partial pressure of water vapor (H2O) $\left({e_w}\right)$ in the mixture to the saturated vapor pressure of water $\left({{e^*}_w}\right)$ 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][: where $T$ is the dry bulb temperature expressed in degrees Celsius (°C), $P$ is the absolute pressure expressed in hectopascals (hPa), and ${{e^*}_w}$ 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. Relative Humidity 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 $\left(f_w\right)$ is defined as the ratio of the saturated vapor pressure of water in moist air $\left(e'_w\right)$ 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.)
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: Lettering Scheme 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.
Humidity 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, $m_w$, per unit volume of total air and water vapor mixture, $V_{net}$, 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.[3] (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. Relative humidity 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 (H2O) $\left({e_w}\right)$ in the mixture to the saturated vapor pressure of water $\left({{e^*}_w}\right)$ 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. Specific 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, $m_v$, per unit mass of dry air $m_a$ . 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: or: 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 or hyperthermia. 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.