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.