Humid subtropical climate
A humid subtropical climate (Köppen climate classification Cfa or Cwa) is a zone of subtropical climate characterized by hot, humid summers and generally mild winters. Under the Köppen climate definition, this category of climate type covers a broad range of attributes, especially in terms of winter temperatures. Under the more modern Trewartha climate classification, most of the climates that have 8 or more months with a mean temperature of 10 C (50 F) are warm to hot much of the year with mild winters.
The Köppen definition of this climate is for the coldest month's mean temperature to be between −3 °C (26.6 °F) and 18 °C (64.4 °F), and the warmest month to be above 22 °C (71.6 °F). Some climatologists prefer to use 0 °C (32 °F) 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).
Under the more modern Trewartha climate classification - climates are termed Humid Subtropical when they have mean temperatures of 10 C (50 F) for eight or more months a year. In most locations classed within this system, the mean temperature of the coldest month is between 4.5 C (40 F) and 7 C (45 F). Most modern climatologist consider the Trewartha grouping of subtropical climates to be more real-world and fitting on a global scale.
Rainfall in humid subtropical climates often shows a summer peak, and where monsoons are well developed like Southeast Asia - a strong summer peak and winter drought is common. Due to the influence of the subtropical highs and subsidence, droughts can be severe and often catastrophic to agriculture. Winter rainfall is associated with large storms and fronts that the westerlies steer from west to east, that on occasion reach down into subtropical latitudes. Most summer rainfall occurs during thunderstorms and weak tropical lows that move in from adjacent warm tropical oceans.
Overview of distribution
Humid subtropical climates normally lie 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.
|Climate chart (explanation)|
In Africa, the humid subtropical climates are found in two separate areas on the southern hemisphere of the continent. The Cwa climate is found 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.
|Climate chart (explanation)|
In East Asia, this climate type is found in 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 near the equatorward boundary of this zone include Hong Kong, Hanoi and Taipei while Qingdao is near 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 display a highly pronounced summer peak, 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.
Southwestern Asia (Northern Middle East and Caucasus)
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 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. Again, during winter, the coastal areas can receive snowfall, which is usually of a short duration.
In North America, humid subtropical climates are almost exclusively the domain of the American Gulf and south Atlantic states, including the following states: the eastern half of Texas, Oklahoma, Louisiana, Arkansas, Alabama, Mississippi, North Carolina, South Carolina, Virginia, Tennessee, Georgia, and Florida. On the Florida peninsula, the humid subtropical climates gives way to the tropical climate of south Florida and the Florida Keys. In the USA, the archetypal humid subtropical climate is best exemplified by a city like Savannah, Georgia - where summers are long and almost tropical, and winters are mild with temperatures reaching freezing on only a few nights. Summers in this zone are hot and humid, with daily averages above 25 °C (77 °F) with average daily maxima above 30 °C (86 °F) .
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.
Characteristics and variants
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, such as Orlando and Tampa and along the Texas coast around Brownsville 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 (using the Koppen climate classification system - as the newer Trewartha climate classification does not class any areas north of North Carolina, southern Tennessee, and southern Arkansas as subtropical), such as New York City, Philadelphia, and Louisville, Kentucky have average winter temperatures (0 C/32 F) at the coldest limit of climates classed as subtropical. These cities generally experience colder winters than the true subtropical climates to the south.
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, New Orleans, and Savannah 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.
Precipitation is plentiful in the humid subtropical climate zone in North America - and tends to be concentrated in the six warmer months (May through October). Inland areas in the south tend to have precipitation spread evenly throughout the year, while there is a semblance of a monsoon pattern (dry winters/wet summers) along the Atlantic coast from southern North Carolina south to Florida. The seasonal monsoon is much stronger on the Florida peninsula, as most locations in Florida have dry winters and wet summers. 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 drought-like nadir in mid-summer.
|Climate chart (explanation)|
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, Curitiba, 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.
|Climate chart (explanation)|
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.
Variations in Australia
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.
|Climate chart (explanation)|
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, particularly in Catalonia, southern France Garonne Valley and Rhone Valley, Adriatic northern Italy, parts of Epirus in Greece around the area of Ioannina, most of the Vardar Valley in the Republic of Macedonia, parts of coastal northern Croatia, coastal Slovenia and Central Serbia 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 in 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.
- Köppen: w — driest winter month receives less than one tenth of the precipitation of the wettest summer month
- Koppen: f — winter months get more than one tenth of the precipitation of the wettest summer month, and summer months get at least 30 mm (1.2 in) per month or more than one third as much the wettest winter month
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- "Brisbane Regional Office". Climate statistics for Australian locations. Bureau of Meteorology. Retrieved 2 August 2011.
- 058037 Lismore (Centre Street) monthly rainfall