A humid subtropical climate is a temperate climate type, characterized by long and hot summers, and cool to mild winters. These climates normally lie on the southeast side of all continents (except Antarctica), generally between latitudes 25° and 40° and are located poleward from adjacent tropical climates, and equatorward from either humid continental (in North America and Asia) or oceanic climates (in other continents). It is also known as warm temperate climate in some climate classifications.
Under the Köppen climate classification, Cfa and Cwa climates are either described as humid subtropical climates or warm temperate climates. This climate features mean temperature in the coldest month between (or ) and and mean temperature in the warmest month or higher. However, while some climatologists have opted to describe this climate type as a "humid subtropical climate", Köppen himself never used this term. The humid subtropical climate classification was officially created under the Trewartha climate classification. In this classification, climates are termed humid subtropical when they have at least 8 months with a mean temperature above .
While many subtropical climates tend to be located at or near coastal locations, in some cases, they extend inland, most notably in China and the United States, where they exhibit more pronounced seasonal variations and sharper contrasts between summer and winter, as part of a gradient between the hotter tropical climates of the southern coasts and the colder continental climates to the north and further inland. As such, the climate can be said to exhibit somewhat different features depending on whether it is found inland, or in a maritime position.
Characteristics
In a humid subtropical climate, summers are typically long, hot and humid. A deep current of tropical air dominates the humid subtropics at the time of high sun, and daily intense (but brief) convective thundershowers are common. Monthly mean temperatures in winter are normally mild, however they may be cold at the northern edges of the humid subtropical zone.
Rainfall often shows a summer peak in most humid subtropical climates, especially where there is a well developed monsoon (seasonal reversal of winds) such as in South Asia. Most summer rainfall occurs during thunderstorms that build up due to the intense surface heating and strong subtropical sun angle. Weak tropical lows that move in from adjacent warm tropical oceans, as well as occasional tropical cyclones often contribute to summer seasonal rainfall peaks such as occurs in the US state of Florida as well as eastern Australia. Normally, winter is a drier season in most humid subtropical climates, however, some higher latitude subtropical climates in North America, East Asia, and Europe, also have modest precipitation in winter as well.
Under the Holdridge life zones classification, the subtropical climates have a biotemperature between the frost or critical temperature line, (depending on locations in the world) and , and these climates are humid (or even perhumid or superhumid) when the potential evapotranspiration (PET) ratio (= PET / Precipitation) is less than 1.The climate of Carpathian Region in the 20th century based on the original and modified Holdridge life zone system In the Holdridge classification, the humid subtropical climates of the Koppen system coincide more or less with the subtropical and warm temperate life zones.
Breakdown of letters Cfa: C = Mild temperate f = Fully humid a = Hot summer
Cwa: C = Mild temperate w = Dry winter a = Hot summer
Locations Africa
In Africa, humid subtropical climates are primarily found in the southeastern part 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.
Asia East Asia
In East Asia, this climate type is found in the southeastern quarter of China from Hong Kong north to Nanjing, the northern half of Taiwan, southern and central Japan (Kyushu, Shikoku and half of Honshu), and the most southern regions of Korea (the south and east Central and Southern Gyeongsang Province and Jeju Island). Cities near the equatorward boundary of this zone include Hong Kong and Taichung; while Iida - Kōfu - Niigata - Sendai of Japan and Gwangju - Daegu - Gangneung - Sokcho of Korea and Qingdao are near the northern boundary.
The influence of the strong Siberian anticyclone in East Asia brings colder winter temperatures than in the humid subtropical zones in The Americas, South Asia, and Australia. The isotherm reaches as far south as the valleys of the Yellow and Wei rivers, roughly latitude 34° N. At Hainan Island and in Taiwan, the climate transitions from subtropical into tropical. In most of this region, the winter monsoon is very well developed, as such eastern Asian humid subtropical zones have a strong winter dry season and heavy summer rainfall.
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. Drought can be severe and often catastrophic to agriculture in the Cwa zone.
The only area where winter precipitation equals or even exceeds the summer rain is around the San'in region at the 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 , and in areas below the Himalayas can be much higher still.
South Asia
Humid subtropical climates can also be found in the Indian subcontinent, predominantly in the northern regions. However, the humid subtropical climates exhibited here typically differ markedly from those in East Asia (and, for that matter, much of the globe). Winters (December–February) are typically cool to mild (sometimes reaching , and ranging from humid and foggy in December to dry in February. These winters are followed by a mild spring (March–April), often accompanied by hail in the northwestern regions. Summers tend to be relatively longer and very hot, starting from mid-April and peaking in June, with high temperatures often exceeding . Summers usually begin dry, complete with dust storms, traits typically associated with arid or semi-arid climates, before eventually transforming into a more humid and stormy June. This is followed by the cooler but still hot and extremely humid monsoon season (July–September), where the region experiences heavy rains almost daily, with humidity usually above 90%. The autumn season (October–November), which immediately follows the monsoon and precedes winter, usually experiences a pleasant climate. Cities such as New Delhi, Dehradun, Lucknow, Kanpur and Chandigarh, among others, exhibit this atypical version of the climate in India. In Pakistan, the cities of Islamabad, Sialkot, Gujranwala and Rawalpindi, among others, feature this weather pattern. Lahore and New Delhi are humid subtropical with some semi-arid influences as they receive slightly less annual rainfall compared to a typical humid subtropical climate at 747 mm and 790 mm respectively.
In Bangladesh, humid subtropical climates are commonly present in the northern half (north of Dhaka). Cities like Rajshahi, Rangpur, Saidpur and Dinajpur in the northern region feature the monsoon variant (Cwa), where rainfall peaks at the monsoon season. Closely resembling the climate patterns of the neighboring Indo-Gangetic Plains, this region shows a distinct three-season pattern: a relatively dry and very hot summer (March–May), extremely wet, cooler monsoon season (June–September), and mild, foggy winter (December–February), with a post-monsoon/autumn transition season (October–November)
Humid subtropical climates can also be found in Nepal. However, the Nepalese version of the climate generally does not feature the extreme hot spells that are commonplace for many other South Asian locations with this climate. In Nepal cities such as Kathmandu, Pokhara, Butwal, Birgunj and Biratnagar feature this iteration of the climate.
In South Asia, humid subtropical climates generally border on continental climates as altitude increases, or on a Mediterranean climate in western areas of Pakistan and northwestern India (e.g. Peshawar in northwestern Pakistan or Srinagar in the Kashmir Valley in India, where the primary precipitation peak occurs in March, not July or August). Further east, in highland areas with lengthier monsoons such as Nepal, seasonal temperature variation is lower than in the lowlands. Further west, moving closer to the Bagar and Great Indian Desert, the climate begins to shift towards a hot semi-arid climate.
Southeast Asia
In Southeast Asia, about 90% of the region has a tropical climate; but humid subtropical climates can also be seen here, such as in Northern Vietnam.
Southeast Asian locations with these climates can feature cool temperatures, with lows reaching during the months of December, January, and February. Unlike a good portion of East Asian locations with this climate however, most of Southeast Asia seldom experiences snowfall. These areas tend to feature hot and wet summers and cool and dry winters, with mean temperatures varying between in summer.
Western Asia
Although humid subtropical climates in Asia are mostly confined to the southeastern quarter of the continent, there are two narrow areas along the coast of the Caspian Sea and Black Sea with humid subtropical climates. Summers in these locations are cooler than typical humid subtropical climates and snowfall in winter is relatively common, but is usually of a short duration.
In Western Asia, the climate is prevalent in the Gilan, Māzandarān and Golestan Provinces of Iran, in parts of the Caucasus, in Azerbaijan and in Georgia wedged between the Caspian and Black seas and coastal (Black Sea) Turkey, albeit having more oceanic influence.
Annual rainfall ranges from around at Gorgan to over at Bandar-e Anzali, and is heavy throughout the year, with a maximum in October or November when Bandar-e Anzali can average in one month. Temperatures are generally moderate in comparison with other parts of Western Asia. During winter, the coastal areas can receive snowfall, which is usually of a short duration.
In Rasht, the average temperature in July and August is around but with near-saturation humidity, whilst in January and February it is around . 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.