Humid subtropical climate

A humid subtropical climate is a zone of climate characterized by hot and humid summers, and cool to mild winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 40° (sometimes 46°) and are located poleward from adjacent tropical climates. It is also known as warm temperate climate.

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 more tropical climates of the southern coasts of these countries and the more continental climates of China and the United States’ northern and central regions (localities around the Ohio and Yangtze rivers exhibiting continental influence from the north, compared to climates around the Gulf of Mexico and the South China Sea, which exhibit tropical influence due to their southern coastal positions). Inland locations in South America also feature humid subtropical climates, though they do not feature quite as pronounced seasonal variations as inland locations with this climate in China and the United States.

Under the Köppen climate classification, Cfa and Cwa climates are either described as humid subtropical climates or mild temperate climates. This climate features mean temperature in the coldest month between 0 °C (32 °F) or −3 °C (27 °F) and 18 °C (64 °F) and mean temperature in the warmest month 22 °C (72 °F) 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 10 °C (50 °F).

Under the Holdridge life zones classification, the subtropical climates have a biotemperature between the frost or critical temperature line, 16 to 18 °C (61 to 64 °F) (depending on locations in the world) and 24 °C (75 °F), and these climates are humid (or even perhumid or superhumid) when the potential evapotranspiration (PET) ratio (= PET / Precipitation) is less than 1. In the Holdridge classification, the humid subtropical climates coincide more or less with the warmest Cfa and Cwa climates and the less warm humid tropical “Köppen” climates (Aw, Am and Af).

In a humid subtropical climate, summers are typically long, hot and humid. Monthly mean summer temperatures are normally between 24 and 27 °C (75 and 81 °F). 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. Summer high temperatures are typically in the high 20s to mid-30s °C (80s or 90s °F), while overnight lows in the summer are typically in the lower 20s °C (70s °F). Monthly mean temperatures in winter are often mild, typically averaging 7.5 to 16 °C (45.5 to 60.8 °F). Daytime highs in winter normally are in the 10 to 16 °C (50 to 61 °F) range, while overnight lows are from 2 to 7 °C (36 to 45 °F), though the poleward boundaries of this climate feature colder temperatures.

Rainfall often shows a summer peak, especially where monsoons are well developed, as in Southeast Asia and South Asia. Other areas have a more uniform or varying rainfall cycles, but consistently lack any predictably dry summer months. 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 infrequent tropical storms often contribute to summer seasonal rainfall peaks. Winter rainfall is often associated with large storms in the westerlies that have fronts that reach down into subtropical latitudes. However, many subtropical climates such as southeast Asia and Florida in the United States have very dry winters, with frequent brush fires and water shortages.

Breakdown of letters

Cfa: C = Mild temperate f = Fully humid a = Hot Summer

Cwa: C = Mild temperate w = Dry Winter a = Hot Summer 

Africa

Durban, South Africa
Climate chart (explanation)
J F M A M J J A S O N D 134 28 21 113 28 21 120 28 20 73 26 17 59 25 14 38 23 11 39 23 11 62 23 13 73 23 15 98 24 17 108 25 18 102 27 20 Average max. and min. temperatures in °C Precipitation totals in mm Source:
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In Africa, humid subtropical climates are primarily found in 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.

Asia

East Asia

Tokyo, Japan
Climate chart (explanation)
J F M A M J J A S O N D 52 10 1 56 10 2 118 14 4 125 19 9 138 23 14 168 26 18 154 29 22 168 31 23 210 27 20 198 22 14 93 16 8 51 12 4 Average max. and min. temperatures in °C Precipitation totals in mm Source: Japan Meteorological Agency
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In East and Southeast Asia, this climate type is found in the southeastern quarter of mainland China from Hong Kong north to Nanjing, the northern half of Taiwan, northern Myanmar, northern Vietnam, north through southern and central Japan (Kyushu, Shikoku and half of Honshu), and the most southern regions of Korea (the south coast and Jeju island). Cities near the equatorward boundary of this zone include Hong Kong, Hanoi and Taipei; while Tokyo, Busan 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 North America, South America, and Australia. The 0 °C (32 °F) 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 1,000 millimetres (39 in), and in areas below the Himalayas can be much higher still.

South Asia

Humid subtropical climates can also be found in South Asia. However, the humid subtropical climates exhibited here typically differ markedly from those 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 (104 °F). They also tend to be extremely dry, complete with dust storms, traits usually associated with arid or semi-arid 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 decrease during the monsoon season, but humidity increases. This results in hot and humid conditions, similar to summers in other 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 Islamabad and Rawalpindi, as well as the city of Swabi in Khyber Pakhtunkhwa, feature this weather pattern.

In Bangladesh, cities like Rangpur and Dinajpur features a more conventional version of the humid subtropical climate, though it is heavily influenced by monsoons.

Rangpur, Bangladesh
Climate chart (explanation)
J F M A M J J A S O N D 9.2 23 8 12 24 11 13 26 17 26 31 21 118 33 23 268 32 25 467 32 26 477 32 26 345 31 25 181 31 20 8.5 27 15 8.1 25 10 Average max. and min. temperatures in °C Precipitation totals in mm Source: Bangladesh Meteorological Department
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Humid subtropical climates can also be found in Nepal. However the Nepalese version of the climate generally do 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 winter-rainfall climates 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.

Western Asia

Kutaisi, Georgia
Climate chart (explanation)
J F M A M J J A S O N D 106 8 1 129 9 2 100 13 5 112 18 8 85 23 12 105 26 16 106 28 19 86 29 20 116 26 16 108 21 12 141 15 8 139 10 4 Average max. and min. temperatures in °C Precipitation totals in mm Source:
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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 and Māzandarān 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 740 millimetres (29.1 in) at Sari to over 2,000 millimetres (79 in) 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 Western Asia. During winter, the coastal areas can receive snowfall, which is usually of a short duration.

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.

Western Georgia (Batumi and Kutaisi) in the Kolkheti Lowland and the northeast coast of Turkey (Giresun), 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 (72 °F) in summer to 5 °C (41 °F) in winter and rainfall is even heavier than in Caspian Iran, up to 2,300 millimetres (91 in) per year in Hopa (Turkey). These climates are a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case.

North America

In North America, humid subtropical climates are found in the American Gulf and lower East Coast states, including Texas, Oklahoma, Louisiana, Arkansas, Alabama, Mississippi, Tennessee, Kentucky, North Carolina, South Carolina, Georgia, and Florida. On the Florida peninsula, the humid subtropical climate gives way to the tropical climate of South Florida and the Florida Keys.

Houston, United States
Climate chart (explanation)
J F M A M J J A S O N D 86 17 6 81 19 8 87 23 11 84 26 15 129 30 20