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Example of Trewartha climate types for the contiguous United States, showing the humid subtropical zone in the Gulf and south Atlantic states

A humid subtropical climate is a zone of climate characterized by hot, humid, and wet summers, and mild, drier winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 35°. They transition to tropical climates on their equatorial side and continental climates on their poleward side. While many subtropical climates tend to be located at or near coastal locations, in some cases they extend inland, most notably in southern China and the southern United States.

The subtropical zone was added to climate classifications after the Koppen climate system was developed. The Trewartha system was a 1966 update of the Köppen climate classification, and sought to redefine middle latitude climates into smaller zones. In the original Köppen system, a vast area between 25 latitude and 55 latitude was grouped into a single climate zone. Thus, locations like Brisbane, Australia, Jacksonville, Florida, and Hong Kong, were located into the same climate zone as London, New York City, or Beijing. The Trewartha classification also sought to better reflected changes in the natural vegetation between latitudes 25 and 55 north and south.

Under the Trewartha climate classification, climates are termed humid subtropical when they have monthly mean air temperatures higher than 10 °C (50 °F) for eight or more months a year and at least one month with mean temperature below 18 °C (64.4 °F). Under the Trewartha system, humid subtropical climates typically occupy the southernmost portions of the temperate zone from 23.5 to 35.0 north and south latitude, and the coldest month has a mean temperature around 7 C (44 F).^[1]

Like tropical climates, seasonal rainfall in the humid subtropics follows the sun. Summer is often the peak of seasonal rainfall, with the cool months (winter) being drier to varying degrees. In some locations summers have very heavy rainfall, especially where monsoons are well developed, as in Southeast Asia. Most summer rainfall occurs during thunderstorms that build up due to the intense surface heating and strong subtropical sun angle. Daily, brief, but intense convective showers are typical at the time of high sun (summer). Weak tropical lows that move in from adjacent tropical oceans, as well as infrequent tropical storms often contribute to summer seasonal rainfall peaks in the humid subtropical zone. 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 or Florida have very dry winters, with frequent brush fires and water shortages.

Africa

According to the Trewartha climate classification map, a small area of humid subtropical climate is found in the narrow coastal sections of southern South Africa, primarily in KwaZulu-Natal and the Durban, South Africa area.

Asia

East and Southeast Asia

According to the Trewartha climate classification map, a humid subtropical climate extends from southern China east through southern and central Japan and extreme southern North Korea. This region includes the area 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 extreme southern tip of South Korea around Busan. Cities near the equatorward boundary of this zone include Hong Kong, Hanoi and Taipei; while Qingdao and Tokyo are near the northern boundary.

In most of this region, the classic hot and wet summers and cooler and drier winters of the humid subtropical climate is quite well developed. Most locations show a heavy summer concentration of rainfall and a strong winter anticyclone. The influence of the strong Siberian anticyclone in East Asia brings colder average winter temperatures than the humid subtropical zones in other parts of the world. The 0 °C isotherm reaches 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 tropical.

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.

A zone of Humid subtropical climate can also be found along the Ganges River. In this area, 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). Winters are typically mild, dry and relatively short. They also tend to be foggy. As is typical with other humid subtropical climates, rainfall peaks in the summer months. Cities such as New Delhi, Lucknow, Kanpur and Patna show very heavy summer rainfall and drier winter. In Pakistan, the twin cities of Islamabad and Rawalpindi, as well as the city of Swabi in Khyber Pakhtunkhwa, feature a humid subtropical climate with similar hot and wet summers, and cool winters with less rainfall.

Western Asia

In Asia Humid subtropical climates are mostly confined to the southeastern quarter of the continent, however there are two narrow areas along the coast of the extreme southern Caspian Sea and southern portions of the Black Sea with humid subtropical climates. Summers in these locations are cooler than typical humid subtropical climates and there is significantly less tropical air mass control throughout the year.

This zone of humid subtropical climate runs in a narrow belt from 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 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 Western Asia. During winter, the coastal areas can receive snowfall, which is usually of a short duration.

North America

In North America, humid subtropical climates are found exclusively in the American Gulf and south Atlantic states, including eastern Texas, southern Oklahoma, Louisiana, Arkansas, Alabama, Mississippi, North Carolina, South Carolina, Georgia, and northern and central Florida. On the Florida peninsula, the humid subtropical climate gives way to the tropical climate of south Florida and the Florida Keys.

In Mexico, there are small areas of Cfa and Cwa climates. The climate can be found in 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 zone in North America lie just north of South Florida and around southern coastal Texas. Cities at the southernmost limits, such as Tampa, Orlando, Jacksonville, and areas along the Texas coast from Corpus Christi to Brownsville generally feature warm weather year-round and minimal temperature differences between seasons. In contrast, cities at the northernmost limits of the climate zone such as Dallas, Texas, Atlanta, Georgia, and Norfolk, Virginia, feature hot, humid summers and chilly winters. These areas have average winter temperatures at the coldest limit of climates classed as humid subtropical.

Precipitation is normally plentiful in the humid subtropical climate zone in North America and is often summer concentrated, however, there are significant variations in terms of wettest/driest months and seasons across parts of the interior of the region. A semblance of a monsoon pattern (dry winters/wet summers) is evident along the Atlantic coast from southern North Carolina (Wilmington, North Carolina area) south to Florida. The seasonal monsoon is much stronger on the Florida peninsula, as most locations in Florida have quite dry winters and wet summers. In the interior parts of the region such as northern Mississippi and Alabama peak rainfall occurs in the winter months. Snowfall is rare in the humid subtropical areas of the United States. Cities along the Gulf Coast, such as Corpus Christi, Houston, and New Orleans, only see a dusting of snow every 15 or 20 years. Locations at the northern fringes such as Atlanta, Birmingham, Charlotte, or Raleigh, might see snow once or twice a season or every few years.

South America

Humid subtropical climates are found in a sizable portion of South America. The climate extends over a few states of southern Brazil, including Paraná, 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, Montevideo and Campo Grande, 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, Mato Grosso do Sul 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.

Australia

The humid subtropical climate zone predominantly lies in eastern Australia. The coastal strip from Mackay, Queensland to the southern coast of Sydney, where it transitions into the cooler and wet oceanic climates.^[7][8]

From Newcastle, approx 167KM NE of Sydney, the Cfa zone would extend to inland New South Wales, excluding the highland regions (which have an oceanic climate), stretching towards Dubbo to the northwest and Wagga Wagga to the south, ending at the New South Wales/Victoria border (Albury–Wodonga).^[9] To note, these places would have characteristics of the semi-arid and/or Mediterranean climates. Furthermore, the inland Cfa climates generally have drier summers, or at least summers with low humidity.^[10]

Extreme heat is more often experienced in Sydney than in other large cities in Australia's Cfa zone, especially in the western suburbs, where highs over 40 °C are not uncommon. Frost is prevalent in the more inland areas of Sydney, such as Richmond. Average annual rainfall in the Sydney region ranges between 800 mm and 1200 mm.^[11]

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 month (September). Temperatures are very warm to hot but are 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 which extends north to the Köppen Cwa classified climate zone of the Atherton Tablelands region. This region has a very pronounced dry winter period with often negligible rainfall between June to October. Winter temperatures generally only fall slightly below 18°C, which would classify the region as a tropical savanna, or Aw, climate.

Annual rainfall within Australia's humid subtropical climate zone can reach as high as 2,000 mm (80 inches) in coastal locations and is generally 1,000 mm (40 inches) or above. The most intense 2-3 day rainfall periods that occur in this coastal zone however are the outcome of east coast lows forming to the north of a large high pressure system, there can be great variation in rainfall amounts from year to year as a result of these systems. As an example Lismore which lies 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.^[12]

See also

• Subtropics


• Subtropical ridge

References