Humid subtropical climate

Warm temperate with hot summer climate zones of the world according to a modified Köppen climate classification that uses a threshold of 0 °C (32 °F) for the coldest month.

  Cfa

  Cwa

A humid subtropical climate is a zone of climate characterised by hot and humid summers where tropical air masses dominate, and mild winters. These climates normally lie on the southeast side of all continents, generally between latitudes 25° and 35° and are adjacent to tropical climates to the south. While many subtropical climates tend to be located at coastal or near coastal locations, in some cases they extend inland, most notably 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 temperatures in the coldest month between 0 °C (32 °F) and 18 °C (64 °F) and mean temperatures 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",^[1] Köppen himself never used this term. The humid subtropical climate classification was officially created under the Trewartha Climate classification.

The Trewartha system was a 1966 update of the Köppen climate classification, and sought to redefine middle latitude climates into smaller zones (the original Köppen system grouped all middle latitude climates into a single zone). 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. ^[2]

Rainfall often shows a summer peak, especially where monsoons are well developed, as in Southeast Asia and Florida (USA). Other areas have a more uniform or varying rainfall cycles, but never have dry months on the summer (climatologically). 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 high sun (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 or Florida have very dry winters, with frequent brush fires and water shortages.

Africa

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.

Asia

East & Southeast Asia

In East and Southeast Asia, this climate type is found in the southeastern quarter of mainland China, the northern half of Taiwan, northern Myanmar, northern Vietnam, narrow areas along the coast of South Korea, and Japan (Kyushu, Shikoku and half 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. 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 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.

South Asia

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 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 decreases during the monsoon season but the 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 Rawalpindi/Islamabad also feature this weather pattern, but with wetter and relatively cooler winters. Sialkot is another major city in Pakistan that features a humid subtropical 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 (i.e. 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

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 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 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.

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.^[5]

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 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). These climates are a Cfb/Cfa (Oceanic climate/Humid subtropical climate) borderline case.

North America

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, Kentucky, North Carolina, South Carolina, Tennessee, Georgia, and Florida. On the Florida peninsula, the humid subtropical climate gives way to the tropical climate of south Florida and the Florida Keys.

Under Köppen's climate classification, the humid subtropical climate can also be found in the Mid-Atlantic, primarily Virginia, the lower elevations of West Virginia, 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 far southern portions of Illinois, Indiana and Ohio.

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 in North America lie just north of South Florida and around southern coastal Texas. Cities at the southernmost limits, such as Tampa and Orlando and along the Texas coast around Corpus Christi down toward Brownsville generally feature warm weather year-round and minimal temperature differences between seasons. In contrast, cities at the northernmost limits of the humid subtropical region (using the Köppen climate classification system) such as Philadelphia and Louisville, Kentucky feature hot, humid summers and chilly winters. These areas have average winter temperatures at the coldest limit of climates classed as humid subtropical.

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, Charlotte, and Raleigh, snow typically falls once or twice a season and is usually three inches or less. However, for the majority of the winter here, temperatures remain above or well above freezing. At the northernmost limits of this zone (according to the Köppen climate classification system), cities such as New York City and Philadelphia typically see snowfall during the winter, with occasional heavy snowstorms. Still, average temperatures during a typical winter hover just above freezing at these locations.

Precipitation is plentiful in the humid subtropical climate zone in North America – but with significant variations in terms of wettest/driest months and seasons. Much of the interior South, including Tennessee, Kentucky, and the northern halves of Mississippi and Alabama, tends to have a winter or spring (not summer) precipitation maximum. Closer to the South Atlantic and Gulf coasts, there is a summer maximum, with July or August usually the wettest month – as at Norfolk, Cape Hatteras and Jacksonville, North Carolina, Charleston, South Carolina, Mobile, Alabama and New Orleans. 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 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 May, a drought-like nadir in mid-summer and a secondary, if not equal, precipitation peak in September or October. Districts further south, along South Texas' Gulf Coast (Corpus Christi and Brownsville), typically have a strong September precipitation maximum, and a tendency toward dry conditions in late winter and spring, with March or April often the driest months.

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 grades into the cooler and wet oceanic climates. ^[8][9]

From Newcastle, just above 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).^[10] 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.^[11]

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.^[12]

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.

Annual rainfall on the coast can reach as high as 2,000 mm (80 inches) in coastal 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.^[13]

Europe

In Central Europe, a small area of humid subtropical climates are located in transitional areas between the oceanic and continental climates in areas where higher summer temperature do not quite qualify it for inclusion in the Oceanic climate schema (but, in the Trewartha climate classification, this type of climate is included in that typical of the other areas of central-west Europe, classified as Do -the Temperate Oceanic climate-) and mild winters don't allow their inclusion into Continental climates. Average summer temperatures in areas of Europe with this climate are generally not hot as most other subtropical zones around the world.

The Po Valley, in Northern Italy, including major cities like Milan, Turin, Bologna and Verona, have a humid subtropical climate, featuring hot, humid summers with frequent thunderstorms; winters are foggy, damp and chilly, with sudden bursts of frost. Some parts of the valley have a mild continental climate. Places along the shores of Lake Maggiore, Lake Lugano, Lake Como (Como and Verbania in Italy and Lugano in Switzerland) have a humid subtropical climate with a distinctive high amount of rainfall during summer.

The coastal areas in the northern half of the Adriatic sea also fall within this climate zone. The cities include Trieste, Venice and Rimini in Italy, Split in Croatia, Koper in Slovenia and Kotor in Montenegro. Other Southern European areas in the Cfa zone include the central valleys and coast of Catalonia of Girona and Barcelona in Spain, some on the north-east of Spain (Huesca), the Garonne Valley (Toulouse) and Rhone Valley (Avignon) in France.

Along the Black Sea coast of Bulgaria (Varna), coast of Romania (Constanta and Mamaia), Sochi, Russia and Crimea, disputed territory that is both claimed by Russia and 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, where they'd be fit to be classed under Cfa. Though they closely border the Humid continental climate zone. 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, warm summers (> 21 or 22 °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 summer precipitation and southerly flow of tropical air-masses in summer is not present in Europe, as it is in eastern Australia or the southern United States.

See also

• Subtropics
• Subtropical ridge
• Köppen climate classification

References