Trewartha Climate Classification

Just like in the Koppen classification, tropical climate is based on the average temperature of all months being higher than 18°C (65°F). This basically means that all regions classified as A in the Trewartha classification, are places with warm weather all year round.

The Ar subtype is characterized by evenly distributed precipitation throughout the year. There are no regular drought or wet periods. The Ar climate type is sometimes also referred to as the climate of the rainforests. It is most commonly found around the equator in areas covered with tropical rainforests. Typical is high humidity and sufficient precipitation. Seasonal variability is very small and because these places are located close to the equator, also the day and night length do not change very much during the year.

Just like in the Koppen classification, tropical climate is based on the average temperature of all months being higher than 18°C (65°F). This basically means that all regions classified as A in the Trewartha classification, are places with warm weather all year round.

The Aw subtype is characterized by a drier period during the “winter” – the term winter here refers to the part of the year with shorter days and longer nights. This climate type (sometimes also called the tropical climate of the savannas) is typical for savannas, most commonly found in Africa, Asia and South and Central America.

In general there are four savanna climate types:

• clearly distinguishable dry and wet periods of approximately same lengths. Most of precipitation falls during the wet period, during the dry period it rains only very little if at all.
• longer dry period followed by a relatively short wet period. In this case there are typically seven or more months of drought followed by five or less months with rain. This creates a relatively large variability and includes places with almost equal length dry and wet periods (5 and 7 months), but also places with a very long drought and then very short wet period, which, however, is typical with very intense heavy rains.
• the third type is a relatively long wet period and a shorter dry period, in other words the exact opposite of the previous type. There are at least seven wet months and maximum of five dry months.
• the fourth type is very rare and is characterized by dry periods, during which however, it can sometimes also rain and so the difference between the dry and wet period is smaller than in the previous three types.

Just like in the Koppen classification, tropical climate is based on the average temperature of all months being higher than 18°C (65°F). This basically means that all regions classified as A in the Trewartha classification, are places with warm weather all year round.

The As subtype is characterized by dry period during the “summer” – this means during the period of longer days and shorter nights. It is a relatively rare subtype and just like the Aw subtype (dry winter), it is sometimes referred to as tropical climate of the savannas.

In general there are four savanna climate types:

• clearly distinguishable dry and wet periods of approximately same lengths. Most of precipitation falls during the wet period, during the dry period it rains only very little if at all.
• longer dry period followed by a relatively short wet period. In this case there are typically seven or more months of drought followed by five or less months with rain. This creates a relatively large variability and includes places with almost equal length dry and wet periods (5 and 7 months), but also places with a very long drought and then very short wet period, which, however, is typical with very intense heavy rains.
• the third type is a relatively long wet period and a shorter dry period, in other words the exact opposite of the previous type. There are at least seven wet months and maximum of five dry months.
• the fourth type is very rare and is characterized by dry periods, during which however, it can sometimes also rain and so the difference between the dry and wet period is smaller than in the previous three types.

The climate types from the group B of the classification are arid (deserts) and semi-arid areas. The BS subtype is sometimes referred to as the climate of the steppes and in comparison to the BW subtype, it is slightly less dry. It can also be defined as area, where the precipitation is lower than potential evapotranspiration, but the difference is not very big. For classification it is first necessary to determine the precipitation threshold based on the average annual temperature and annual distribution and amount of precipitation. In Koppen classification there are two subtypes of BS, the BSh and BSk. BSh is a hot semi-arid climate, BSk is cold semi-arid climate.

Semi-arid regions are characterized by bushes and grasslands. In case of the hot semi-arid zones, the summer tends to be extremely hot and winters very mild. Snow is extremely rare. Hot semi-arid climate can be found for example in Western Africa, India, parts of Mexico and some states in the U.S. The cold semi-arid zones are usually located more inland and also have relatively warm summers, but colder than the hot semi-arid zones. Winters are colder and often accompanied by snow. This type is also common for areas at higher altitudes and typical is bigger day-night temperature variation. In general it can be said that areas with cold semi-arid climate that are at higher altitudes tend to have drier summers and more precipitation during spring, autumn and winter. Cold semi-arid regions can be found for example in Central Asia, on the West of the U.S., in some parts of Northern Australia, parts of Spain or Southern part of South America.

Desert, in other words arid, climate is characterized by extremely small precipitation. In Koppen classification it is further subdivided into types h and k. Deserts can be both hot and cold. It could be said that it basically includes areas, which although are not classified as polar regions, are in some aspects quite similar to them, especially by the almost complete absence of vegetation due to water insufficiency.

Arid regions have annual precipitation of less than 250 mm, but often much less and there are even areas where there are years with absolutely no rain at all.

Hot desert climate is most often found at around 30 ° latitude, bordering the subtropical zones. There is minimal cloudiness all year round and temperatures are extremely high, often above 40°C (105°F), in some places even more than 45°C (113°F). It in fact includes the warmest places on Earth, however, this is not to say it is not cold here at all. Due to the minimal cloud cover, it cools down very rapidly after sunset and sometimes even freezes in the early morning hours, just before sunrise. Hot deserts are found especially in Northern Africa, the most well-known is the largest desert on Earth, the Sahara desert. Then there are also hot deserts in the Middle East, Central and West Australia or some parts of Southwest U.S. and Southwest Africa.

Cold desert regions have a slightly misleading name because the word “cold” must be considered in context – summers here are still extremely hot and dry. In the winter however, temperatures drop quite significantly, sometimes well below freezing point. Cold desert regions are often found in places with higher altitude. Regions with this climate type are for example in Central Asia, the most well-known is the Mongolian Gobi desert.

Subtropical in the Koppen and Trewartha classification is defined as a region, where at least eight months has an average temperature of more than 10°C (50°F), but the coldest month has an average temperature of less than 18°C (65°F). The individual subtypes then differ in their annual precipitation distribution. In Trewartha classification there is a condition of maximum annual precipitation of 900 mm.

In general the subtropical climate is usually found in areas polewards from the tropical zones. It can also be found inside tropical zones at places with higher altitude. Typical are for example citrus trees or palm trees.

The Cs subtype in the Trewartha classification represents a subtropical climate with dry summer, sometimes referred to as the Mediterranean climate. It is characterized by hot and dry summers and mild winters, during which most of the precipitation is observed. High pressure is the dominating factor in the summer, when precipitation is very small and if there is some, it is usually in the form of very short, but intense thunderstorms. In the winter it can sometimes snow, especially at higher altitudes, but in general the winters are mild.

This climate type is most common in western coasts of continents in the middle latitudes, for example it includes areas on the West coast of the U.S. such as California, Southern Europe or Southwest tip of Australia.

Subtropical in the Koppen and Trewartha classification is defined as a region, where at least eight months has an average temperature of more than 10°C (50°F), but the coldest month has an average temperature of less than 18°C (65°F). The individual subtypes then differ in their annual precipitation distribution. In Trewartha classification there is a condition of maximum annual precipitation of 900 mm.

In general the subtropical climate is usually found in areas polewards from the tropical zones. It can also be found inside tropical zones at places with higher altitude. Typical are for example citrus trees or palm trees.

The Cw subtype is characterized by more precipitation in the summer and relatively dry and mild winter. In regions with this climate type, which are affected by the monsoons, there is often very intense precipitation during the summer and in contrast very profound dry periods during winter, which can potentially have quite negative effects for agriculture. Most of the summer precipitation falls during shorter intense thunderstorms.

Subtropical climate with dry winter can be found for example in some parts of Central South America, in the center of Southern part of Africa and also in some parts of India and Southeast Asia.

Subtropical in the Koppen and Trewartha classification is defined as a region, where at least eight months has an average temperature of more than 10°C (50°F), but the coldest month has an average temperature of less than 18°C (65°F). The individual subtypes then differ in their annual precipitation distribution. In Trewartha classification there is a condition of maximum annual precipitation of 900mm (35in).

In general the subtropical climate is usually found in areas polewards from the tropical zones. It can also be found inside tropical zones at places with higher altitude. Typical are for example citrus trees or palm trees.

The Cf subtype is characterized by precipitation evenly distributed throughout the year.

It can be found for example in Eastern part of South America, in large areas in Southeast U.S., Northern Italy, the Southwest tip of Russia, in Southeast Asia, some parts of Japan or East coast of Australia.

In the Trewartha classification, the category D is like in the Koppen classification, assigned to regions in the temperate zone. The criterion for this type of climate is four to seven months with average temperature of more than 10°C (50°F). In general it is further subdivided into two subtypes: oceanic and continental. These differ in their average temperature of the coldest month.

Temperate climate is characterized by four distinct seasons and there are substantial differences between the temperatures in the summer and in the winter, but the absolute values are not so extreme and the change is gradual.

In case of the oceanic type, winter is milder than that of the continental type and the temperature of the coldest month usually above 0°C (32°F). The weather conditions are to a large extent influenced by the nearby ocean, which causes smaller temperature variation between the individual seasons. Typical is Western wind and so we often find this type of climate on Western coasts of continents, such as for example in Western Europe or Western coast of the U.S., in latitudes between 40 and 60° in America and 40 and 65° in Europe. These regions tend to be densely inhabited.

In the Trewartha classification, the category D is like in the Koppen classification, assigned to regions in the temperate zone. The criterion for this type of climate is four to seven months with average temperature of more than 10°C (50°F). In general it is further subdivided into two subtypes: oceanic and continental. These differ in their average temperature of the coldest month.

Temperate climate is characterized by four distinct seasons and there are substantial differences between the temperatures in the summer and in the winter, but the absolute values are not so extreme and the change is gradual.

The continental subtype is typical for regions located more inland and this also means the temperature variation is larger than that in case of the oceanic subtype. The coldest month has an average temperature of less than 0°C. Summers are usually warmer, while winters colder with snow being quite common.

Continental D climate type can be found for example in parts of Northeast U.S., Central and Eastern Europe and usually these areas are quite densely inhabited.

Boreal climate is sometimes also referred to as subarctic. It is colder all year round and characterized by a very large temperature amplitude throughout the year, in other words very big difference between temperatures in the summer and in the winter. Winters often experience temperatures as low as –30°C (-22°F). In the summer on the other hand, values of even more than 30°C (85°F) are not uncommon. These warm summers however only last very short, not more than three months. The criterion that is used for boreal climate classification is at least one month with average temperature of at least 10°C (50°F). Five to seven months, the average temperature is below freezing point and the soil is frozen, in some areas permanently (permafrost).

Boreal climate usually has relatively little annual precipitation not exceeding 400mm (15in). Places further from the coast tend to have most of precipitation in the winter months, in areas near the coast most precipitation is usually observed during autumn.

Typical vegetation includes conifers, ferns and several other tree species, which can withstand the very low temperatures in the winter. Even though the overall species diversity is smaller, vegetation is abundant, which can be seen for example in large forested areas referred to as taiga. In fact, taigas are the absolute largest tree covered regions on Earth and can be found especially in Russia and Canada.

Just like in case of the temperate climate C, the Trewartha classification divides this category into two subtypes – the oceanic and continental. The oceanic type (EO) does not have such extreme differences between individual seasons. Boreal oceanic climate can be found for example in some parts of Scotland, Kamchatka and in regions closer to the coast in Canada, Siberia, Alaska and Northern Europe.

Boreal climate is sometimes also referred to as subarctic. It is colder all year round and characterized by very large temperature amplitude throughout the year, in other words very big difference between temperatures in the summer and in the winter. Winters often experience temperatures as low as –30°C (-22°F). In the summer on the other hand, values of even more than 30°C (85°F) are not uncommon. These warm summers however only last very short, not more than three months. The criterion that is used for boreal climate classification is at least one month with average temperature of at least 10°C (50°F). Five to seven months, the average temperature is below freezing point and the soil is frozen, in some areas permanently (permafrost).

Boreal climate usually has relatively little annual precipitation not exceeding 400mm (15in). Places further from the coast tend to have most of precipitation in the winter months, in areas near the coast most precipitation is usually observed during autumn.

Typical vegetation includes conifers, ferns and several other tree species, which can withstand the very low temperatures in the winter. Even though the overall species diversity is smaller, vegetation is abundant, which can be seen for example in large forested areas referred to as taiga. In fact, taigas are the absolute largest tree covered regions on Earth and can be found especially in Russia and Canada.

Just like in case of the temperate climate C, the Trewartha classification divides this category into two subtypes – the oceanic and continental. The continental subtype (EC) is characterized by extreme differences between temperatures in the summer and in the winter. An example of this is the Russian Oymyakon, where in July temperatures can be above 30°C (85°F), while in January, it can even drop below -50°C (-58°F), absolute record being around -70°C (-94°F)! In general therefore we can find boreal continental climate in large inland areas on the Northern hemisphere in Canada and Russia.

Polar climate includes the coldest places on Earth. The general criterion for this classification is average monthly temperature of all months less than 10°C (50°F). In Trewartha classification this type is further subdivided into two subtypes – polar tundra climate and ice cap climate.

Polar regions are characterized by absence of the summer as we know it. Each month has an average temperature of less than 10°C (50°F), quite often much less. In total, the Polar Regions include approximately 20% of land. Summers are characterized by very long days and the opposite is true for the winter, when it is dark for most part of the day if not the entire day.

The tundra subtype is slightly warmer than the ice cap type. At least one month in the year, the average temperature is above freezing point. For trees however, this is still too cold, so the vegetation here is usually small bushes or ferns. Precipitation tends to be very small.

Tundra can be found for example in the northernmost part of Canada, West and North coast of Alaska, the northern tip of Europe and Russia, coastal areas in Greenland and also a very narrow strip along the coast of Antarctica.

Polar climate includes the coldest places on Earth. The general criterion for this classification is average monthly temperature of all months less than 10°C (50°F). In Trewartha classification this type is further subdivided into two subtypes – polar tundra climate and ice cap climate.

Polar regions are characterized by absence of the summer as we know it. Each month has an average temperature of less than 10°C (50°F), in case of the ice-cap subtype, it is even less than 0°C.

Ice-cap polar climate is characterized by very low temperatures all year round and many places lie behind the arctic circle, meaning there is the polar day and polar night. Average temperature in the entire year is below freezing point and the areas are permanently covered with snow and ice. Only very few animal species live here, which rely on gathering food from the see (for example polar bears or penguins). No plants can grow here.

Ice cap polar climate includes areas in most of Antarctica and inland Greenland. The absolute minimum temperature recorded on Earth was recorded in the polar station in Vostok, Antarctica (-89.2°C/-128.6°F). Satellite images however suggest that in some parts of this coldest continent, the temperatures are during specific weather conditions even lower.