Trewartha’s Classification of Climate

Glen T. Trewartha was an eminent American climatologist. The Trewartha climate classification scheme is considered a modified version of the Koeppen system.

Besides being ‘simple and explanatory, Trewartha’s classification combines the basic fundamentals of the empirical as well as the genetic classification schemes. Trewartha, while proposing his climatic classification, was conscious of the fact that the classification systems of Koeppen and ‘Thornthwaite being based on certain statistical parameters of certain weather elements were cumbersome and complex.

Trewartha recognised only a limited number of principal climatic types. He made use of the two most important and basic weather elements, i.e., temperature and precipitation as the basic for his own classification.

Besides these, the effects of land and water surfaces on the climate of an area have also been taken into consideration. He has classified world climates in seven climatic groups out of which six—A, C, D, E, F and H—are based on temperature criteria, and the seventh—B—is the dry group based on precipitation.

Climatic Groups Based on Temperature Criteria:

Group A:

This is the tropical climate group. This type of climate is found in the low latitudes on each side of the equator in an irregular belt 20° to 40° wide. There is -no winter season in-this climatic-group. Temperature is uniformly high throughout the year with adequate annual rainfall. In marine areas, the average temperature for the coldest month is around 18°C to 20°G. This climatic group is subdivided into two climatic types:

(i) Ar:

Ar. is tropical wet climate. This type of climate is characterised by less than two dry months. The climate is under the influence of
intertropical convergence zone and the equatorial westerlies. The belt is distinguished by constant low pressure and is also known as the tropical rainforest.

(ii) Aw:

Aw is tropical wet-and-dry climate. At the time of the low sun, two months are usually dry. The climatic regions are dominated by the dry trade winds or subtropical anticyclones. During the high sun period, the equatorial westerlies and intertropical convergence control the weather. The duration of the dry season is usually longer than that of the wet season. Temperature remains uniformly high in this type of climate.

Group C:

This category encompasses sub­tropical climate with temperatures above 10 °C for only eight or more months. Frosts occur occasionally in continental parts, but the marine locations remain frostless, On the basis of the seasonal distribution of precipitation, the sub­tropical climate is further classified into two climatic types:

(i) Cfw:

Cfw. is sub-tropical humid climate. This type of climate is found on the eastern side of continents. It has no distinct dry season and rains fall throughout the year. During summer season, this type of climate comes under the influence of unstable air in the western end of a sub-tropical anticyclone. But & ring winter, the climate is influenced by temperate cyclones.

(ii) Cs:

Cs. is sub-tropical dry summer climate. It is characterised by moderate to scanty amount of precipitation. Winter is the rainy season, while summers are nearly or completely dry. This climate type represents a transition zone between the tropical dry climates towards the equator and the temperate climates towards the poles. The average annual precipitation is less than 890 mm (35 inches).

Group D:

This group represents temperate climates. The climatic group is also known as the micro-thermal climatic type. The average temperature is around 10 °C for 4 to 8 months. This type of climate is found in the middle- latitudes between the sub-tropical and boreal climates. The two types of climate that are included in the temperate group of humid climates are:

(i) Do:

Do. is temperate marine climate. With mild winters, the average temperature for all the 12 months is 0 °C or above. A humid climate with adequate precipitation at all seasons, it is found on the western windward side of the continents in the temperate zone.

(ii) Dc:

Dc. is temperate continental climate found in the continental interiors of the middle- latitude continents. The climate is basically land- controlled. The climatic type is charaterised by severe winters and summers. Annual temperature ranges are, therefore, high throughout this climate, Cold waves, heat waves, blizzards, and heavy downpours are all yearly events in this category, of climate. Precipitation occurs throughout the year with maximum concentration during summers.

Group E:

The group represents sub-arctic or boreal climate found in the higher middle latitudes. Super-continental in temperate features, here the summers are short and comparatively cool. The winters are, however, long and very cold with a very short frost-free season. Average temperature hovers around 10 °C for one to three months during the year. The rest of the year has an average temperature below 10 °C. These regions are characterised by the lowest annual means of temperature for any part of the earth. Even though boreal climates are classified as humid, annual precipitation is comparatively very less. Precipitation occurs throughout the year, most during the warmer months when the amount of water vapour present in the air is highest.

Because of the severity of climate, population is sparse.

Group F:

The group consists of polar climate found in the high latitudes. The climate is confined to northern hemisphere only. The average temperature in this type of climate seldom exceeds -1O °C. There is no summer season. The polar climates are classified into the following two climatic types:

(i) Ft:

Ft. is Tundra climate found only in the northern hemisphere, where it occupies the coastal sides of the Arctic Ocean, and many Arctic islands and ice-free shores of northern Iceland and southern Greenland. No tundra climate is found in the southern hemisphere because of complete absence of extensive land areas.

The Tundra region, essentially a region of grasses, mosses and lichen, is characterised by the absence of trees. The average temperature of the warmest month is recorded between 0 °C and 10 °C. Being a region of permafrost where the subsoil is permanently frozen, precipitation occurs throughout the year, with the warmest season
getting the maximum precipitation. In the summer months, precipitation occurs partly as snow and partly as rain. During winters, precipitation occurs in the form of snowfall which is usually dry and powdery. .

(ii) Fi:

Fi. is ice-cap climate in which the average temperature for all the months is below freezing. There is no vegetation of any kind. The land is permanently covered with ice and snow. The climate is exclusively confined to the ice-caps of Greenland and Antarctica.

Precipitation is meagre, and falls as snow, most of which is in the form of dry, hard, sand-like particles. Most of the snow falls along the seaward margins of the continent. The air close to the ice­cap being intensely chilled, surface inversions are the common features of this type of climate.

Group H:

The group represents highland climates in which altitude plays a role in determining climate classification. The temperature under normal conditions decreases with altitude, with the summit area of a mountain being always cooler than its base.

Windward slopes force the incoming air to rise up with the resultant condensation, cloud formation and precipitation. The leeward slopes are characterised by descending air which is warmed up and produces little precipitation.

East-facing slopes have more sunshine in the mornings which are relatively warmer, and cool afternoons. On the contrary, west-facing slopes have cool mornings and warm afternoons.

Trewartha says there is no such thing as a highland type of climate because various types of local climates exist in every significant mountain range. There are no typical temperature and rainfall regimes in the highland climates. They may be considered as the cool, moist islands in the midst of zonal climates that occur in the area around them. The highland areas are also said to be biotic islands with a natural vegetation and animal life adapted to cooler and more moist conditions than those of the surrounding low lands.

The highland type of climate are found on the Tibet Plateau, Karakoram Range, the Armenian mountain knot commonly shared by Turkey and Russia, the Alps in Europe, the volcanic highlands of Ethiopia and Ruwenzori Range in Africa, and the Andes in South America.

Climate Group based on Precipitation Criteria:

Group B:

This group represents dry climate. The boundaries of this type of climatic group are fixed by precipitation values. The characteristic feature of a dry climate is that the loss of moisture through evapotranspiration is far in excess of the annual water gain collected from precipitation. Because of clear and calm weather and the dry atmosphere, the dry climates are quite severe for their latitudes with large annual ranges of temperature.

Precipitation is comparatively very low in this type of climatic group. Relative humidity, high potential evaporation, abundant sunshine and small cloudiness are some of the common features of Group B classification. They are further classified into two climatic groups.

(i) BW:

BW is arid or desert type of climate.

(ii) BS:

BS is semi-arid or steppe type of classification.

BW and BS are further classified into the following subdivisions on the basis of temperature:

(a) BWh:

BWh is tropical-subtropical hot deserts;

(b) BWK:

BWK is temperate boreal cold deserts;

(c) BSh:

BSh is tropical-boreal steppes; and

(d) BSk:

BSk is temperate-boreal and cold.

The BWh and BWK climates are constantly dry and are under the influence of sub-tropical high and dry trades. The BWh type of condition lasts for 8 months or more with average temperature over 10 °C while the BWK lasts fewer than 8 months with average temperature above 10 °C. The BSh is characterised by a short moist season and is greatly influenced by sub­tropical high and dry trades. The BSK type of climate receives most of its meagre annual precipitation during the warmer season.