• Trewartha climate classification was published in 1966 by Glenn Thomas Trewartha.
• It is a modified version of the Koeppen system.
• Trewartha’s classification combines empirical and genetic classification schemes.
• Trewartha’s classification is simpler and more explanatory than Koeppen and Thornthwaite systems.
• Trewartha considered temperature, precipitation, and effects of land and water surfaces for his classification.
• Trewartha classified world climates into seven groups.
• Six groups (A, C, D, E, F, and H) are based on temperature criteria.
  • Group A: Tropical climates.
  • Group C: Subtropical climates.
  • Group D: Temperate and continental climates.
  • Group E: Boreal climates.
  • Group F: Polar climates.
  • Group H: Highland climates.
• The seventh group (B) is the dry group based on precipitation.
  • Group B: Dry (arid and semi-arid) climates.

Climatic Groups Based on Temperature Criteria

Group A

• This is the tropical climate group.
• It 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°C.
• This climatic group is subdivided into two climatic types:
  • (i) Ar:
    • Tropical wet climate
    • Less than two dry months
    • Influenced by the intertropical convergence zone and equatorial westerlies
    • Known as tropical rainforest
  • (ii) Aw:
    • Tropical wet-and-dry climate
    • Two dry months during low sun
    • Dominated by dry trade winds or subtropical anticyclones
    • Equatorial westerlies and intertropical convergence control weather during high sun
    • Dry season is usually longer than wet season
    • Temperature remains uniformly high

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:
    • 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 the summer season, this type of climate comes under the influence of unstable air in the western end of a subtropical anticyclone.
    • But during winter, the climate is influenced by temperate cyclones.
  • (ii) Cs:
    • It is characterized by a 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.
• Also known as the micro-thermal climatic type.
• Average temperature around 10°C for 4-8 months.
• Found in middle latitudes between sub-tropical and boreal climates.
• Two types of climate included:
  • Do:
    • temperate marine climate with mild winters
    • adequate precipitation at all seasons
    • found on western windward side of continents in temperate zone.
  • Dc:
    • temperate continental climate found in continental interiors of middle-latitude continents,
    • characterized by severe winters and summers, 
    • high annual temperature ranges, precipitation occurs throughout the year with maximum concentration during summers.

Group E

• This group represents sub-arctic or boreal climate found in the higher middle latitudes.
• Summers are short and comparatively cool while winters are long and very cold with a very short frost-free season.
• The 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 characterized 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 vapor present in the air is highest.
• Because of the severity of the climate, the population is sparse.

Group F

• The group consists of polar climate found in the high latitudes.
• The climate is confined to the 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:
    • 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.
  • (ii) Fi:
    • 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.

Group H

• The group represents highland climates where altitude plays a role in determining climate.
• The temperature generally decreases with altitude, with the summit area of a mountain being cooler than its base.
• Windward slopes cause incoming air to rise, resulting in condensation, cloud formation, and precipitation.
• Leeward slopes have descending air, which is warmed and produces little precipitation.
• There is no typical temperature and rainfall regime in the highland climates, and various types of local climates exist in every significant mountain range.

Climate Group based on Precipitation Criteria:

Group B

• This group represents a dry climate.
• The boundaries of this type of climatic group are fixed by precipitation values.
• The loss of moisture through evapotranspiration is far in excess of the annual water gain collected from precipitation.
• 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: BW and BS.
  • BW is an arid or desert type of climate, while BS is a semi-arid or steppe type of classification.
• BW and BS are further classified into subdivisions based on temperature: BWh, BWK, BSh, and BSk.
  • The BWh and BWK climates are constantly dry and are under the influence of subtropical high and dry trades.
  • The BSh is characterized by a short moist season and is greatly influenced by subtropical high and dry trades.
  • The BSK type of climate receives most of its major annual precipitation during the warmer season.

Trewartha Climatic Classification of India

• Trewartha’s classification of climate is a modified form of Koeppen’s scheme and corresponds well with the vegetative, agricultural, and geographical regions of India.
• Four major climatic groups (A, B, C, and H) with seven climatic types have been recognized.
• A: Tropical Rainy Climatic Group, with two subtypes: Am (Tropical Monsoon) and Aw (Tropical Savannah).
• B: Dry Climatic Group, with three subtypes: BS (Tropical Steppe), Bsh (Sub-Tropical Steppe), and Bwh (Sub-Tropical Desert).
• C: Humid Mesothermal Climatic Group, with one subtype: Caw (Sub-Tropical Humid – Dry Winters).
• H: Mountain Climate, with no subtypes.
• The letters A, B, C, and H stand for major climate groups, and the other letters indicate sub-divisions of major groups.
• A is a tropical rainy climate with persistently high temperature.
• B is a dry climate where evaporation is greater than moisture received from precipitation.
• C represents a humid sub-tropical or humid mesothermal climate.
• Subtypes are indicated by lowercase letters, such as a (hot summers with the warmest month over 22°C), h (mean annual temperature of 18°C), m (heavy but seasonal monsoon rainfall; short dry period), s (steppe or semi-arid climate), and w (desert).
• Am (Tropical Monsoon) type of climate is found over the Western Ghats, western Nagaland, and Tripura.
• Aw (Tropical Savannah) covers almost the whole of the Deccan Plateau.
• BS (Tropical Steppe) type of climate covers interior Karnataka, central Maharashtra, western Andhra Pradesh, and interior Tamil Nadu.
• Bsh (Sub-Tropical Steppe) type covers an area from Punjab to Kachchh.
• Bwh (Sub-Tropical Desert) prevails over western Rajasthan and Kachchh.
• Caw (Sub-Tropical Humid – Dry Winters) covers the Punjab foothills, Uttar Pradesh, Bihar, West Bengal, Assam, and Arunachal Pradesh.
• The mountain type of climate (H) prevails in mountain areas of Kashmir and Arunachal Pradesh