The Köppen Climate Classification System is the most widely used system for classifying the world's climates. Its categories are based on the annual and monthly averages of temperature and precipitation. The Köppen system recognizes five major climatic types; each type is designated by a capital letter.
A - Tropical Moist Climates: all months have average temperatures above 18° Celsius.
B - Dry Climates: with deficient precipitation during most of the year.
C - Moist Mid-latitude Climates with Mild Winters.
D - Moist Mid-Latitude Climates with Cold Winters.
E - Polar Climates: with extremely cold winters and summers.
Tropical Moist Climates (A)
Tropical moist climates extend northward and southward from the equator to about 15 to 25° of latitude. In these climates all months have average temperatures greater than 18° Celsius. Annual precipitation is greater than 1500 mm. Three minor Köppen climate types exist in the A group, and their designation is based on seasonal distribution of rainfall. Tropical wet is a tropical climate where precipitation occurs all year long. Monthly temperature variations in this climate are less than 3° Celsius. Because of intense surface heating and high humidity, cumulus and cumulonimbus clouds form early in the afternoons almost every day. Daily highs are about 32° Celsius, while night time temperatures average 22° Celsius. During the dry season very little rainfall occurs. The tropical wet and dry or savanna (Aw) has an extended dry season during winter. Precipitation during the wet season is usually less than 1000 millimetres, and only during the summer season.
Dry Climates (B)
The most obvious climatic feature of this climate is that potential evaporation and transpiration exceed precipitation. These climates extend from 20 - 35° North and South of the equator and in large continental regions of the mid-latitudes often surrounded by mountains. Minor types of this climate include:
Dry arid (desert) is a true desert climate. It covers 12% of the Earth's land surface and is dominated by xerophytes vegetation. The additional letters h and k are used generally to distinguish whether the dry arid climate is found in the subtropics Mediterranean or in the mid-latitudes, respectively.
Dry semiarid (steppe). Is a grassland climate that covers 14% of the Earth's land surface? It receives more precipitation than the BW either from the inter-tropical convergence zone or from mid-latitude cyclones. Once again, the additional letters h and kare used generally to distinguish whether the dry semiarid climate is found in the subtropics Mediterranean or in the mid-latitudes, respectively.
Moist Subtropical Mid-Latitude Climates (C)
This climate generally has warm and humid summers with mild winters. Its extent is from 30 to 50° of latitude mainly on the eastern and western borders of most continents. During the winter, the main weather feature is the mid-latitude cyclone. Convective thunderstorms dominate summer months. Three minor types exist: Humid subtropical; Mediterranean; and marine. The humid subtropical climate has hot muggy summers and frequent thunderstorms. Winters are mild and precipitation during this season comes from mid-latitude cyclones. A good example of a climate is the south-eastern USA. Marine climates are found on the western coasts of continents. They have a humid climate with short dry summer. Heavy precipitation occurs during the mild winters because of the continuous presence of mid-latitude cyclones. Mediterranean climates receive rain primarily during winter season from the mid-latitude cyclone. Extreme summer aridity is caused by the sinking air of the subtropical highs and may exist for up to 5 months. Locations in North America are from Portland, Oregon to all of California.
Moist Continental Mid-latitude Climates (D)
Moist continental mid-latitude climates have warm to cool summers and cold winters. The location of these climates is pole ward of the C climates. The average temperature of the warmest month is greater than 10° Celsius, while the coldest month is less than -3° Celsius. Winters are severe with snowstorms, strong winds, and bitter cold from Continental Polar or Arctic air masses. Like the climates there are three minor types: dry winters; dry summers; and wet all seasons.
Polar Climates (E)
Polar climates have year-round cold temperatures with the warmest month less than 10° Celsius. Polar climates are found on the northern coastal areas of North America, Europe, Asia, and on the landmasses of Greenland and Antarctica. Two minor climate types exist. Polar tundra is a climate where the soil is permanently frozen to depths of hundreds of meters, a condition known as permafrost. Vegetation is dominated by mosses, lichens, dwarf trees and scattered woody shrubs. A polar ice cap has a surface that is permanently covered with snow and ice.