climate classification

Description of different types of climate, taking into account the averages, extremes, and frequencies of all meteorological elements such as temperature, atmospheric pressure, precipitation, wind, humidity, and sunshine, together with the factors that influence them.

The different types of climate can be described as tropical (hot), warm temperate (or subtropical), cool temperate, cold (or polar), and arctic.

Tropical (hot) climates There are three distinctive tropical climates: equatorial, tropical continental, and hot deserts. The mean monthly temperature in these climate areas never falls below 21°C/70°F.

Areas with an equatorial climate are located within 5° north and south of the Equator. These areas include the Amazon and Congo basins and the coastal lands of Ecuador and West Africa. Temperatures are high and constant throughout the year because the Sun is always high in the sky. Each day has approximately 12 hours of daylight and 12 hours of darkness. Annual rainfall totals usually exceed 2,000 mm/6.5 ft; most afternoons there are heavy showers. There is high daytime humidity and winds are light and variable.

Areas with a tropical continental climate are mainly located between latitudes 5° and 15° north and south of the Equator, and within central parts of continents. These areas include the Campos (Brazilian highlands), most of Central Africa surrounding the Congo basin, and parts of northern Australia. Temperatures are high throughout the year but there is a short season, slightly cooler than the equatorial climate, when the sun is overhead at either the Tropic of Cancer or the Tropic of Capricorn. The annual temperature range is slightly greater than that of the equatorial climate due to the sun being at a slightly lower angle in the sky for part of the year. The higher temperature is also due to tropical continental areas being at a greater distance from the sea and having less cover from cloud and vegetation. The main feature of this climate is the alternate wet and dry seasons.

Hot deserts are usually found on the west coast of continents between 15° and 30° north or south of the Equator and in the trade wind belt. The exception is the extensive Sahara-Arabian-Thar desert which owes its existence to the size of the Afro-Asian continent. Desert temperatures are characterized by their extremes. The annual range is often 20-30°C/36-54°F and the diurnal (daytime) range over 50°C/90°F. Due to the lack of cloud cover and the bare rocks or sand surface, daytimes receive intense insolation (exposure) from the overhead sun. In contrast, nights may be extremely cold with temperatures likely to fall below 0°C/32°F. No deserts are truly dry even though they suffer from extreme water shortages. The amount of precipitation is extremely unreliable; some desert areas may receive rain only once every two or three years. Rain, when it does fall, is heavy.

Warm temperate (subtropical) climates Warm temperate climates can be classified into two areas: Mediterranean climate and the Eastern margin climate.

The Mediterranean climate is found on the west coasts of continents between 30° and 40° north and south of the Equator - that is, in Mediterranean Europe, California, parts of southern Australia, Cape Province (South Africa), and central Chile. The characteristics of the climate are hot, dry summers and warm, wet winters. Summers in southern Europe are hot with little cloud cover, and the winters are mild. Other Mediterranean climate areas are less warm in summer.

Eastern margin climates, which are found in southeast and eastern Asia, are dominated by the monsoon. Temperature figures and rainfall distributions are similar to those of tropical continental areas although annual amounts of rain are much higher.

Cool temperate climates There are two main cool temperate areas, which can be classified as continental climate and western margins. Periods of one to five months are below 6°C/43°F in these climate areas.

Continental climates are to be found in the centre of continents between approximately 40° and 60° north of the Equator. The annual range of temperature is high as there is no moderating influence from the sea. Maximum mean monthly summer temperatures are around 20°C/68°F. Precipitation decreases rapidly as distance from the sea increases. Annual rainfall averages at around 500 mm/20 in and there is a threat of drought. The ground is snow-covered for several winter months.

Western margin climates are often referred to as ‘northwest European’, and are found on west coasts between approximately 40° and 60° north and south of the equator. Other areas with similar climatic characteristics are northwest USA and British Columbia, southern Chile, New Zealand's South Island, and Tasmania. Summers are cool with the warmest month at a temperature of 15-20°C/59-68°F. This is due to the low angle of the sun in the sky combined with frequent cloud cover and the cooling influence of the sea. Winters are mild in comparison. Mean monthly temperatures remain a few degrees above freezing due to the warming effect of the sea. Autumn is usually warmer than spring; seasonal temperature variations depend on prevailing air masses. Precipitation often exceeds 2,000 mm/6.5 ft annually and falls throughout the year, the highest amount falling during winter when depressions are more frequent and intense. Snow is common in the mountains.

Cold or polar climates The subarctic regions 60° north of North America, Europe, and Asia are said to have a cold or polar climate. This type of climate also occurs at higher altitudes in more temperate latitudes and in southern Chile. For over six months of the year the mean temperature remains below 6°C/43°F. There is a period each year in the area north of the Arctic Circle during which time the Sun never rises. Winters are long and cold; the minimum mean monthly temperatures may be as low as −30°C/−22°F. The wind-chill factor is high with strong winds. Summers are short but the long hours of daylight and clear skies mean they are relatively warm. Precipitation is light as the cold air can only hold a limited amount of moisture, and the small amount of winter snowfall is frequently blown about in blizzards.

Arctic climates Antarctica, all of Greenland, the north of Alaska, Canada, and Russia are areas with continuous permafrost. Winters are severe and the sea freezes; summers have continuous periods of daylight but the monthly temperatures struggle to rise above freezing point. Nearer each pole the climate is constant frost. Precipitation is light and falls mainly as snow.

History of climate classification The earliest known classification of climate was that of the Ancient Greeks, who based their system on latitude. In recent times, many different systems of classifying climate have been devised, most of which follow that formulated by the German climatologist Wladimir Köppen (1846-1940) in 1900. These systems use vegetation-based classifications such as desert, tundra, and rainforest. Classification by air mass is used in conjunction with this method. This idea was first introduced in 1928 by the Norwegian meteorologist Tor Bergeron, and links the climate of an area with the movement of the air masses it experiences.

In the 18th century, the British scientist George Hadley developed a model of the general circulation of the atmosphere based on convection. He proposed a simple pattern of cells of warm air rising at the Equator and descending at the poles (see Hadley cells). In fact, due to the rotation of the Earth, there are three such cells in each hemisphere. The first two of these consist of air that rises at the Equator and sinks at latitudes north and south of the tropics; the second two exist at the mid-latitudes where the rising air from the sub-tropics flows towards the cold air masses of the third pair of cells circulating from the two polar regions. Thus, in this model, there are six main circulating cells of air above ground producing seven terrestrial zones. There are three rainy regions (at the Equator and the temperate latitudes) resulting from the moisture-laden rising air interspersed and bounded by four dry or desert regions (at the poles and subtropics) resulting from the dry descending air.