Climate zones


Lying between the Tropics of Cancer in the northern hemisphere and Capricorn in the south, equatorial climates are home to the world's rainforests, where rainfall and humidity are high. Surprisingly, temperatures are not that extreme, generally 25-35 °C, and vary little. The hottest months are only two or three degrees warmer than the cooler times of the year. Because these regions are so close to the Equator, the length of day and night hardly varies throughout the year.


Our deserts - the hottest, driest and most inhospitable places on Earth - are found mainly across the subtropical continents. Here, descending air forms large, almost permanent areas of high pressure leading to cloud-free skies virtually all year round. Annual rainfall is low and, in some deserts, almost non-existent. No rain has fallen in the Atacama Desert in South America for 400 years. Because they're so dry, the temperature range in our deserts is huge, regularly exceeding 45 °C by day in summer and often falling to below freezing overnight in winter.


The hot dry summers of the Mediterranean are caused by a seasonal shift of the descending air that also creates our deserts. Low summer rainfall is matched by many months of warm, sunny weather. But, at times, dangerously hot spells of weather engulf the region with fiercely high temperatures of up to 45 °C. In winter, there is more rain and cooler temperatures, but little frost.


In the higher northern latitudes, vast areas of the continental interior endure long, hard winters with short, bountiful summers, separated by rapid climatic changes during spring and autumn. The landscape here is contrasting. On one hand there is one of the world's largest terrestrial ecosystems - the vast areas of fir and spruce of the Boreal forest. But to the north, where summer temperatures are lower, there is the relatively featureless tundra. Here, the land will not thaw even during the brief summer. Typical summer temperatures are around 15 °C but there could already be frosts by August and ice on lakes by September.


The poles experience the coldest temperatures on Earth but the two poles' climates are different. The Arctic is mostly frozen ocean, while Antarctica is a vast continent of mountains and high plateaus buried under more than 3 km of ice. The Arctic climate is moderated by the relatively warm Atlantic Ocean. Winter temperatures fall to below -60 °C in the coldest regions, while summers range from a few degrees below zero to about 20 °C. Temperatures in the south are colder: winter temperatures often dip below -80 °C. The Antarctic interior is very dry - drier than many deserts. This is because the interior is a long way from the ocean and, as the temperature falls, so does the atmosphere's capacity to hold the water vapour needed to make snow.


This classification covers a range of climates from near-Mediterranean climates and humid, sub-tropical zones to maritime climates influenced by the oceans - like ours in the UK. The former are mostly found on the western side of continents at 30-45° latitude. Summers can be either hot or warm, but they are always markedly drier than other times of the year. Humid, subtropical climates tend to be in the middle or on the eastern side of continents at 25-45° latitude. Summers here are humid with plenty of rain, but winters are usually dry. Some temperate climates have wet and dry seasons while others have no marked dry season at all. But all have four distinct seasons.