Hot and dry. These are the terms that characterise the desert climate in most people's minds, but is this an accurate depiction? As a general rule, areas subject to an average rainfall of less than 100mm a year can be classed as deserts. Sometimes a broader classification is used whereby deserts are split into three categories - semi-arid, arid and hyper-arid. Areas of the first kind receive less than 600mm of rainfall a year, areas of the second kind receive less than 200mm, whilst rainfall in hyper-arid desert regions never exceeds 25mm.

But low average annual rainfall is only half the story. Not only do deserts experience very little rain, but to make matters worse, this small amount is highly irregular, both in terms of time and space. Sometimes an area of desert will be without rain for years and then, seemingly for no reason at all, a few large storms will provide enough rain in a short period to bring the average annual rainfall back to normal. Desert storms can be extremely localised, centring upon one area and leaving adjoining parts entirely dry. These are some of the factors that contribute to making deserts some of the most inhospitable environments on the planet.

Water shortage in the desert is not limited to low rainfall. Humidity and consequently cloud cover are also in short supply. This in turn means that the surface, and any living things on it, are continually exposed to direct sunlight, causing intense evaporation of any water faster than the rain can replenish it.

Where deserts are to be found close to the sea, fogs often occur in the early mornings as a result of cold sea water meeting warm air. When the warm air, laden with moisture, comes into contact with the surface of the land, which is still cold from the previous night, it cools and condenses, leaving a film of water. The Namib Desert of southern Africa and Chile's Atacama Desert are both cases in point.

Contrary to popular belief, high temperature is not the most important feature of deserts: lack of water, as we have seen, is of much greater significance. However, it is true that most of the world's deserts are subject to extreme heat for at least part of the year. Many people are surprised by how much temperatures can vary. With cloud cover so scarce, air temperatures in the desert frequently reach 40° C, and in some circumstances they can soar to 50° C. The temperatures of the rocks and sand are even higher, up to 75° C. But these maximum figures can vary by up to 30° C. Seasonal temperature variation can also be considerable in the desert, particularly those furthest from the equator - and in some places, winter nights can even bring frost.

Deserts often experience high winds which, coupled with the sand and dust particles which are typical of most deserts, leads to the formation of the distinctive desert landscape.

photo courtesy of DigitalVision

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