Regions of dry climate are found on every continent. In even Europe, some areas have under 20 inches of rainfall per year and can be described as semi-desert. Hot deserts (B1) are, however, mainly found in in those zones of almost permanent high pressure and clear skies between about 20 and 35 degrees latitude. Deserts such as the Sahara and the Arabian are the result of the global pattern of air circulation. Dry air descends over these regions bringing clear skies and hot sunshine. These hot deserts have little rain at any season and no real cold weather although temperature drops sharply at night. Sand or rocks in direct sunlight will easily reach 60-70 degrees C (140-160 degrees F). But at night temperatures may drop to below freezing point.

Some hot deserts are the result of local factors. The west coast dry zones of southern Africa and South America are made even drier by the effect of cool ocean currents flowing along the coast. In semi-desert (B2) or tropical steppe areas, such as the drier parts of India and the Sahel region of Africa, the rain can fail several years in succession, causing severe drought. Large areas of the driest deserts are seas of sand dunes. The constantly shifting dunes prevent even the most drought resistant plants from gaining a foothold Not all deserts are hot; some are cool or even cold. Cold deserts (B3) occur in higher latitudes in the interior of large continents. The best examples are parts of central Asia and western China where although very hot in summer, are bitterly cold in winter.