GCSE Geography | Global Pressure and Surface Winds (Weather Hazards 2)

Global pressure belts and surface wind patterns influence the weather, and in particular, precipitation.

Pressure belts

Descending air = high pressure = no clouds and rain

Rising air = low pressure = clouds and rain

The atmospheric circulation results in high pressure at the poles, so they are very dry. A low-pressure belt runs around the globe at about 60°, making locations such as the UK wet. There is a band of high pressure around the tropics, so they are dry, with most of the world’s hot deserts found in these regions. Finally, the Equator has a band of low pressure and so is very wet, meaning much of it is covered in rainforests. These pressure belts move north and then south over a year. The movement of the low-pressure band to the north and then south of the Equator explains why some countries are dry for most of the year but then have two rainy seasons.

Surface winds

Wind is the movement of air from high pressure to low pressure. Surface winds are part of the global circulation and are vital in moving heat and water vapour from one place to another. Surface winds blowing from the tropics to the Equator are dry, affecting the climate of the countries they blow over. Surface winds are also known as trade winds as they were used to determine where sailing ships could travel easily to trade. These winds curve as they blow across the surface because they are distorted by the rotation of the Earth. Trade winds in the tropics blow tropical storms from east to west.

Global variations in weather

• At the Equator rainfall is high and constant all year round - here water vapour gets converted into convectional rain droplets.
• The intertropical convergence zone (ITCZ) is a low pressure zone around the Equator - short bursts of torrential rainfall are caused by air rising which forms areas of low pressure. Tropical storms occur here and can travel away from the Equator once they have gathered enough energy.
• In coastal areas of Western Europe rainfall can be high - this is due to the jet stream moving across the Atlantic Ocean, picking up moisture and forming depressions, which can lead to storm formation.
• Around the Tropic of Cancer and Tropic of Capricorn rainfall is often quite low - this is due to the dry air descending within the Hadley Cell, causing arid conditions (and is where many of the world's deserts are located).
• At the poles very little precipitation occurs, and what does falls mainly as snow - this is because of limited evaporation.