Anti-cyclones and Depressions

Anti-cyclones (high pressure)

These are areas of high pressure which result when the air sinks to give high pressure at the surface.

Anti-cyclones bring with them dry, hot weather in the summer. This is because the sinking air warms as it descends and any water droplets turn to water vapour which in turn leads to a lack of precipitation. Also during this time the sun is at a high angle in the sky.

Anti-cyclones bring with them dry, cold, foggy and freezing weather in the winter. The sun is at a low angle in the sky, so therefore it struggles to heat up the earth. During the long nights heat is quickly lost into the cloudless skies and fog forms as the water vapour in the air condenses into tiny water droplets (fog). If temperatures continue to fall water freezes to form frost on any available surface.

Depressions (low pressure)

These are areas of low pressure which form when air rises leaving low pressure on the ground. Frontal depressions form where the warm air mass (from the equator) meets the cold air mass(from the poles).

Ahead of the warm front high clouds begin to form because of the lighter warm air being forced to rise above the denser cold air. A thick sheet of cloud then develops and heavy rain begins to fall due to the moisture in the rising warm air condensing. In between the fronts (the warm sector) the temperature increases and the rain stops The warm air is now at ground level and is not being forced to rise. As the cold front arrives, dense cold air undercuts lighter, warmer air forcing it to rise. Tall clouds begin to develop and there is heavy rain and often thunder storms. Once the cold front has passed temperatures begin to fall and the weather is a mixture of sunshine and showers.