Depression

A low pressure system also known as a Depression occurs when the weather is dominated by unstable conditions.

Formation:

Under a depression air is rising, forming an area of low pressure at the surface. This rising air cools and condenses and helps encourage cloud formation, so the weather is often cloudy and wet. In the Northern Hemisphere winds blow in anticlockwise direction around a depression. Isobars are normally closely spaced around a depressions leading to strong winds.

Depressions are often accompanied by fronts. Most depressions have a warm and cold front, more mature depressions may also have an occluded front.

Maturity:

The warm air rises up above the sinking colder air . A warm area develops between the warm and cold fronts. The mature stage of a depression often occurs over the UK.

Occlusion:

The cold front travels at speed of 40 to 50 miles per hour, compared to the warm front which travels at only 20 to 30 miles per hour. Therefore the cold front eventually catches up with the warm front. When this occurs an occlusion is formed.

Relationship with airmass:

The weather of south and south east Asia are mainly associated with MARITIME TROPICAL AIRMASSES known as summer monsoons, warm, humid and unstable which yields torrential rainfall when forced to ascend by mountain barriers. being originated in southern oceans they move northeast and after entering main land they are heated from below cuz of warm ground surface and hence become unstable and conventional currents are produced.