Formation of a depression

Depressions follow a life cycle in which 3 main stages can be identified:

1) Embryo stage

• The embryo depression begins as a small wave on the polar front. It is here that warm, moist (TM) air meets colder, drier polar air (PM).
• The convergence of these two air masses results in the warmer, less dense air rising upwards in a spiral movement.
• This upward movement results in less air at the earth's surface, creating an area of below average pressure.
• The developing depression, with it's warm front (the leading edge of the tropical air) and the cold front (the leading edge of the polar air) usually moves in a North Easterly direction under the influence of the upper westerlies i.e, the polar fron jet stream.

• Pressure continues to fall as more warm air in the warm sector is forced to rise.
• As the pressure fals, the pressure gradient steepens, the inward blowing winds increase in strength.
• Due to the coriolis force, these anticlockwise bloing winds come from the South West. As the relatively warm air of the warm sector continues to rise along the warm front, it eventually cools to dew point.
• Some of it's vapour will condense to release large amouts of latent heat, and clouds will develop. Continued uplifting and cooling will cause orecipitation as clouds become both thicker and lower.
• As temperatures rise and the uplift of air decreases within the warm sector, there is less chance of precipitation and the low cloud may break to give some sunshine. The cold front moves faster and has a steeper gradient than the warm front.
• Progressive undercutting by cold, denser air at the rear of the warm sector gives a second wave of precipitation - although with greater intensity and a shorter duration than at the warm front.
• This band of meso-scale precipitation may be only 10-50km wide.
• Although the air behind the cold front is colder than that of the warm front (as it originated in higher lattitudes) it becomes unstable froming towering cumulonimbus clouds + heavy showers
• Winds often reach their max strength at the cold front and change to a more NW direction

The depression begins to decay when the cold front catches up with the warm front to form an occulsion or an occluded front. By this stage, the TM air will have squeezed upwards leaving no warm sector at ground level.

As the uplift of air is reduced, so too is the amount of condensation, the release of latent heat and the amount and pattern of precipitation - there may be one episode of rain.

Cloud cover begins to decrease, pressure rises and wind speeds decrease as the colder air replaces th uplifted air and 'infills' the depression