The life cycle of a depression

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Before a depression forms

Over the Atlantic at the polar front warm and moist tropical maritime air meets with colder polar maritime air 

The two air masses DON'T mix

(http://www.metlink.org/wp-content/uploads/2013/11/weather-systems/Fig_1.gif)

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Stage 1: Embryo

(http://pages.bangor.ac.uk/~oss006/meteorology/depression1.gif)

A kink appears in the Polar front (cold) when the jet stream passes overhead 

It diverges and the warmer less dense air starts to rise

The corolis force leads to upward spiraling motion of warm air leaving low pressure at the surface 

Developing depressions head NE following the Jet stream and the winds begin to circle round depression towards the low pressure 

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Stage 2: Maturity

Pressure continues to fall as more warm air at the warm front is forced to rise 

Pressure falls and pressure gradients steepen (inward blowing winds increase)

Warm air rises and cools to dew point forming clouds and latent heat release 

Continued uplift leads precipitation and clouds becoming thicker and lower 

The band of rain at the warm front is up to 150km in width

(http://www.s-cool.co.uk/assets/learn_its/alevel/geography/weather-conditions/united-kingdom-weather-and-climate_/a-geo-weacon-dia19b.gif)

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Stage 2: Warm sector and Cold front

Warm sector:

  • Sme cloud cover but air is the same temp (so not rising and thefore unlikely to rain)
  • Warm dur to tropical air mass

Cold sector:

  • Cold air is moving faster, undercutting warm air and lifting it
  • Warm air is forced to rise, cool and condense at the cold front 
  • Both bring rain at cold front air is froced to rise so forms cumulonimbus clouds+thunderstorms 
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Stage 3: Decay or occlusion

Depression begins to decay when cold front catches up with the warm front (occluded front)

Tropical maritime squeezed upwards leaving no warm sector at ground level

There is one episode of rain

Cloud cover decreases, pressure rises and wind speeds decrease as cold air replaces uplifted air at the surface

(http://pages.bangor.ac.uk/~oss006/meteorology/depress3.jpg)

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