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  • Formed way out in Atlantic Ocean when cold Pm air moves south and meets warm Tm air moving north
  • Lighter Tm air rises up over the colder. much denser Pc air as the two cannot mix
  • The rising warm air then reduces the atmospheric pressure leading to baroclinic instability which continues to develop into a front
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  • Move from SW towards NE
  • Pressure is low and falling
  • Winds blow anticlockwise
  • Wind speeds increase- isobars close together
  • Temps vary on weather overhead
  • Wide range of clouds and corresponding precipitation that varies as depression is overhead
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Associated Weather

1. Ahead of Warm Front- Clear skies with pressure falling steadily. The temperature is quite cold as cold air is overhead. Cloud base beings to drop getting thicker- cirrus and altostratus as the warm air is just beginning to rise over the cold air. Wind speed increases with wind direction changing to SE and there is little rain due to little cloud

2. Passage of Warm Front- Clear skies with low pressure due to uplift of warm air. Temperatures rise due to Tm air overhead. Clouds getting lower and thicker- nimbostratus. Wind become much stronger and direction changes for SE to SW. Some drizzle due to these nimbostratus clouds

3. Warm Sector- Pressure steadies with an increase in temp as warm sector is overhead. Some clear skies with light stratus clouds. Calm conditions with direction going SW. Drizzle with some dry conditions

4. Passage of Cold FrontPressure rises again with cold temperatures Pm is overhead. Clouds being to thicken with some towering cumulonimbus clouds due to convection currents. Wind speed increases to gale and direction changes from SW to NW. Thunderstorms can result

5. Cold Sector- Pressure rises with cold temps as cold sector overhead. Cumulus clouds form with falling wind speed and direction now to NW. Light rain showers which ease as depression decays

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+Bring warm air into UK in winter and increased cloud cover means higher temps

+Can bring valuable water supplies following a drought

-Can bring bad weather including heavy rain, hail and cloud limiting outdoor pursuits can causing cancellation of them

-Can take place one after another, so bad weather can be prolonged


+Bring water which farmers need for their crops to grow

+Clouds during winter months keep temps up reducing frost which prolongs growing season

-High winds can damage crops and property, cancellation of ferries- delay of goods

-Flooding resulting from high amounts of rainfall can damage property

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Case Study: February 1994 Storm in NI


  • Embryo low pressure system formed over Atlantic to west of Ireland
  • Pm collided with Tim along Polar Front
  • Next two days moved eastwards and intensified into a deep mid latitude depression- Feb 3rd pressure dropped to 950mb
  • Cold front rapidly overtook warm front lifting it off ground to form an occlusion which resulted in 25mm of rain for Co. Down and Antrim
  • Winds peaked at 80mph and 90mph for Bangor and Donegal respectively


  • Two dies- farmer electrocuted by cable, other fell to their death
  • 27 crew of the Christinaki ship were lost at sea when it sank
  • NIE reported 400,000 homes without power- worst cut for 50 years
  • Travel links disrupted by fallen trees and ferry services and air travel affected on 3rd and 4th February
  • Flooding in on a Newry farm and around Dublin where coastal defences were undermined 
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