UK Weather Systems

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  • UK Weather Systems
    • Depressions
      • 1) Warm air moves from the tropics to the poles - this is the warm front.
      • 2) Cold air moves out of the poles towards the tropics - this is the cold front.
      • 3) Where the air masses meet, the warm air rises above the cold air, this creates low pressure.
      • 5) Cold fronts move more quickly, so the cold front catches up with the warm front.
      • 6) The warm air behind the warm front is undercut by the incoming cold air and is lifted away. It now sits above the cold air.
      • 7) The warm air rises into the upper atmosphere and the depression dissolves.
      • 4) Strong winds blow from the surrounding areas of high pressure and the whole system rotates as air continues to rise.
    • Anticyclones
      • Anticyclones are areas of high atmospheric pressure caused by a large mass of falling air.
      • The air falls from the upper atmosphere, and warms on its way down.
      • This causes humidity to decrease b/c air masses can hold more moisture as they get warmer. Clouds don't develop and conditions are dry.
      • Air pressure on the centre and edges is similar, the winds are weak and flow gently outwards. They flow clockwise from the centre.
      • Winter Weather
        • Low Temps during the day, because the sun is at a low angle.
        • Extremely cold overnight, with frosts b/c clear skies allow loss of heat through radiation.
        • Low level cloud and radiation fogs - fogs form overnight. As the ground cools, mositure in the air close to the ground condenses, forming fog.
        • High levels of atmospheric pollution in urban areas. Pollutants are trapped by temp inversions - high atlitudes are warmer than low. Lack of wind means no dispersion.
      • Summer Weather
        • Intense insolation due to lack of clouds.
        • Rapid radiation at night causes temp inversions, dew and morning mist.
        • Costal areas may get fogs and strong breezes. Highlands may get strong winds due to heating of valley sides (creating a pressure gradient)
        • After several days, there's a risk of thunderstorms, due to large amounts of rising warm air.
  • Pattern of weather from depressions
    • When the cold front catches up to the warm front, all of the warm air has been squeezed upwards so there's no warm air at the bottom, There's less rain b/c the uplift of air has decreased.
    • As the cold front passes, cool air lowers the temp. The cold dense air behind the cold front undercuts the warm air mass in front.
      • This forces the warm air to rise rapidly and condense, forming clouds and heavy showers. Rapidly rising air makes it very windy at the ground.
    • When the warm air is overhead, its warm. The's no cloud b/c warm air holds a lot of water vapour.
    • As the warm front passes, conditions on the ground get warmer. The wam air is pushed over the cold air and condenses, forming low, thick clouds and sustained drizzle.
    • Ahead of the warm front, it's cool b/c the cold air is overhead. Thin clouds from high up as warm air is pushed upwards.
    • There is little condensation, so cloud cover decreases. Wind speed also decreases, but air pressure rises as the dense cold air replaces the uplifted air and the depression dies out.


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