Weather Systems

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  • Weather Systems
    • Depressions
      • Form over Atlantic ocean then move East over UK usually tropical maritime and polar maritime meet
        • HOW?
          • 1. Warm air constantly moves out of topical areas towards poles. Warm front=leading edge of warm air mass
          • 2. Cold air moves out of polar regions towards tropics. Cold front=leading edge of cold air mass
          • 3. Where air mass meet warm air heading towards poles rises above cold air (warm air is less dense)
          • 5. Strong winds blow areas of high pressure towards area of low pressure= system rotates and continues to rise
          • 6. Cold fronts move more quickly than warm fronts so cold front catches up with warm front
          • 4. Rising warm air means atmospheric pressure is reduced
          • 7. When this happens warm air behind warm front is undercut by incoming cold front and is lifted away from ground
            • it now sits above cold air= occluded front
              • warm air rises into upper atmosphere and the depression dissolves
      • Weather
        • distinct pattern of weather, which usually lasts for 2-3 days
        • 1. Ahead of warm front its cool because cold air is overhead. Thin clouds form high up as warm air is pushed up
        • 2. As warm front passes, conditions on ground get warmer. Warm air is pushed up over cold and condenses, forming low thick cloud and sustained drizzle
        • 3. When warm air is overhead its warm. There's no cloud because warm air holds alot of water vapour
        • 4. As cold front passes, cool air brings temp. down again. The cold dense air behind the cold front undercuts warm air mass in front
          • this forces warm air to rise rapidly and condense, forming clouds and heavy showers.
            • rapidly rising air makes it very wind at surface
        • 5. When cold front catches up with warm front, all of warm air has been squeezed upwards so there's no warm air left at bottom
          • there's less rain because the uplift air has decreased
            • there is little condensation, so cloud cover decreases
              • wind speed decreases
                • but air pressure rises as the dense cold air replaces uplifted air and depression dies out
    • Anticyclones
      • Areas of high atmospheric pressure caused by a large mass of falling air
      • air falls from the upper atmosphere, and warms on its way down
        • causes humidity to decrease because air masses can hold more moisture as they get warmer
          • clouds don't develop and very dry
      • in anticyclones there's not much difference in air pressure between centre and edges.
        • because pressure is similar, winds are weak and flow gently outwards
          • In UK wind flows clockwise around centre
        • Low temp. during the day because sun is at low angle
        • extremely cold overnight temps. with frost because clear skies allow loss of heat through radiation
        • low level cloud and radiation fogs- radiation fog forms overnight.
          • as ground cools, moisture in air close to ground condenses  forming fog
        • high levels of atmospheric pollution in urban areas
          • pollutants are trapped by temp. inversions(when air at higher altitudes is warmer than air at lower levels)
        • Absence of clouds leads to intense insolation which means its hot, sunny and no rain
        • rapid radiation at night can cause temp. inversions, dew and morning mist
        • coastal area may get fogs and strong breezes
          • highlands may experience strong winds due to heating of valley sides=pressure gradient
          • advection fog
        • after several days risk of thunderstorm due to large amounts of rapidly rising warm air


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