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  • Created on: 22-03-15 15:36
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  • Depressions
    • A depression is an area of low atmospheric pressure which rotates in an anticlockwise direction
    • Britain is located at 50-59N which positions us at the polar front, this marks the boundary between warm tropical air from the south (TM) and the cold polar air from the north (PM) of the tricellular model, this means Britain is placed in the low pressure zone
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    • Depression triggers
      • One of the triggers is when the Polar jet stream kinks, the coriolis affect can sometimes cause the jet stream to turn northwards, the jet stream then accelerates because of the coriolis effect, this then pulls up air from ground level creating a low pressure system
      • Contrasting air masses is also a trigger, when warm air meets colder air it is forced upwards because it is less dense.
    • Formation - The warm tropical maritime air comes from the south and forms a bulge in the cold polar maritime air because the warm air is less dense it begins to rise, the coriolis effect causes the rising air to rotate. The whole low pressure system now starts to rotate anti-clockwise, because the coriolis affect causes air in the northern hemisphere to turn to the right, as the air try's to reach and replace the rising air in the centre.  This is called the embryo depression. As it travels across the north Atlantic, the depression grows larger and reduces in pressure, forming a mature depression. At this point, the two meeting points of air are where the warm sector is pushing against the cold polar air, called the warm front and where the cold sector is pushing the tropical maritime air called the cold front.  The pressure is lowest in the centre of the depression  because air is rising fastest.
    • Frontal rain occurs at the cold and warm front. At the warm front the warm tropical maritime air is rising steadily over the cold sector, as the air expands it cools, when dew point is reached, condensation on condensation nuclei occurs, nimbostratus clouds form and the water droplets collide and converge together, large droplets form to over come weak uplifts, resulting in a long spell of drizzle.
      • Frontal rain occurs at the cold front, however in this area, the cold air is pushing the air up at a steeper gradient and at a faster rate, the air expands and starts to cool, dew point is reached and condensation occurs, cumulonimbus clouds form and produce short heavy showers
    • Surface pressure at the fronts are low, the lowest is at the cold front.
    • Wind speed is high at the fronts, it is stronger at the cold front because there is a steep gradient of pressures
    • Occlusions
      • A depression has a life span of several days on average. As a cold front moves faster than the warm front it eventually catches the warm front to form a occluded front. Warm tropical maritime air in the warm sector is lifted completely off the ground as it is undercut by the cold front.
        • Over time there is an increase in cloud cover and rain until it is pushed up and cooled, condensation will eventually stop


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