WEATHER

  1. Warm fronts are formed when warm air rises over a mass of cold air. As the air lifts into regions of lower pressure, it expands, cools and condenses the water vapour as wide, flat sheets of cloud.

  2. Cold fronts are usually associated with depressions. A cold front is the transition zone where a cold air mass is replacing the warmer air mass. The cold air is following the warm air and gradually moves underneath the warmer air. When the warm air is pushed upwards it will rain heavily. Often more rain will fall in the few minutes the cold front passes than it will during the whole passage of a warm front. As the cold front passes, the clouds roll by and the air temperature is cooler.

  3. Occluded fronts occur at the point where a cold front takes over a warm front or vice versa. If a cold front undercuts a warm front it is known as a cold occlusion and if the cold front rises over the warm front it is called a warm occlusion. Occluded fronts bring changeable weather conditions.

    On a synoptic chart occluded fronts are represented by semi-circles and triangles positioned next to each other. The triangles are in blue and the semicircles are in red, or both are purple (mixing both red and blue colours together).

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  • Created by: caz
  • Created on: 11-06-10 18:28

ANTICYCLONES

Anticyclones

Anticyclones are the opposite of depressions – they are an area of high atmospheric pressure where the air is sinking.

  • As the air is sinking, not rising, no clouds or rain are formed. This is because as the air sinks it warms - meaning it can hold more water.
  • The absence of fronts means winds may be very light.
  • Consequently, high-pressure areas are often associated with settled, dry and bright conditions.
  • In summer, anticyclones bring dry, hot weather. In winter, clear skies may bring cold nights and frost.
  • In cold conditions, anticyclones may also bring fog and mist. This is because the cold forces moisture in the air to condense at low altitudes.
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Hurricane

How hurricanes form

  • Warm air rises quickly, causing towering clouds, heavy rainfall and intense low pressure.
  • The low pressure sucks in air, causing very strong winds which spiral - anti-clockwise in the northern hemisphere - around the centre of the low, at speeds of around 120 km/h (75 mph).
  • Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300miles) wide. The cloud brings heavy rain, thunder and lightning.
  • In the centre is the eye of the hurricane, about 45 km across (30miles) across. Often there will be no cloud in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where air is sinking.
  • In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.
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