WEATHER REVISION

• In the examination you may be asked to describe and explain the weather changes at a location as a depression passes.

• As the depression approaches and passes, the following sequence of changes in the weather would be typical.

As the depression approaches.

• Weather is bright and high level cloud appears.

• Winds will be light and south-westerly (because winds rotate anticlockwise around a depression).

• Winds strengthen (isobars get closer) and the cloud base falls as warm front approaches. Light rain falls.

As the warm front passes overhead

• Rain becomes persistent (heavy drizzle is common). Temperature rises and winds increase, becoming westerly (closer isobars and as centre of the depression moves eastwards, the wind direction changes).

In the warm sector

• The rain eases, although showers can occur.

• Temperatures stay warm and winds still strong.

As the cold front passes overhead.

• Heavy rainfall, sometimes thundery with hail.

• Winds may strengthen and turn to a north-westerly direction.

• There is a sudden drop in temperature as winds from the north west (colder, Arctic air) are brought in.

After the cold front

• Winds begin to ease and turn to northerly (cold).

• Sunny with (sometimes) showers.

ANTICYCLONE

• 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, as the cold forces moisture in air to condense at low altitudes.