Grade 12 Climatology

Mid-latitude Cyclones

Low Pressure Cells

L

Pressure lower than 1000 hPa is regarded as low.

Pressure is lowest at the centre and increases outwards.

Winds blow into the low pressure cell in a clockwise direction.

Between 40° and 60° the polar air mass meets the warm subtropical air. Warm air rises and condensation occurs.

General Characteristics of M.L.C’s

Also known as extratropical

cyclones, temperate

depressions, or frontal systems.

They occur as a pair of fronts – a warm and a cold front –linked to a

central area of low pressure.

They carry lots of moisture,

they can bring high winds and heavy rainfall

near the coast.

Their winds spiral clockwise in the

southern hemisphere and anti-clockwise in

the NH.

They can stretch up to about

2000km across.

They travel from the west to east

with the Jetstream.

Travel about 30 km/h (1200

km/day).

Conditions necessary for M.L.C formation

• Mid-latitude cyclones are powered by large temperature differences in the atmosphere.

• There needs to be a large temperature difference between the subtropical Westerlies and the Polar Easterlies so that a front develops.

• Where these masses meet (polar front), there must be a disturbance in the jet stream that causes the cold air to push into the warmer air.

• The warmer air mass pushes up above the cold air mass creating a low pressure cell into which the wind spirals

M.L.C - stages of developmentsInitial stage *Disturbance in polar front as warm

air pushes up over cold air. *A wave is formed and the low pressure cell begins to form. *Winds strengthen and clouds begin to form.

Mature stage *Wave deepens and pressure gradient increases. *Warm and cold front formed.*Warm/cold sector split by fronts.*Strong winds and rain.

Occluded stage *Cold sector wedges below warm sector and warm sector narrows. *Two fronts combine to form an occluded front (occluded front).*Nimbostratus cloud and rain.

Degeneration stage *All warm air is lifted off the ground and the cyclone weakens.*Air is cold and gusty as skies clear.