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Summary Grade 12 Geography - Climatology
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An in-depth summary for Climatology.
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GENERAL CHARACTERISTICS Mid-latitude cyclones are cells of low pressure.
They bring cold, windy and wet weather.
Are caused by pressure and wind patterns of mid-latitude cyclones and air masses.
General Characteristics of Mid-Latitude Cyclones
Are due to pressure, wind patterns and air masses involved in the formations.
Effects of pressure and wind pattern on mid-latitude cyclones:
Cell of low pressure.
Isobar patterns and winds converge in a mid-latitude cyclone.
Pressure is low at the centre and increases outwards.
Low pressure is below 1000 hPa.
Winds blow around and into low pressure cell in clockwise direction.
Winds do not blow straight along pressure gradient but are deflected by Coriolis force.
The effect of air masses on mid-latitude cyclones:
Mid-latitude cyclones develop in the middle latitudes. Between 40 and 60 degrees north
and south.
Where cold polar air mass meets warm subtropical air mass in zone called polar front.
When they meet, warm subtropical air rises and cold polar air wedges in underneath.
As it rises warm air cool and condenses water vapour.
Produces clouds and rain.
Can be between 1 500 and 3 000km in diameter.
Can move 1 200km in a day, lasts about five days in one region.
AREAS WHERE MID-LATITUDE CYCLONES FORM
Develop in the middle
latitudes from 37o-70o
N & S.
In region called the
polar front.
Mid-lat’s move from
west to
Occur mainly over
oceans and coastal
areas.
, Mid-latitude cyclones in South Africa:
Cold fronts only cross the southern Cape because we sit at 37 o S.
Warm fronts are bent southwards away from South Africa.
Cold fronts only reach SA in winter time because pressure belts and wind systems move
slightly north.
Rain is brought by cold fronts passing over the region from west to east.
CONDITIONS NECESSARY FOR THE FORMATION OF MID-LATITUDE CYCLONES
At around 50o S, only a short distance separates warm tropical air masses from cold polar
mass.
Transition zone is called the polar front.
Polar front is where warm, moist air of mid-latitude cell meets cold air of polar cell.
Warmer air gets pushed up polar front while colder air wedges in underneath.
Pressure difference exists between the warm tropical air and cold polar air.
Results in pressure gradient.
Westerly winds and polar easterlies blow along pressure gradients towards polar front.
Sometimes a bend forms in polar front - due to disturbance in winds in jet stream.
Change in condition causes isobars to form more of a circular pattern = a low-pressure cell.
Winds deflect and blow into low pressure cell.
STAGES OF DEVELOPMENT AND RELATED
WEATHER CONDITIONS
Initial Stage:
Bend forms and winds start to deflect and blow
into low pressure cell.
Weather features: winds strengthen, clouds
appear.
Warm, moist air is starting to rise, cool down
and water vapour is condensing.
Mature Stage:
Bend in polar front deepens and pressure
gradient becomes steeper.
Air blows in clockwise direction around and into
low
Polar front forms cold front and warm front.
Fronts divide area into warm and cold sector.
There is cold weather with strong winds, overcast skies and rain.
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