Grade 12 Climatology

Subtropical Anticyclones

Subtropical Anticyclones• Cells of high pressure• Subtropical areas are found between Tropic of

Capricorn/Cancer and 40°N/S.• They tend to bring clear, calm weather.• In an anticyclone air descends and pressure

increases. This brings very light winds which blow out from the centre in an anti-clockwise direction in the S.H.

H

Planetary wind blow out of and around high pressure cells towards low pressure cells along the pressure gradient. Coriolis

force deflects these winds to the left in the SH.

Pressure Cells Around SA

South Atlantic anticyclone

Indian Ocean anticyclone

Kalahari anticyclone

Anticyclonic influence on climate in winter• S.A.A, S.I.A and Mid-latitude cyclones all move

further northwards.• In the northern hemisphere summer the sun is

directly overhead causing the heat distribution and pressure belts to move northwards.

• S.A.A & S.I.A move closer to the coasts. • Winds travelling out of these cells have a shorter

distance to travel, therefore they pick up less moisture and bring less rain at this time of year.

Weather in the Interior (Winter)• We see clear skies, sunny weather and no rain.• Temperatures range from freezing to mid

twenties.• We often see very little rain or drought for a few

months.• These conditions are caused by the Kalahari

anticyclone.• Inversion Layer: a layer of air where temperature

increases with altitude.

Weather in summer• Kalahari .A. has risen verticaly by about 500m• Hot conditions in summer cause a low pressure at

the surface – this is why we see a low pressure.• The inversion layer rises with the K.A.A. this

allows moist air from the Indian Ocean to penetrate inland = summer rain.

• Pressure belts and S.A.A & S.I.A have shifted further southwards.

• S.A.A & S.I.A have shifted further from the coast. Winds travel further and pick up moisture.

• Warm current = more moisture = more rain

Position of the Kalahari High in Winter

Position of the Kalahari High in Summer

The Southeaster • S.A.A moves south and often elongates to

form a ridge south of the country.• This produces south easterly winds over the

cape.• The wind can be funnelled through valleys,

streets and can be very strong.• Wind is known as the Cape Doctor as it blows

in and can cool off the cape and blow pollution away.