REVISION

GRADE 11

GLOBAL AIR CIRCULATION

DIFFERENCE BETWEEN HIGH AND LOW PRESSURE:

LOW PRESSURE:

• Air rises in the middle;

• Air move clockwise in southern hemisphere;

• Unstable conditions with clouds and rain;

• Isobars decrease towards the middle

HIGH PRESSURE:

•Air descends in the middle;

•Air move anticlockwise in southern

hemisphere;

•Stable conditions with clear skies and

sunshine;

• Isobars increase towards the middle.

FOUR GLOBAL PRESSURE BELTS:

EQUATORIAL LOW PRESSURE

ZONE:

• 5° N and S

• High amount of isolation leads to warm air rising.

• Rising moist air cause cloud formation and high

rainfall.

SUBTROPICAL HIGH PRESSURE

ZONE:

•30° - 40° N and S

•Air cools down and descend (sink);

•Descending air is warm and dry and leads

to the formation of deserts;

•Cause warm days and low rainfall.

SUBPOLAR LOW PRESSURE ZONE:

•60° N and S

•Different air masses meet;

•Ascending (rising) air due to

convection. Warm air is forced to rise.

•Low pressure cell is formed;

POLAR HIGH PRESSURE ZONE:

•90° N and S

•Caused by cold and dense sinking air;

•Stable, clear and cold conditions.

TRI-CELLULAR CIRCULATION:

Ensure that no surplus energy is stored at the

equator.

THREE CELLS:

1. Hadley cell: 0° - 30 ° N and S

2. Ferrel cell: 30 ° - 60 ° N and S

3. Polar cell: 60 ° - 90 ° N and S

HADLEY CELL:

•0° - 30 ° N and S

•Air rises from equator to form

equatorial LP system and descends at

30 ° to form subtropical HP system.

•Tropical easterlies blow from hp to a

lp.

•Forms of ITCZ at equator (area of

convergence of tropical easterlies)

•Forms large cumulonimbus clouds

where ITCZ form.

•Cause heavy rain and thunderstorms.

FERREL CELL:

• 30 ° - 60 ° N and S

•Warm air from the subtropical sinks and

collide with cold air from the polar region.

• Polar front is formed here.

•Westerlies blow from a hp to a lp.

•Dry and stable conditions;

POLAR CELL:

• 60 ° - 90 ° N and S

• Air from the 90° sinks at form the HP cell.

• Polar easterlies blow from a hp system to a

lp system.

•Mid latitude cyclones form at the polar

front.

WARM LIGHTER AIR IS

FORCED TO RISE

FERREL CELL

POLAR CELL

Cold dense air

force warm air

to rise

CORIOLIS FORCE

•As air move from a high pressure to a low

pressure it is deflected and change direction.

•Reason: rotation of the earth.

•The winds deflect from north to south.

FERREL’S LAW

•Air is deflected to the LEFT in the southern

hemisphere and to the RIGHT in the northern

hemisphere

GEOSTROPHIC FLOW:

• Wind that blow parallel to isobars;

• Wind move from a high pressure to a low

pressure;

• Coriolis effect cause winds to deflect.

• When Coriolis force and pressure gradient is

equal and in opposite effect it will blow parallel

to the isobars.

WINDS RELATED TO GLOBAL AIR CIRCULATION:

TROPICAL EASTERLIES WESTERLIES POLAR EASTERLIES

Between 5° and

30/40° N and S.

Between 40° - 60° N

and S.

Move from poles to

60° N and S.

Move from Subtropical

HP to Equatorial LP.

Move from

Subtropical HP to

Subpolar LP.

Move from Polar HP

to Subpolar LP

Migrate North and

South as the ITCZ move

with season.

Winds gather latent

heat and is warm and

moist.

Very cold winds.

AIR MASS CHARACTERISTICS:

1. Air can heat up and cool down.

2. Air can shrink or expand.

3. Air has a mass. Air can be heavy or light.

4. Air can move vertically and horizontally.

SYNOPTIC WEATHER MAPS:

Definition: Map that shows a range of weather conditions for a particular area on a particular day.

Isobars: Lines on synoptic map that join points with equal pressure.

• Measured in hectopascals (hPa) with a barometer.

TWO TYPES OF PRESSURE CELLS:

HIGH PRESSURE: LOW PRESSURE:

HIGH PRESSURE CELL:

- Pressure increase towards the middle.

- Warm, stable and sunny conditions.

LOW PRESSURE CELL:

- Pressure decrease towards the middle.

- Rainy, unstable and cloudy conditions.

WEATHER ELEMENTS ON SYNOPTIC MAPS:

Air temperature

Dew point temperature

Type of precipitation

Cloud cover

Wind direction

Wind speed

TYPES OF PRECIPITATION:

CLOUD COVER:

WIND SPEED:

TYPES OF FRONTS:

COLD FRONT: WARM FRONT:Leads with warm air and brings

cold air behind.Leads with cold air and brings

warm air behind.

In front: Warm air In front: Cold air

Behind: Cold air Behind: Warm air

Impact on weather: Bring cold weather to South Africa.

Impact on weather: Little to noeffect on climate.

WINTER SYNOPTIC MAP:

FEATURE OF A WINTER MAP:• The date on the map;

• High pressure cell over the interior of South Africa;

• Other high pressure cells are closer to land and more North;

• Low temperatures over the land;

• Cold fronts are present.

SUMMER SYNOPTIC MAP:

FEATURE OF A SUMMER MAP:• The date on the map;

• Low pressure cell over the interior of South Africa;

• High temperatures over the land;

• Occlusion of cold and warm front are present (Tropical cyclone).