This topic focuses on Geography Grade 11 content on Global Air Circulation.

1. GLOBALAIR CIRCULATION
Presented by : Mr T Mokgopa
: t.mokgopa9@gmail.com

2. GLOBAL WIND AND PRESSURE BELTS AS A RESPONSE TO
THE UNEQUAL HEATING OF THE ATMOSPHERE
• Equatorial Regions receive more insolation
than Polar Regions.
• Land/ Earth heats up and cools in a cycle of
a day.
• The mechanism that sets up the Earth’s
general circulation pattern is the
Latitudinal heat transfer.

3. AIR PRESSURE AND THE RELATIONSHIP BETWEEN
AIR PRESSURE AND TEMPERATURE
• Air pressure is created by molecules that constitute air through their motion, side, and number.
• Pressure and density both decrease with an increase in altitude.
• Relationship between Air Pressure and Temperature:
• Low Pressure System: Warmer Temperatures
• High Pressure System: Colder Temperatures

4. WIND, WIND SPEED, WIND DIRECTION
AND NAMING OF WINDS
• Simply put, Wind refers to the horizontal motion of air.
• A vertical component of wind results from turbulences.
• Properties of wind: Speed and Direction.
• Anemometer: Measures wind speed.
• Wind Vane: Determines wind direction.
• WINDS ARE NAMED FROM THE DIRECTION FROM WHICH THEY ORIGINATE.

5. DRIVING FORCES WITHIN THE
ATMOSPHERE
1. Gravitational Force.
2. Pressure Gradient Force.
3. Coriolis Force.
4. Friction Force

6. PRESSURE GRADIENT FORCE
• This force drives air from higher barometric pressure to lower barometric pressure.
• A Gradient is the rate of change in some property over distance.
• Unequal heating of Earth results in the existence of High-pressure and Low-pressure systems.
• When the two varying air masses are near each other, a pressure gradient force develops.
• Vertical air movement can also cause pressure gradient force.
• High-pressure: Subsiding and diverging air.
• Low-pressure: Rising and converging air.

7. CORIOLIS FORCE
• This a force that deflects wind on Earth’s rotating surface.
• If Earth was not rotating, wind would travel in a straight pattern as they move from areas of high-
pressure, to areas of low-pressure.
• The Coriolis Force deflects anything flying across the Earth’s surface.
• The strength of the Coriolis Force varies at different latitudes.
• There is no Coriolis Force at the Equator.

8. FRICTION FORCE
• The Frictional force drags wind as it moves across the surface of the Earth.
• Without the frictional force, the wind would continually flow at higher speeds.
• Generally, as wind flows through rougher surfaces, the presence of friction is higher.

9. PRIMARY PRESSURE AREAS AND ASSOCIATED
WINDS
JANUARY AVERAGE SURFACE BAROMERTIC
PRESSURES
JULY AVERAGE SURFACE BAROMETRIC
PRESURES

10. EQUATORIAL LOW/ ITCZ: Warm and Rainy
• Located at the 0° latitude.
• Receives constant high altitude of the Sun.
• Associated with the Low-pressure system.
• Intertropical convergence zone (ITCZ): Identified by bands of clouds along the equator.
• Associated with TRADE WINDS
• Winds are calm within the ITCZ. REASON? Weak Pressure Gradient.
• These equatorial calm conditions are referred to as Doldrums.

11. SUBTROPICAL HIGHS: Hot and Dry
• Located between 20° and 35° latitudes in both hemispheres.
• These subtropical anticyclones generally form as air above subtropics in pushed downward.
• Since warm air can absorb water vapour, the descending warm air is relatively dry.
• Several high-pressure areas are dominant in the Subtropics.
• Bermuda High: The Atlantic subtropical high in the NH.
• In the SH, three large high-pressure centres dominate the Pacific, Atlantic, and Indian Oceans.
• Associated with the Westerlies.

12. SUBPOLAR LOWS: Cool and Moist
• Two low-pressure cyclonic cells exist over the oceans around 60° N.
• Aleutian Low & Icelandic Low.
• These cells are dominant in winter.
• Polar front: areas of contrast where masses of air with different characteristics meet.

13. POLAR HIGHS: Frigid and Dry
• Characterized by weak cells.
• The polar regions receive little energy from the Sun.
• Associated with the Polar Easterlies.
• Characterized by cold and dry winds that move away from Polar Regions in an anticyclonic
direction.
• The Antarctic has stronger and persistent high-pressure system, the Antarctic High.

14. THE TRI-CELLULAR CIRCULATION WITH WINDS

15. THE END!!!