This comprehensive IEB geography summary will help you achieve a distinction in your Grade 11 year. It was created by a learner who achieved 93 % for his grade 11 geography year.
Temperature differences on a global scale create differences in atmospheric pressure.
Atmospheric pressure can be categorised as a high pressure or a low pressure.
Atmospheric pressure is the force exerted on the surface of the Earth by the air molecules
above as a result of the Earth’s gravitational pull.
Atmospheric pressure is measured in hectopascals (hPa) or Milimeters
High Pressure Low pressure
Air movement Associated with Associated with rising
sinking (subsiding) and (ascending) and
diverging air. converging air.
Shape Circular Oval
Direction of air movement Rotate in an anticlockwise Rotate in a Clockwise
direction direction in the Southern
in the Southern Hemisphere
Hemisphere
Weather Clear, cloudless and Cloud cover and rainfall.
dry conditions. Stable. Unstable weather
conditions.
Isobar Values Increase towards Decrease towards centre
centre of cell of cell.
Othername Anti-cyclone Cyclone
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,World Pressure belts
● Air does not flow directly from the poles to the Equator; it circulates in three cells per
hemisphere and is thus known as the tri-cellular pattern of air circulation.
● Air masses with different temperatures and characteristics do not mix.
● Where there are high temperatures the air expands and rises, so that an area of low
pressure is formed.
● Air is drawn towards the low pressure to replace the air that is rising and so is an
area of converging air.
● Where there are low temperatures the air contracts and sinks, so that an area of high
pressure is formed.
● The descending air moves outwards from the high pressure area and divergence
occurs.
● Sinking (descending) air creates high pressure areas (stable conditions).
● Rising air (ascending) creates low pressure areas (unstable conditions).
- Polar High
- Subpolar Low (polar front)
- Subtropical High
- Equatorial low-pressure belt (ITCZ –
intertropical convergence zone).
- The polar front is the latitude at 60°North
and South where the polar easterlies
bringing cold dry air meet the westerlies
bringing warm moist air from the equator.
- ITCZ or inter-tropical convergence zone is
where the two sets of tropical easterlies
converge close to the equator. The ITCZ
moves more south in summer and further
north in winter (in the southern hemisphere)
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,Atmospheric temperature, pressure and winds
High-temperature cause air to heat up and rise to leave an area of low pressure at the
surface.
Low temperatures cause air to sink, leaving an area of high-pressure
PRESSURE GRADIENT AND CORIOLIS FORCE
Pressure Gradient Force (PGF) is the force that results when there is a difference in
pressure across a surface.
Pressure Gradient: the amount of change in atmospheric pressure between high and
low-pressure areas.
- Wind is the result of this movement of air from a high pressure (HP) area to a low pressure
(LP) area down a pressure gradient.
- The PGF acts at right angles to the isobars and the speed of the air movement (Wind) is
determined by how close or far apart the isobars are from each other.
- If the isobars are close together then the wind speed is faster (steep PGF). If the isobars
are far apart then the wind speed is slower (gentle PGF).
- Pressure gradient is the difference in atmospheric pressure between a high and
low-pressure area.
Coriolis Force (CF) causes objects (wind/water) that are freely
moving to deflect towards the right in the
northern hemisphere and to the left in the southern hemisphere as a
result of the Earth’s rotation.
Developed and tested by scientist William Ferrell and is also
known as Ferrell’s Law.
- The Coriolis force affects wind, ocean current, aeroplane,
and missile direction.
- The amount of deflection of the wind is directly related to
both the speed at which the air is moving and the latitude.
3
, - Slow moving winds will be deflected only a small amount, while stronger winds will be
deflected more.
- Winds blowing closer to the poles will be deflected more than winds blowing at the same
speed closer to the equator.
- The Coriolis Force is zero at the equator (tropicalcyclones only form from 50 to 30° N and S
of the equator where the force becomes strong enough to cause the air to start spinning to
form a vortex).
Synoptic Maps
Synoptic weather maps give a summary of the recorded weather conditions at a specific
time.
All Readings are taken at 14:00 South African time.
Weather data is collected through:
- Meteosat, a weather satellite.
- Weather balloons that carry instruments to record temperature and humidity at different altitudes.
- Official weather stations all over the country and surrounding seas.
- Important South African weather stations are also found on Gough and Marion islands.
● Temperature: Air and Dew point in *C on the left of the weather station top = Air
bottom =Dew
● Weather indicated between temp known diff WW and WW]
● Precipitation if any between temps
● Wind direction look for the arrow and where the arrow connects with a dot is where
the wind is blowing
● Windspeed, long feather = 10 knots short = 5 knots
● Pressure is shown by isobars with intervals of 4hPa between them.
● Cloud cover inside the circle. Shaded in 8ths according to cloud cover seen.
Atmospheric pressure on synoptic weather
maps
● Isobars are lines on weather maps
that join places of equal pressure.
● Atmospheric pressure is measured in
hectopascals with the average being around
1000hPa.
● Pattern and numbering of isobars
show us whether there is a high or a low
pressure.
● By studying the patterns shown by
isobars, forecasters can make predictions
about how the weather conditions will
develop.
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