Pressure, Wind and Weather Systems
WINDS are horizontal flows of air; winds blow from areas of high pressure
to areas of low pressure (nature tries to equalise pressure)
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PRESSURE describes the tendency of the air to rise or to sink at any given
place or time.
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Air tends to rise or sink as a result of its density.
Air density varies with altitude but, at the ground level, air density is
governed by its temperature.
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Thus, variations in radiation and temperature control pressure and wind.
Pressure & Wind
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atschool.eduweb.co.uk
This powerpoint has been altered by Mr. Smith!
GLOBAL PRESSURE & WIND
Global circulation depends on differential heating over the globe. The system is driven by strong
equatorial heating, causing LOW PRESSURE. Equatorial air rises, diverges and descends over the
tropics, where HIGH PRESSURE dominates; where it diverges at ground level. This tropical air blows
towards the equator, completing the equatorial cell, or towards the mid-latitides where it meets cold,
dense polar air blown out from the polar HIGH PRESSURE. These contrasting tropical and polar air
masses meet at the POLAR FRONT LOW PRESSURE BELT, where the warmer air is forced upwards
by the polar air. At high level, this air again diverges towards the pole or to the tropic.
POLAR HIGH
POLAR FRONT (LOW PRESSURE)
TROPICAL HIGH
EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW
Wind strength depends on the
difference in pressure between the
high and low pressure systems,
and the distance between them.
� This is called the PRESSURE
GRADIENT; it is a similar
concept to the physical slope
between two places, shown on a
contour map. Pressure is shown
by ISOBARS on a weather map.
� Pressure difference essentially
depends on the temperature
difference between the two places.
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A steep pressure gradient results
from a large pressure difference
or short distance between places
and causes strong wind.
WIND DIRECTION & STRENGTH
Farmers plant trees to protect orchards,
houses, stock or prevent soil erosion
Locally, wind is channelled
down streets (wind
canyons).
Strong winds also occur
in low latitudes due to
stronger heating and
steeper presure gradients.
Hurricanes and
tornadoes are both
tropical phenomena.
Strong polar winds due to low
friction
Beach windbreaks reduce
windspread by increasing friction
Hurricane in Florida
Tornado in USA
High
CORIOLIS FORCE
Theoretical wind
which would result
solely from pressure
gradient
Actual wind
which blows, as
diverted by
Coriolis Force
Low
Pressure gradient wind blows from
high presure towards low pressure.
� The earth’s rotation diverts this
wind direction laterally. This force is
called the CORIOLIS FORCE.
� The Coriolis force diverts wind the
the right in the northern
hemisphere; to the left in the south.
� The effect is stronger at high
altitude where ground level friction
is less significant.
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LOW
In the north, winds blow
clockwise out from a high
pressure. (In the south, they
blow anti-clockwise).
HIGH
In the north, winds blow anticlockwise into a low pressure
system. In the south, they blow
clockwise.
HIGH PRESSURE
High Pressure means that air tends to sink. Sinking air is
compressed, warms up as a result and its relative humidity
falls below saturation. Any clouds evaporate. Rainfall is
unlikely, apart from occasional short, intense convectional
storms due to insolation with lack of clouds in daytime.
LITTLE
WIND
CLEAR SKIES
High pressure systems have clear skies, little or no wind,
little rainfall and tend to be stable and slow moving.
� Visibility is initially good, but rapidly deteriorates as dust
is trapped by sinking air and is not washed out by rainfall.
� Cloud cover is slight, resulting in a high diurnal ranges of
temperature (hot days, cold nights). Due to the trapped dust
particles and cold nights, dew, frost, fog or smog are
common.
� Air quality is low as all forms of pollution are retained in
the lower atmosphere.
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VISIBILITY IS
POOR
FEW CLOUDS
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN MOUNTAINS - LESS
POLLUTION
LOW-LATITUDE LOW PRESSURE SYSTEMS
Low pressure systems involve air that tends to rise, thus
causing clouds and precipitation. Those near the equator tend
to be high energy due to strong ground heating
(convectional). Low pressure systems may develop tornadoes
and sometimes develop into hurricanes, fuelled by warm,
very humid air evaporated from tropical oceans in summer.
They tend to be fast moving, with plenty of cloud cover that
reduces diurnal temperature range, strong winds and high
rainfall.
Hurricane off USA
fueled by hot
humid air over
the Caribbean
A convectional cumulo-nimbus
cloud results from strong ground
heating at the equator
A tornado may
develop from a
cumulo-nimbus cloud
WARM FRONTS
A FRONT is the boundary between two air masses. A depression
has two, a warm (the front of the warm air) and a cold.
1
TROPICAL MARITIME AIR
POLAR MARITIME AIR
2
MAINLY STRATUS
CLOUDS
The warm front is
angled gently due to
ground level friction
which slows the air at
low level as the whole
system moves
eastwards.
POLAR
MARITIME
COLD FRONTS
TROPICAL MARITIME
The cold front is steeper, also due to ground level friction
slowing the lower air, so uplift is more rapid than along the
warm front.
� This causes cumulo-nimbus clouds and possible
thunderstorms rather than thick stratus cloud.
� Eventually, the two fronts meet, forcing the warm air off
the ground. This is an OCCLUDED FRONT (occlusion),
and happens to all depressions as they ‘fill’.
� The whole system takes about 24 hours to pass.
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OCCLUSION
Tm air forced up
Pm air
meets at
ground level