Pressure, Wind and Weather Systems

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Pressure, Wind and Weather Systems
Air stops rising when it meets air of equal
density, then diverges at high level to produce
more wind which eventually sinks elsewhere to
complete the circulation cell

• WINDS are horizontal flows of air winds blow
  from areas of high pressure to areas of low
  pressure (nature tries to equalise pressure)
• PRESSURE describes the tendency of the air to
  rise or to sink at any given place or time.
• 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.
• Thus, variations in radiation and temperature
  control pressure and wind.

Insolation
Air heated by contact with ground expands
becomes less dense and rises
Denser air drawn in at low level to replace
rising, less dense air
LOW PRESSURE
Denser air drawn in at low level to replace
rising, less dense air
Sun heats up ground
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GLOBAL PRESSURE WIND
ZONE of least heating produces HIGH PRESSURE
HIGH
ZONE of greatest heating produces LOW PRESSURE
LOW
ZONE of least heating produces HIGH PRESSURE
HIGH
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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.
Rising air diverges at the tropopause, where a
permanent temperature inversion results in warmer
air above.
POLAR HIGH
POLAR FRONT (LOW PRESSURE)
TROPICAL HIGH
EQUATORIAL (Inter-tropical convergence zone -
ITCZ) LOW
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WIND DIRECTION STRENGTH

• 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.

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.
A steep pressure gradient results from a large
pressure difference or short distance between
places and causes strong wind.
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CORIOLIS FORCE
High

• Pressure gradient wind blows from high presure
  towards low pressure.
• The earths 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.

Theoretical wind which would result solely from
pressure gradient
Actual wind which blows, as diverted by Coriolis
Force
Low
LOW
In the north, winds blow anti-clockwise into a
low pressure system. In the south, they blow
clockwise.
In the north, winds blow clockwise out from a
high pressure. (In the south, they blow
anti-clockwise).
HIGH
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GLOBAL PRESSURE WIND
POLAR HIGH PRESSURE
POLAR FRONT MID-LATITUDE LOW PRESSURE
TROPICAL HIGH PRESSURE
INTER-TROPICAL CONVERGENCE ZONE -LOW PRESSURE
TROPICAL HIGH PRESSURE
POLAR FRONT MID-LATITUDE LOW PRESSURE
POLAR HIGH PRESSURE
GLOBAL WIND BELTS (trade winds) are controlled by
the major pressure belts, which relate
fundamentally to temperature. Regional wind
systems (eg the Indian Monsoon) relate to
continental heating effects, and seasonal
changes. Local winds relate to smaller scale
temperature contrasts (ie Aspect, Albedo,
Altitude etc).
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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.

• In Britain, high pressure systems have clear
  skies, little or no wind, little rainfall and
  tend to be stable and slow moving.
• Visibility is intially 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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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.
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MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure systems are called
depressions in Britain. They also involve rising
air, clouds, strong winds and rainfall and are
fast moving.
ARCTIC MARITIME from Arctic Ocean Cold, humid.
POLAR CONTINENTAL from E.Europe Cold, dry in
winter Warm, dry in summer.
POLAR MARITIME from Greenland Cool, humid.

• Depressions result from the convergence of warm
  air from the tropical high pressure belt with
  cold air from the poles along the Polar Front.
• The energy of the depresion is a result of the
  difference in temperature and humidity between
  the two air masses.
• This contrast varies with the exact origin of
  the air mass, the season and the nature of the
  surface over which they have passed.

POLAR FRONT this shifts polewards in summer and
equatorwards in winter, hence British seasonal
contrasts.
POLAR MARITIME RETURN Coolish, very humid.
TROPICAL CONTINENTAL From N.Africa Hot, dry
TROPICAL MARITIME from Atlantic near tropic Warm,
humid
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MID-LATITUDE LOW PRESSURE SYSTEMS
Mid-latitude low pressure systems are called
depressions in Britain. They also involve rising
air, clouds, strong winds and rainfall and are
fast moving.
OCCLUDED FRONT Cold warm fronts meet
Depressions (L) over Europe showing FRONTS
COLD FRONT
POLAR MARITIME AIR
WARM FRONT
WARM FRONT
COLD FRONT
TROPICAL MARTIME AIR
COLD FRONT

• Depressions result from the convergence of warm
  air from the tropical high pressure belt with
  cold air from the poles along the Polar Front.
  The systems move rapidly across the Atlantic
  before filling and drifting north-eastwards to
  Scandinavia from Britain.
• The energy of the depresion is a result of the
  difference in temperature and humidity between
  the two air masses.
• This contrast varies with the exact origin of
  the air mass, the season and the nature of the
  surface over which they have passed.

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WARM FRONTS
A FRONT is the boundary betwen two air masses. A
depression has two, a warm (the front of the warm
air) and a cold.
The warm front is angled gently due to ground
level friction which slows the air at low level
as the whole system moves eastwards.
TROPICAL MARITIME AIR
POLAR MARITIME AIR
As the warm tropical maritime air moves eastwards
towards Britain, it is forced upwards by colder,
denser polar maritime air. The speed of uplift
depends on the relative temperature of the two
air masses. Uplift causes expansion, cooling,
falling relative humidity until dew point
temperature is reached when condensation starts
to occur on particles. The amount of
precipitation depends on the hunidity and
temperature of the warm air mass, and the
particles available.
MAINLY STRATUS CLOUDS
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COLD FRONTS
As the depression moves eastwards, the warm
tropical air continues to be forced upwards by
the colder, denser polar air mass.
POLAR MARITIME
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.