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CGS Ground School
Meteorology
The atmosphere
© Crown copyright 2012.
No part of this presentation may be reproduced without the permission of the issuing authority.
The views expressed in this presentation do not necessarily reflect the views or policy of the MOD.
The atmosphere
The Earth is surrounded by an ocean of air called
the atmosphere extending out almost 200 miles.
The atmosphere is made up of four layers.
The troposphere - The lowest layer of the
atmosphere(0 – approx 7 miles). This is where
the majority of weather occurs.
The stratosphere - This extends from the top of
the troposphere to 30 miles above the Earth’s
surface. The Ozone layer is found in the upper
levels of the stratosphere.
The mesosphere - This extends from 30 miles
to 50 miles above the Earth’s surface. It is the
coldest part of the atmosphere.
The thermosphere – This is made up of two
very similar parts – the ionosphere (lowest) and
the exosphere (highest). This part of the
atmosphere stretches to 200 miles above the
Earth’s surface.
The atmosphere
The atmosphere is comprised of approximately
78% Nitrogen, 21% Oxygen and 1% other gases.
It contains a variable amount of
both visible water and invisible
water vapour, and numerous
small particles of salt, dust, soot
etc.
The weight of the air varies as
waves, ripples and whirlpools of
air meander over the Earth,
exerting a pressure in the region
of 14 PSI (29 inches of mercury
or 1013 hPa).
O2
N2
Atmospheric Pressure
The weight of the air (air pressure) can be
measured using either:
A mercury barometer
Vacuum
30
Or
An aneroid barometer
Mercury
on the
Atmospheric pressure squeezes
open
tube forces
mercury
up
the
partially
evacuated
capsule,
on the sealed
side of pointer.
the
moving
the attached
tube.
Partially evacuated
aneroid capsule
29
28
Atmospheric Pressure
Atmospheric pressure reduces
with height, because there is
less air above as you climb.
Near the ground the pressure
drops approximately 1 hPa for
every 30 ft climbed.
Whilst at 20,000 ft the pressure
drops approximately 1 hPa for
every 50 ft climbed.
These figures are only
approximate because the
temperature also affects the
rate of change of pressure.
1hPa = 50ft
20,000ft
1hPa = 30ft
Atmospheric Pressure
Cold air is more dense
than warm air, therefore a
climb of 1,000 ft in cold air
will cause a greater
pressure reduction than
the same climb in warm
air.
This means that an aircraft
flying at a constant
pressure level will climb
when flying from cold air to
warm air.
Likewise, it will descend
when flying from warm air
to cold.
COLD AIR
WARM AIR
700 hPa
700 hPa
10,000ft
indicated
10,000ft
indicated
1000 hPa
1000 hPa
International Standard
Atmosphere
To ensure that all altimeters read the same in any
given pressure conditions they are calibrated using
the International Standard Atmosphere.
This assumes that:
Temperature at mean sea level = 15°C
Pressure at mean sea level
= 1013.25 hPa
Temperature reduces at
1.98°C / 1000 ft
until 36,090 ft where it remains constant at -56.5°C
Global Atmospheric
Pressure
In meteorology the atmospheric
pressure recorded at any site is
converted to a mean sea level
reading.
This allows worldwide pressure
comparisons to be made.
Polar high
If the pressure patterns were
Temperate low
recorded over the Earth at any
given moment in time, they would Sub-tropical
appear similar to those as shown high
on the right.
Equatorial low
With distinct bands of
high and low pressure.
Surface pressure charts
The recorded pressures can be plotted on a
synoptic (surface pressure) chart.
Areas of equal pressure can then be joined by
lines called isobars.
These lines will be variable and complex but will
usually produce well defined shapes which are
given names.
Surface pressure features
The depression (low or cyclone) - an area of
relatively low pressure, marked by more or less
circular, concentric isobars enclosing the centre
where the pressure is the lowest.
Surface pressure features
The Secondary Depression - a small area of low
pressure within the area covered by a larger
(primary) depression.
Surface pressure features
The anti-cyclone (high) - an area of relatively high
pressure, marked by more or less circular,
concentric isobars enclosing the centre where the
pressure is the highest. The isobars are generally
further apart than a depression.
Surface pressure features
The Col - an area of little pressure change
between two highs and two lows.
Surface pressure features
The trough - an area of low pressure extending
out from a depression. The isobars may bend
sharply to give a "V" shape.
The ridge - an area of high pressure extending
out from an anti-cyclone. The isobars are always
rounded and are never “V” shaped.
Pressure systems
Each of the different pressure distributions will
have particular weather associated with them.
The pressure systems will constantly change
with anticyclones subsiding (pressure reducing)
or intensifying (pressure increasing)
and depressions deepening (pressure reducing)
or decaying (pressure increasing).
By studying these changes in the pressure
systems, the forecaster can predict the future
pressure distribution and therefore the likely
weather.
By measuring the distance between the isobars
the windspeed can also be predicted (see
presentation on Wind).
THE END
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