1. CGS Ground School Meteorology The atmosphere
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2. 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.
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3. 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
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4. Partially evacuated aneroid capsule
Atmospheric Pressure
The weight of the air (air pressure) can be measured using either:
A mercury barometer Or
An aneroid barometer
Vacuum 30 29
Mercury 28
Atmospheric pressure on the open tube forces mercury up on the sealed side of the tube.
Atmospheric pressure squeezes the partially evacuated capsule, moving the attached pointer.
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Partially evacuated aneroid capsule

5. 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
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1hPa = 30ft

6. 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
1000 hPa
700 hPa
10,000ft indicated
10,000ft indicated
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7. 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 = hPa
Temperature reduces at 1.98°C / 1000 ft
until 36,090 ft where it remains constant at -56.5°C
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8. 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.
If the pressure patterns were recorded over the Earth at any given moment in time, they would appear similar to those as shown on the right.
With distinct bands of
high and low pressure.
Polar high
Temperate low
Sub-tropical high
Equatorial low
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9. 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.
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10. 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.
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11. Surface pressure features
The Secondary Depression - a small area of low pressure within the area covered by a larger (primary) depression.
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12. 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.
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13. Surface pressure features
The Col - an area of little pressure change between two highs and two lows.
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14. 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.
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15. 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).
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16. THE END
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