Lesson 01 Atmospheric Structure Composition, Extent & Vertical Division
Jan 16, 2016
Lesson 01Atmospheric Structure
Composition, Extent & Vertical Division
Composition
By Volume Nitrogen (N2) 78%
Oxygen (O2) 21%
Others (1%)Carbon Dioxide (CO2) 0.035%
Ozone + Others(Neon, Xenon etc) Water Vapour (variable from 0 - 4% locally)
Composition con’t
Analysis reveals no variation in its composition up to at least 60km
except for increasing concentrations of Ozone in the upper Stratosphere.
At higher altitudes, the force of gravity being less causes the proportions to change.
Composition con’t
None of these gases changes its state within the normal temperature range of the atmosphere so dry air remains invisible.
Only water vapour changes its state
water gas solid
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The water cycle, illustrating evaporation, condensation & precipitation
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The 3 M’s:-
Moisture Mass
Movement
Properties of the Earth’s Atmosphere
The earth’s atmosphere varies both vertically and horizontally in terms of:
- Pressure- Temperature - Density
- Humidity
Properties of the Earth’s Atmosphere, contd.
The atmosphere is also a poor conductor of heat and being a gas is extremely fluid and only supports life in the lower levels.
Fifty percent of the atmosphere is below the 500mb pressure level (about 18,000 feet).
Vertical Division
The atmosphere extends to about 1000 km above the earth’s surface and
consists of several layers, each with its own properties.
The layers are defined by the temperature profile as shown in the following diagram..
Vertical Division
Ionosphere
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Layer in contact with surface Contains 80% of atmosphere Temperature decreases with height Until Tropopause reached Zone of vertical mixing by convection and
turbulence Contains most of water vapour Zone of clouds and weather Heated from below
Troposphere
Tropopause
Indicated by marked change in lapse rate
Upper limit of cloud and weather
Lowest tropospheric temperatures at this level
Strongest winds aloft just below it
Height varies according to season and latitude
H2O decreases, O3 increases
Typical Average Tropopause Heights and
Temperatures
TropopauseHeight
TropopauseTemperature
Equator 56 000 ft -75C
Latitude 50N 37 000 ft -55C
Poles 27 000 ft -45C
Variation in Height of Tropopause
The thickness of the Troposphere will vary according to temperature.
Lower when the air is cold (i.e. polar latitudes) Higher when air is warm (i.e. equatorial latitudes) Height therefore depends on season and latitude Coldest tropopause temperatures over the equator Breaks in tropopause caused by jet streams
Variation of Surface Pressure with Height of Tropopause
High tropopause high air column high surface pressure more prevalent in summer near equator
Low tropopause low air column low surface pressure more prevalent in winter near polar latitudes
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Height/temperature graph for tropopause
JA, JB, JC = Jetstreams
Layered structure defined by the temperature profile (ELR) through the atmosphere.
(Radio sounding)
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Radio Sonde
Stratosphere Temperature increases with height to
about 50 km. Contains warm Ozone region in upper
levels at 40-50 km. Weather does not penetrate but some
severe CB’s may penetrate several 1000’s of feet.
Nacreous clouds form at about 20-30 km
Nacreous Clouds
Stratopause
Marks upper limit (50 km), of the stratosphere.
Maximum temperature just below freezing.
Mesosphere
Region of decreasing temperature Noctilucent clouds form at
approximately 80 - 85 km. Usually visible in summer in
Northern sky near midnight.
Noctilucent Cloud
North Ayrshire Scotland
1988 July 02-03 00005 UTC
Mesopause
Marks the upper limit of the Mesosphere.
Height approximately 80-90 km. Temperatures between -120°C
(summer) & -50°C (winter) at high latitudes
Thermosphere
Zone of increasing temperature Upper limit undefined. Merges into the
Exosphere/Ionosphere.
Ozone in the Stratosphere
90% of Ozone found here.
Absorbs harmful Ultra Violet (UV) radiation.
UV Radiation causes skin cancers. destroys DNA
molecules. Ozone filters in
aircraft
Strongly absorbs IR earth radiation
Absorbed energy re-radiated back to earth
Helps maintain the thermal balance in the atmosphere
Ozone Production
Ozone produced by interaction of molecular O2 and UV radiation
O2 + UV O + O
O2 + O O3
Ozone forms mainly above 25 km but drifts down producing a peak concentration at 25 km.
Ozone Destruction
Ozone is also destroyed by UV radiation.
O3 + UV O2 + O
O3 + O 2O2
Ozone is also destroyed by the collision between two ozone molecules.
O3 + O3 3O2
Ozone Balance
Ozone between 25 and 35 km is maintained by a delicate natural balance
Ozone can be reduced by natural events e.g. solar flares and cosmic rays.
Pollutants such as fluorocarbons also destroy ozone.