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.