2. Lesson 01 Atmospheric Structure n Composition, Extent & Vertical Division

3. Composition n By Volume u Nitrogen (N 2 ) 78% u Oxygen (O 2 ) 21% u Others (1%) F Carbon Dioxide (CO 2 ) 0.035% F Ozone + Others(Neon, Xenon etc) u Water Vapour (variable from 0 - 4% locally)

4. Composition con’t n Analysis reveals no variation in its composition up to at least 60km H except for increasing concentrations of Ozone in the upper Stratosphere. n At higher altitudes, the force of gravity being less causes the proportions to change.

5. Composition con’t n None of these gases changes its state within the normal temperature range of the atmosphere so dry air remains invisible. n Only water vapour changes its state u water  gas  solid

6. 5 The water cycle, illustrating evaporation, condensation & precipitation

7. 6 The 3 M’s:- X Moisture X Mass X Movement

8. Properties of the Earth’s Atmosphere n The earth’s atmosphere varies both vertically and horizontally in terms of: - Pressure - Temperature - Density - Humidity

9. Properties of the Earth’s Atmosphere, contd. n 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).

10. Vertical Division n The atmosphere extends to about 1000 km above the earth’s surface and n consists of several layers, each with its own properties. n The layers are defined by the temperature profile as shown in the following diagram..

11. Vertical Division Ionosphere

12. 11

13. n Layer in contact with surface n Contains 80% of atmosphere n Temperature decreases with height n Until Tropopause reached n Zone of vertical mixing by convection and turbulence n Contains most of water vapour n Zone of clouds and weather n Heated from below Troposphere

14. Tropopause n Indicated by marked change in lapse rate n Upper limit of cloud and weather n Lowest tropospheric temperatures at this level n Strongest winds aloft just below it n Height varies according to season and latitude n H 2 O decreases, O 3 increases

15. Typical Average Tropopause Heights and Temperatures

16. Variation in Height of Tropopause n The thickness of the Troposphere will vary according to temperature. n Lower when the air is cold (i.e. polar latitudes) n Higher when air is warm (i.e. equatorial latitudes) n Height therefore depends on season and latitude n Coldest tropopause temperatures over the equator n Breaks in tropopause caused by jet streams

17. Variation of Surface Pressure with Height of Tropopause n High tropopause u high air column u high surface pressure u more prevalent in summer u near equator n Low tropopause u low air column u low surface pressure u more prevalent in winter u near polar latitudes

18. 17 Height/temperature graph for tropopause J A, J B, J C = Jetstreams

19. Layered structure defined by the temperature profile (ELR) through the atmosphere. (Radio sounding)

20. 19 Radio Sonde

21. Stratosphere n Temperature increases with height to about 50 km. n Contains warm Ozone region in upper levels at 40-50 km. n Weather does not penetrate but some severe CB’s may penetrate several 1000’s of feet. n Nacreous clouds form at about 20-30 km

22. Nacreous Clouds

23. Stratopause n Marks upper limit (50 km), of the stratosphere. n Maximum temperature just below freezing.

24. Mesosphere n Region of decreasing temperature n Noctilucent clouds form at approximately 80 - 85 km. n Usually visible in summer in Northern sky near midnight.

25. Noctilucent Cloud North Ayrshire Scotland 1988 July 02-03 00005 UTC

26. Mesopause n Marks the upper limit of the Mesosphere. n Height approximately 80-90 km. n Temperatures between -120°C (summer) & -50°C (winter) at high latitudes

27. Thermosphere n Zone of increasing temperature n Upper limit undefined. n Merges into the Exosphere/Ionosphere.

28. Ozone in the Stratosphere n 90% of Ozone found here. n Absorbs harmful Ultra Violet (UV) radiation. n UV Radiation u causes skin cancers. u destroys DNA molecules. n Ozone filters in aircraft n Strongly absorbs IR earth radiation n Absorbed energy re- radiated back to earth n Helps maintain the thermal balance in the atmosphere

29. Ozone Production n Ozone produced by interaction of molecular O 2 and UV radiation u O 2 + UV  O + O u O 2 + O  O 3 n Ozone forms mainly above 25 km but drifts down producing a peak concentration at 25 km.

30. Ozone Destruction n Ozone is also destroyed by UV radiation. u O 3 + UV  O 2 + O u O 3 + O  2O 2 n Ozone is also destroyed by the collision between two ozone molecules. u O 3 + O 3  3O 2

31. Ozone Balance n Ozone between 25 and 35 km is maintained by a delicate natural balance n Ozone can be reduced by natural events e.g. solar flares and cosmic rays. n Pollutants such as fluorocarbons also destroy ozone.