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Lesson 01 Atmospheric 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 The water cycle, illustrating evaporation, condensation & precipitation 5 The 3 M’s:- Moisture Mass Movement 6 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 11 Troposphere 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 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 Tropopause Height Tropopause Temperature 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 JA, JB, JC = Jetstreams Height/temperature graph for tropopause 17 Layered structure defined by the temperature profile (ELR) through the atmosphere. (Radio sounding) Radio Sonde 19 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 reradiated 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.