Lecture notes, lecture 1 - atmospheric composition PDF

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Atmospheric Composition ...

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THE EARTH’S ATMOSPHERE A. COMPOSITION OF THE ATMOSPHERE 1. General.  The atmosphere is a continuously moving sea of air around the earth.  Dry air is a mixture of nitrogen (about 78%), oxygen (about 21%) by volume, and small quantities of many other gases.  Air is never usually completely dry because it contains water.  Approximately 50% of the earth’s atmosphere, by weight, is contained in the first 18,500 ft. 2. Composition of the Atmosphere by Volume  Oxygen (O 2) approx. 21%  Nitrogen (N2 ) approx. 78%.  Other gases approx. 1%  Water Vapour  Water vapour is present in the atmosphere in varying proportions (generally between 1% and 4%)  is responsible for the weather around the earth.  Hygroscopic Nuclei (Condensation Nuclei)  Water vapour needs a “solid” on which to condense (salt, smoke, dust etc).  These solid particles are known as hygroscopic nuclei.  Ozone (O 3)  Ozone is a form of oxygen which is found in a layer in the upper atmosphere.  This layer shields and absorbs ultra violet (UV) radiation.  Ozone is caused the breakup of oxygen molecules at altitudes above 30km by ultraviolet radiation from the sun (O2  O + O).  These separated atoms may then collide with other oxygen molecules to create ozone. (O2 + O  O3 ).  These collisions are rare above 80 km because of the very low density of the atmosphere.  below 35km most of the incoming ultraviolet radiation has already been absorbed at higher levels.  Ozone is unstable, it is easily destroyed by oxygen, hydrogen, nitrogen, chlorine, photochemical cycles, etc., releasing low energy radiation (e.g. heat) in the process. B. 1.   

Structure Of The Atmosphere The Troposphere The lowest layer of the atmosphere in contact with the surface of the earth. Temperature tends to decrease as height increases within the troposphere. There may be minor irregularities in the rate of temperature decrease with height (Environmental Lapse Rate, ELR), but in general the lapse rate is uniform until the

tropopause is reached.

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 The lapse rate then remains constant (isothermal) or the temperature may even increase slightly.  This marked change in the lapse rate marks the tropopause which defines the upper limit of the troposphere. 2. The Tropopause  inhibits the vertical movement of the air and therefore little weather occurs above the tropopause.  marks the upper limit of normal weather and cloud (although large thunderstorms do sometimes extend above it).  The lowest temperatures are found at this level, and the strongest winds are usually found just below it.  The height of the tropopause and its temperature both vary with latitude and the season.  In general it is lowest at the poles and highest at the equator.  in latitudes higher than 30 it is lowest in winter and higher in summer.  Nearer to the equator the seasonal trend is reversed and the tropopause is slightly higher in winter than in summer.  Typical approximate values are:  at the equator, 56,000 ft, temperature as low as - 75C;  at 50S, 37,000 ft, temperature above - 55C;  at the poles, 27,000 ft, temperature about - 45C.  The following table is for the Southern Hemisphere. July (winter) January (summer) Pole 28,000’ 32,000’ 30,000’ 35,000’ 60S 55,000’ 53,000’ 30S Equator 57,000’ 55,000’  The coldest air at tropopause levels is found above the Equator, and the warmest above the poles. 3. The Stratosphere  Above the tropopause, temperature tends to increase to values similar to those experienced on the surface, up to a level known as the stratopause, or warm ozone region.  Ozone is merely a particular variety of oxygen, but one which absorbs ultra-violet radiation, turning harmful radiation into heat.  Although turbulence in the stratosphere is less frequent than in the troposphere, it is by no means negligible. It can be found in or above Cb cloud. Mountain waves and waves above Cb which protrude into the stratosphere can be turbulent at times.  Concentrations of O3 above the tolerable limit are normal from 50,000’ to 100,000’. 4. Definitions  Lapse Rate: is the rate at which the temperature increases or decreases with an increase in height.  positive lapse rate = temperature decreases with height.  negative lapse rate = temperature increases with height.  Inversion: (negative lapse rate) Warm air on top of cold air. Very stable.  Isothermal: temperature remains constant with a change in height.

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5. Temperature and height – Season and Latitude

6. Atmospheric Structure...