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Part 1. Energy and Mass Chapter 1. Composition and Structure of the Atmosphere ENSC201 Introduction The Atmosphere A mixture of gas molecules, suspended particles, and falling precipitation The atmosphere strongly affects our day-today lives ENSC201 ENSC201 Meteorology The study of the atmosphere and the processes that cause “weather” Climatology Examines weather elements over long time periods ENSC201 ENSC201 Variable Gases Water Vapor Most abundant variable gas Added/ removed to air through the hydrologic cycle Concentrations = nearly 0% to nearly 4% Important to energy balance and many atmospheric processes ENSC201 ENSC201 Carbon Dioxide A trace gas • 0.038% of atmosphere’s mass • Important to Earth’s energy balance Added through biologic respiration, volcanic activity, decay, and natural and human-related combustion Removed through photosynthesis Increasing at a rate of 1.8 ppm/year ENSC201 Temporal increases due to human activities Seasonal variations related to biological activity ENSC201 Ozone Tri-atomic form of oxygen Absorbs ultraviolet radiation Chlorofluorocarbons (CFCs) destroy ozone Destruction peaks over southern hemisphere Antarctic circumpolar vortex limits latitudinal mixing • Leads to an O3 “hole” ENSC201 Vertical Structure of the Atmosphere Density Mass (kg) per unit volume (m3) • Sea level average = 1.2 kg/m3 Near surface air is more dense • Compressibility • Mean free path – At surface = 0.0001 mm – At 150 km = 10 m ENSC201 Compressibility of gases relates to density ENSC201 Thermal Layers of the Atmosphere Four distinct atmospheric layers • Troposphere • Stratosphere • Mesosphere • Thermosphere Each has particular temperature characteristics with height. ENSC201 Troposphere Lowest layer Steady temperature decrease with height • -6.5oC/km (-3.6oF/1000ft) Virtually all weather processes • Contains 80% of atmospheric mass Tropopause = top of troposphere ENSC201 Violent updrafts may penetrate cloud tops into the stratosphere. The flattened top of this cumulonimbus cloud is in the stratosphere. ENSC201 Stratosphere Little actual “weather” Temperature inversion • Caused by absorption of UV radiation by O3 Stratopause = top of stratosphere ENSC201 Mesosphere and Thermosphere Combined = 0.1% of total mass The mesosphere • Decreasing temperatures with height • Coldest layer The thermosphere • Slowly merges into space • Increasing temperatures with height – “Temperature” = molecular kinetic energy ENSC201 ENSC201 Some Weather Basics Weather information is abundant Pressure and wind Wind = horizontal movement of air • Caused by unequal pressures ENSC201 Pressure units • Millibar (mb), Kilopascal (kPa) • Isobars Station Models • Portray weather information ENSC201 ENSC201 Temperature Most obvious weather component • Fronts Humidity May be expressed as relative humidity • Dew point temperature ENSC201 ENSC201 ENSC201
