Composition and Structure of the Atmosphere

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1 Chapter 1 Lecture Understanding Weather and Climate Seventh Edition Composition and Structure of the Atmosphere Frode Stordal, University of Oslo Redina L. Herman Western Illinois University

3 Thickness of the Atmosphere Although the density of the atmosphere decreases rapidly with height, the top of the atmosphere is undefined. Overall, the majority of atmospheric mass is contained in a thin layer near the surface. The atmosphere still contains an impressive sum of mass.

4 Factors Affecting the Composition and Vertical Structure of the Atmosphere The atmosphere is a mixture of gases, suspended particles, and liquid/solid water. Gases and particles are readily exchanged between the Earth s surface and the atmosphere through physical (volcanic) and biological (photosynthesis, respiration) processes. These gases are categorized in two types: permanent and variable gases.

5 Composition of the Atmosphere The Permanent Gases

6 Composition of the Atmosphere Variable Gases

7 2016 Pearson Education, Inc.

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10 Composition of the Atmosphere Variable Gases: Water Vapor Water vapor is the most abundant variable gas, as it is added or removed from atmosphere through the hydrologic cycle. Concentrations exist from nearly 0 percent over desert and polar regions to nearly 4 percent near tropics. Water vapor is a contributor to Earth s energy balance and many important atmospheric processes.

11 Composition of the Atmosphere Variable Gases: Water Vapor Water vapor image showing broader distribution of moisture than the image of actual clouds (below).

12 Composition of the Atmosphere Variable Gases: Carbon Dioxide Carbon dioxide (CO 2 ) is a trace gas accounting for percent of the atmosphere s volume. CO 2 is important to Earth s energy balance. CO 2 is added through biologic respiration, volcanic activity, decay, and natural and human-related combustion. CO 2 is removed through photosynthesis, the process of plants converting light energy to chemical energy.

13 Composition of the Atmosphere Variable Gases: Carbon Dioxide The steady increase of atmospheric CO 2.

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16 Composition of the Atmosphere Variable Gases: Ozone Ozone is the tri-atomic form of oxygen and is essential to life on Earth. Ozone near the surface is a pollutant but in the stratosphere is an essential absorber of ultraviolet radiation. Chlorofluorocarbons (CFCs), specifically chlorine atoms, react with ozone in the stratosphere and destroy ozone. Ozone destruction peaks over the southern hemisphere and persists through spring.

17 Composition of the Atmosphere Variable Gases: Ozone Images showing reduction of ozone over Antarctica over time.

18 Composition of the Atmosphere Variable Gases: Methane Methane is a variable gas in small but recently increasing concentrations. Released to the atmosphere through fossil-fuel activities, livestock digestion, and agriculture cultivation (esp. rice). Methane works as a very effective absorber of terrestrial radiation and plays an active role in near-surface warming. Annual increases in atmospheric methane.

20 Composition of the Atmosphere Aerosols Aerosols are any solid and/or liquid particles, other than water, which exist in the atmosphere (synonymous with the term particulate). Aerosols are both natural (sea spray, dust, combustion) and human (combustion) produced products. Due to their small sizes, they can easily remain in suspension for longer periods. Aerosols contribute to precipitation processes as condensation nuclei.

21 The Atmosphere, Weather, and Climate The atmosphere is a mixture of gas molecules, small suspended particles of solid and liquid, and falling precipitation. Meteorology is the study of the atmosphere and the processes that cause what we refer to as weather. Weather refers to the short-term phenomena. Climate refers to the long-term patterns. Climatology is the long-term study of the atmosphere and the processes that cause what we refer to as climate.

Atmospheric Density and Pressure Density Density is mass (kg) per unit volume (m 3 ). Due to compressibility, nearsurface air is more dense than that above.

22 Atmospheric Density and Pressure Density Density is mass (kg) per unit volume (m 3 ). Due to compressibility, nearsurface air is more dense than that above. This may be expressed in terms of the mean free path, or average distance a molecule travels before colliding with another molecule. less overlying weight more overlying weight Due to compressibility, atmospheric mass gradually thins out with height.

23 Atmospheric Density and Pressure Pressure Pressure is a result of the atmosphere s mass. Pressure decreases vertically but not at a constant rate.

24 Layers of the Atmosphere Layering Based on Temperature Profiles Thermal Layers of the Atmosphere Four distinct layers of the atmosphere emerge from identifiable temperature characteristics with height.

25 Layers of the Atmosphere Troposphere The troposphere is the lowest layer, named so as this region promotes atmospheric overturning. Layer of virtually all weather processes. Warmed at the surface by solar radiation. Identified by a steady temperature decrease with height and as the thinnest layer (but contains 80 percent of the mass). Due to thermal expansion, the tropopause is roughly 16 km over the tropics but only 8 km at poles.

26 Layers of the Atmosphere Troposphere Most clouds exist in the troposphere. Occasionally, violent updrafts penetrate cloud tops into the stratosphere. The flattened top of this cumulonimbus cloud is in the stratosphere.

27 Layers of the Atmosphere Stratosphere The stratosphere is an area of little weather ( stratified ), and the temperature increases with height. Inversion caused by the absorption of ultraviolet radiation by ozone. Although the ozone layer exists through an altitude between 20 and 30 km (12 18 mi), actual concentration of ozone can be as low as 10 ppm.

28 Layers of the Atmosphere Mesosphere and Thermosphere Mesosphere is characterized by decreasing temperatures with height and is the coldest atmospheric layer. Thermosphere, the upper most layer, slowly merges with interplanetary space and is characterized by increasing temperatures with height. Temperatures approach 1500 o C, however, this only measures molecular kinetic energy as the sparse amount of mass produces actual heat content. Combining the two layers account for only 0.1 percent of total atmospheric mass.

Layers of the Atmosphere A Layer Based on Electrical Properties: The Ionosphere Located within the meso- and thermospheres, the ionosphere is supplied with ions, electrically charged particles.

29 Layers of the Atmosphere A Layer Based on Electrical Properties: The Ionosphere Located within the meso- and thermospheres, the ionosphere is supplied with ions, electrically charged particles. Interactions between the ionosphere and subatomic particles emitted from the Sun excite atmospheric gases causing the aurora borealis (northern lights) and the aurora australis (southern lights)

30 Origin and Evolution of the Atmosphere Earth s First Atmosphere The early atmosphere was likely composed of hydrogen and helium. Light gases either exited the atmosphere to space through high escape velocities or through collisions with large celestial bodies.

31 Origin and Evolution of the Atmosphere Earth s Second Atmosphere A secondary atmosphere formed through volcanic outgassing and by material, largely water, gained from comets. These cosmic snowballs might be the source of most of the world s water vapor.

32 Origin and Evolution of the Atmosphere Further Evolution of the Atmosphere Precipitation removed, and continues to remove, excess water vapor, while high concentrations of CO 2 were replaced by oxygen through photosynthesis and dissolution in water. Due to its stability, nitrogen concentrations slowly grew to present-day levels.

33 Some Weather Basics Atmospheric Pressure and Wind Gas molecules are constantly in motion. These molecules exert a pressure (force per unit area) when they strike a surface. Molecules move in all directions, so pressure is exerted in all directions. Air tends to: blow away from regions of high pressure to low pressure (wind). rise in areas of low surface pressure and sink in high pressure. rising motions favor cloud formation, while sinking promotes clear skies.

34 Some Weather Basics Mapping Air Pressure Pressure equals force/area SI Unit: 1Pa = 1 Nm -2 Atmospheric pressure is plotted on maps using lines, called isobars Pressure often measured using hektopascals 1 millibar (mb) = 1 hektopascal (hpa)

36 Some Weather Basics Temperature Temperature varies in time and location. Drastic temperature differences are usually caused by fronts. Cold fronts Warm fronts Occluded fronts Precipitation is common along fronts, so when a front approaches, it is not unusual to have rain or snow in addition to a change in temperature.

37 Some Weather Basics Humidity Humidity is the term used to describe the amount of water vapor present in the atmosphere. Relative humidity: the amount of water vapor present in relation to the maximum for a given temperature. It is reported in percentages. Dew point: the higher the dew point the greater amount of water vapor in the air.

38 A Brief History of Meteorology Meteorology is the science that studies the atmosphere. Weather forecasting is both an art and a science. Forecasters use computer modeling (science) and their own subjective opinion and knowledge (art). There is no precise routine by which a weather forecast is created.

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ATMOSPHERIC STRUCTURE. The vertical distribution of temperature, pressure,

ATMOSPHERIC STRUCTURE. The vertical distribution of temperature, pressure, density, and composition of the atmosphere constitutes atmospheric structure. These quantities also vary with season and location