1. Atmosphere: Origin, Composition and Structure
Chapter 2
Atmosphere: Origin, Composition and Structure

2. Driving Question
What is the composition and structure of the atmosphere?

3. Key Terms
Atmosphere – a thin envelope of gases that encircles the earth and other particles (aerosols)
Weather – the state of the atmosphere at a specific place and time
Meteorology – the study of the atmosphere and the processes that cause weather
Climate – weather conditions at some place averaged over a specific time period
Climatology – the study of the climate

4. Various Models
Scientific Model – an approximate representation of a real system (Earth-Atmosphere System)
Conceptual
Graphical – weather map
Physical
Numerical – run by computers (used to predict weather)
All models simulate reality and are subject to error

5. Primeval Atmosphere Initially Hydrogen and Helium
Outgassing – release of gases from rock via volcanoes and meteorites
Added CO2, N2, water vapor, Ar, among others
Photosynthesis converts CO2 to O2
Presence of O2 allowed ozone layer to form
Atmospheres of Venus and Mars are predominately CO2

6. Modern Atmosphere
Homosphere – lowest 80km of the atmosphere where principal gas ratios remain relatively constant
Heterosphere – above 80km where concentrations of heavier gases decrease more rapidly

7. Important Gases Nitrogen (N2) ~ 78% Oxygen (O2) ~ 21% Argon (Ar) ~ 1%
Greenhouse Gases – trace amounts but important role on Earth
Water Vapor (H2O) ~ 0-4%
Carbon Dioxide (CO2)
Ozone (O3)
Methane (CH4)

8. Aerosols Tiny solid and liquid particles suspended in the atmosphere
Wind erosion of soil
Ocean spray
Forest fires
Volcanic eruptions
Agricultural and industrial activities

9. Pollution
Air Pollutant – gas or aerosol that at a certain concentration adversely affects organisms and the environment
Natural pollutant: carbon monoxide
Non-natural pollutant: benzene
Primary – harmful immediately upon emission into atmosphere (car exhaust)
Secondary – results of chemical reactions between primary pollutants (smog)

10. Gases in the atmosphere

11. Monitoring the Atmosphere
Surface Observations
First done in 1800’s by Army Medical Corps - now run by the National Weather Service (NWS)
Automated Surface Observing System (ASOS)
About 1700 across the US
Fully automated year round
NWS Cooperative Observer Network
About 8000 across the US
Monitored by volunteers with equipment supplied by NWS
Doppler Radar
About 113 operated by the NWS
Others operated by television stations, military, private companies

12. Monitoring the Atmosphere
Upper Air Observations
Radiosonde – measuring device carried through the atmosphere by a weather balloon
Transmit data immediately
Balloons are launched simultaneously at 0000 and 1200 UTC
Data transmitted (temperature, pressure, dewpoint, wind) are plotted on a chart creating a sounding
Dropwindsonde – same thing except dropped by an airplane instead of launched by a balloon
Satellites

14. The atmosphere is divided into several different concentric layers
The atmosphere is divided into several different concentric layers. This plot shows the variation in average air temperature with height.

15. Layers of the Atmosphere
Troposphere
Layer at the surface
Layer where weather occurs (except some high clouds)
About 6km high at the poles and 20km at the equator
Temperature generally decreases with height
Tropopause – boundary between troposphere and stratosphere (average height near 12km)

16. Layers of the Atmosphere
Stratosphere
About 40km thick (extends to an altitude near 50km)
Layer where airplanes fly (location of jet stream)
Layer where ozone is present
Temperature generally increases with height
Stratopause – boundary between stratosphere and mesosphere

17. Pressure decreases exponentially with height
Pressure decreases exponentially with height. This means that the majority of the molecules that make up the atmosphere are in the lower layers. About 50% of the atmosphere's mass is within 6km of the surface and 99.9% is within 50km.

18. Layers of the Atmosphere
Mesosphere
Base is about 50km above the surface
Temperature again decreases with height
Coldest layer of the atmosphere
Extends to the mesopause which is about 80km above the surface of the earth
Thermosphere
Extremely thin air (particles range from 1-10km apart)
Increasing temperature because first particles to receive sun’s energy
Estimated top is about 500km
Exosphere (outer space)

19. Ionosphere Base is about 70-80km above surface
Primarily located in the thermosphere
Home to a high concentration of ions and electrons
Location of the Aurora Borealis