Atmosphere composition

• upper atmosphere 80-480 km:
• stratified by density – lighter gases higher
• molecules have high energy, moving very rapidly = high “temperature”, but:
• very thin – less than 1/100,000 of total mass of atmosphere, so a thermometer would not measure high temp.
• lower atmosphere 0-80 km
• well-mixed, similar composition throughout, except:
• “ozone layer” 19-50 km = stratosphere
• less than 0.25 ppm (parts per million) ozone = O₃
• absorbs longer wavelength UV (ultraviolet radiation)
• chlorine from CFCs (chlorofluorocarbons, used as propellants, refrigerants, plastic foam mfg.) reacts w/ ozone, breaks down to O₂

Temperature structure

• stratosphere – temperature warms with altitude b/c of absorption of UV radiant energy
• troposphere
• temperature decreases with altitude
• thicker at equator than poles
• thickness changes with seasons, weather
• gaseous composition:
• nitrogen (N₂) 78%
• oxygen (O₂) 21%
• argon (Ar) 0.9%
• carbon dioxide (CO₂) 0.037% = 370 ppm (and rising!); pre-industrial CO₂ ~280 ppm
• normal lapse rate – average decrease in temp. w/ altitude = 6.4°C/1000 m
• temperature inversions
• when warm air overlies colder air near surface
• prevents rise of warmed air and mixing
• traps pollutants

Water in the Atmosphere

Dew-point temperature = temperature at which given air mass will become saturated, water condenses as tiny droplets, forms clouds

Polar H₂O molecule - small positive and negative electrostatic charges cause water molecules to be attracted to each other, therefore:

• water forms droplets and raindrops
• water requires energy input to cause change from liquid to gas (evaporation) to break molecules apart from each other
• energy is latent heat of vaporization
• latent heat is released when water changes back into liquid from gas

Humidity = measure of the water content of air

• Relative humidity = RH = % of saturation at a give temperature
• saturation = 100% relative humidity

Normal lapse rate = change in temperature w/ altitude, average observed in atmosphere globally & over time, dry and moist calm air = 6.4°C/1000 m

Environmental lapse rate = ELR = actual observed change in temperature w/ altitude at a given place and time

Adiabatic changes in temperature (without heat input or output) due to expansion of rising air (or compression of sinking air)

• Dry adiabatic rate when RH < 100% = 10°C/1000 m
• Moist adiabatic rate when RH = 100% = 6°C/1000 m
• An important result:
1. air rising up the coastal side of a mountain range cools, causing precipitation
2. moisture is removed from air as it rises
3. adiabatic cooling rate of moist air is less than warming rate of dry air descending continental interior side of range, therefore:
4. air on continental interior side of range usually much drier than on windward coastal side = rainshadow effect

Stability of airmasses

• Stable
• ELR less than adiabatic lapse rates (air relatively warm at altitude)
• rising air cools more than surrounding air, becomes denser than surrounding air, sinks back down
• Unstable
• ELR greater than adiabatic lapse rates (air relatively cold at altitude)
• rising air cools, but still stays warmer and less dense than surrounding air, and keeps rising (often to condensation and precipitation)
• Conditionally unstable
• air will become unstable and continue rising if dew-point temperature is reached and it becomes saturated (then moist adiabatic rate will apply)

Airmass stability diagram

· Temperatures in white on right side of diagrams show decrease of temperature with height above the surface – the environmental lapse rate

· Rising parcels of air cool as a function of the adiabatic lapse rate

· Text within boxes explains whether air parcel is warmer or cooler than surrounding air, so that parcel rises or sinks

Atmospheric pressure and winds

Mercury (Hg) barometer

· measures atmospheric pressure by equivalent force of column of mercury

· normal sea-level pressure = 29.92 inches Hg =1013.2 mb (millibars)

isobar = line of equal pressure

Forces controlling wind direction

• Pressure gradient force – wind flows from high to low pressure
• Coriolis force (click to view animation)
• results from Earth rotation
• deflection to right in N Hemisphere, left in S Hemisphere
• greater for faster winds
• greater near poles than equator
• Friction force - resistance of rough Earth surface slows wind to height of ~500 m above surface

Geostrophic winds (above influence of friction)

• Balance between pressure gradient and Coriolis forces
• Flow parallel to upper-level pressure isobars

Circulation around pressure centers

• High pressure
• denser, colder descending air
• clockwise circulation around center of high in N Hemisphere (b/c of Coriolis force)
• opposite circulation in S Hemisphere
• divergence at surface, convergence aloft
• Low pressure
• warmer, less dense, rising air
• counterclockwise circulation around center of low in N Hemisphere (b/c of Coriolis force)
• convergence at surface, divergence aloft

Atmospheric circulation

Hadley cells: Large-scale convection cells from equator to subtropics

• Equatorial low pressure - warmer, moist, rising air, heavy precip (rain forests such as Amazon, Congo)
• Subtropical high pressure - air moving N and S from tropics cools becomes denser, descends, warms and dries – forms subtropical deserts (Sahara); horse latitudes with lesser winds
• Surface flow toward equatorial low pressure from north, south deflected to west by Coriolis force - producing easterly trade winds (blow from east to west)
• Convergence at/near InterTropical Convergence Zone (ITCZ, near equator)

Midlatitude eastward-moving winds (westerlies)

• Result from northward-moving air deflected to right in N Hemisphere
• Polar front jet at northern boundary of westerlies (30-70°N)
• at boundary of cold polar air and warmer midlatitude airmasses
• acts as storm track for most of US
• height 25,000-35,000 feet, speed often 200 mph
• Rossby waves in jet stream
• may loop strongly north and south, or jet stream may be straighter
• Subtropical jet stream at southern extent of westerlies (20-50°N)

Polar front - very cold descending air, high pressure, weaker easterly winds

Polar vortex - ~Closed circulation around polar high pressure esp. in Antarctic winter