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Lecture 21:ATMOSPHERE

Composition -- review, human impact
Pressure
Zones
Vertical motion
Global wind patterns -- causes

Powerpoint Lecture Slides


IMPORTANCE
O2 and CO2 for life processes
Greenhouse warming
Filters harmful radiation (ultraviolet)
Ocean-atm. interactions
Heat exchange
Winds drive ocean surface currents

COMPOSITION
Dry atmosphere
N2 78%
O2 21%
Ar 1%
CO2 0.036%
All other gases
(He, H2, etc)
< 0.002%
Variable constituents
Gases: ozone (O), CO, CH4, N-O gases, S-O gases
Aerosols: dust, pollen, water droplets
Water vapor: "Humidity" depends on T
Human modification
CO2 release -- fossil-fuel burning, deforestation
Enhanced greenhouse effect
Predicted global warming = 2-4°C
Ozone destruction in stratosphere by CFC compounds (e.g., freon)
Increased uv penetration -- skin diseases
Possible reduction in global photosynthesis

ATMOSPHERIC PRESSURE P = F/A
Weight of overlying atmosphere
"Hydrostatic" -- equal in all directions (like the oceans)
Pressure at sea level:
76 cm-Hg = 29.92 in-Hg
1013 millibars (1 bar = 106 dynes/cm2)
Variations in pressure
Decreases with altitude (~100 mb/km) -- atm. near surface is more dense, more compressed by gravity.
Surface pressure -- changes with vertical air movement
high pressure = descending air
low pressure = ascending air

ATMOSPHERIC ZONES
Troposphere -- 0-12 km
Heated at its base (surface) -- T decreases with altitude
Vertical motion -- convective, turbulent
Heat exchange with Earth surface
gain (warming) -- warm air rises
loss (cooling) -- cool air sinks
Water-vapor content (humidity)
humid air rises (low molecular wt. of H2O)
dry air sinks
Stratosphere -- 12-50 km
Stratified and stable -- little vertical mixing
"Lid" on the turbulent troposphere
Ozone absorbs uv radiation

GLOBAL WIND SYSTEM

Prevailing winds occur in latitude belts
Trade Winds: Low latitudes, E --> W
Westerlies: Mid latitudes, W --> E
Polar Easterlies: High latitudes, E --> W

What accounts for this pattern?
1) Latitude radiation imbalance
2) Earth's rotation -- Coriolis effect (next lecture)

Radiation imbalance
Equatorial zone (gain>losses): max. solar heating
and high evaporation --> rising air masses.
Polar zones (losses>gain): min. solar heating
and low evap. --> sinking air masses.
Net result:
Global convection cells
Winds are surface components of circulation


(Detailed Notes Start Here)

ATMOSPHERE

Composition -- review, human impact
Pressure
Zones
Vertical motion
Global wind patterns -- causes

The atmosphere is an essential component of Earth's dynamic system. For example, it is the souurce O2 and CO2 for life processes (these gases are continuously exchanged between atmosphere and oceans). Through the green-house effect, the atmosphere is an insuulating blanket that warms Earth's surface. It also filters out (absorbs) harmful ultraviolete (uv) radiation from the Sun. Finally, there are important physical interactions between atmosphere and oceans: (a) heat exchange, (b) global wind systems drive surface ocean currents.

Composition

The major components of the atmosphere were described previously -- nitrogen, oxygen, argon, and carbon dioxide comprise more than 99.998 % of the atmosphere. Variable constituents include other gases (e.g., ozone [O3], carbon monoxide, sulfur-oxide, nitrogen-oxide) from both natural processes and human activities; aerosols (e.g., dust, pollen, water droplets); and water vapor (absolute humidity of the atmosphere increases with temperature.

Industrialization over the past 150 years has altered the composition of the atmosphere in signficant ways.

(1) CO2 has increased due both to fossil-fuel burning and deforestation. Because CO2 is a greenhouse gas, there is valid concern that atmospheric temperatures will increase. Current models suggest an average global warming of 2-4 deg. C by the middle of the 21st century. (it should be noted, however, that the effect of increased CO2 on global climate remains a controversial issue.)

(2) Ozone in the stratosphere has been depleted by reactions with man-made gases called CFC's (freon is an example). The biggest effect of ozone destruction has been over Antarctica. If ozone depletion continues, uv radiation from the Sun will increase at the surface, leading to skin diseases in humans and probably other organisms. In addition, higher uv levels may lead to a reduction in photosynthesis rates on land and in the oceans, with potentially diasterous effects on the biosphere.

Atmospheric pressure

In physical terms, pressure is equal to force per unit area. Atmospheric pressure is due to the force (or weight) of the overlying atmosphere. In interesting property of atmospheric pressure is that is directed equally in all directions -- up and sideways as well as down. The same is true about pressure at depth in the oceans. This type of behavior is called "hydrostatic" pressure.

Atmospheric pressure can be expressed in a variety of ways. For example, pressure at sea level (where it is at its maximum) is 76 cm-Hg (=29.92 in-Hg); that is, atmospheric pressure will raise a column of mercury (Hg) to a height of 76 cm in an evacuated tube. Another unit for expressing pressure is the "bar" (1 bar = 10^6 dynes/cm^2). Atmospheric pressure at sea level is 1.013 bars, or 1013 millibars (mb).

Atmospheric pressure decreases with altitude (by about 100 mb/km). This occurs because the atmosphere near the surface is compressed by gravity and is therefore more dense -- the atmosphere becomes progressively "thinner" with altitude. Atmospheric pressure at Earth's surface also changes in response to vertical air movements. High-pressure zones occur under descending air; low-pressure zones under ascending air.

Zones of the atmosphere

Earth's atmosphere is a series of concentric layers, or zones. The structure of the atmosphere is defined based on how temperature varies with altitude. We will focus only on the two zones closest to Earth's surface.

Troposphere (0-12 km). Temperature in the troposphere decreases with altitude because it is heated at its base by interactions with Earth's surface (evap.-condens.; conduction; radiation, etc.). Where there is net warming of the atmosphere at the surface, warm air rises creating low-pressure zones. Where there is net cooling, cool air sinks creating high-pressure zones. The water-vapor content (humidity) is also important in vertical motions in the troposphere. Because of the relatively low molecular weight of H2O, humid air is less dense than dry air and will therefore tend to rise; dry air tends to sink.

Stratosphere (12-50 km). Temperature increases with altitude in this zone. Therefore, as the name implies, the stratosphere is a stratified and stable zone -- it does not mix vertically. The stratosphere acts as a "lid" on the turbulent troposphere. In addition, ozone is concentrated in the stratosphere and absorbs ultraviolet radiation.

Global Wind System

Prevailing winds occur in three latitude bands.

Trade winds are at low latitudes (0-30 deg. in both N.H. and S.H.). These are generally easterly winds, that is, they blow from the east to west.

Westerlies (west-to-east winds) occur at mid-latitudes in both hemispheres (30-60 deg.)

Polar easterlies occur at high latitudes (above 60 deg. in both hemispheres).

Two factors or processes account for the pattern of global wind systems.

Change in the radiation balance with latitude

Recall that low latitudes receive more solar radiation than they loose (by terrestrial infrared radiation back to space). At high latitudes, it is just the opposite. This latitude imbalance is the ultimate driving force for atmospheric circulation and winds.

In equatorial zones, maximum solar heating plus high evaporation lead to rising air masses.

At the poles, minimum solar heating and evaporation lead to sinking air masses.

The result is global convection cells, with winds as the surface component of that convective circulation. If the Earth did not rotate, the convection cells would cover an entire hemisphere (from Equator to pole), and winds would blow from the pole to the Equator. But the Earth does rotate, and that leads to the second factor. This is the Coriolis Effect (see next lecture).

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