The Atmosphere

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• Weather (defn) - the state of the atmosphere at any given time; a short period of time

• Climate (defn) - a description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region; it also includes the extremes

• air temperature
• humidity
• cloud cover - type & amount
• precipitation - type & amount
• atmospheric pressure
• wind - speed and direction

• 100 pascals = 1 millibar (mb)

• 1 inch of mercury = 33.86 mb

[average atmospheric pressure at sea level = 1013.25 mb = 1 std. atmosphere = 29.92 in mercury (Hg) = 14.7 pounds/square inch (psi)]

• 1084 mb - highest ever recorded (Agata, Siberia Dec. 31, 1968)

• 1050 mb - a strong high

• 970 mb - a strong low

• 870 mb - lowest press. ever recorded (Typhoon, Pacific Ocean, Oct. 12, 1979)

Range of Sea-Level Pressure

• a quantity that tells us how hot or cold something is relative to some standard value

• a measure of the average kinetic energy of the air molecules

• a measure of the average speed of the atoms and molecules that comprise air (higher temps correspond to faster average speeds)

We have 3 temperature scales:

a) Degrees Fahrenheit:

    32 ⁰F = freezing point of water
    212 ⁰F = boiling pt. of water

b) Degrees Celsius (⁰C):

    Zero ⁰C = freezing point of water
    100 ⁰C = boiling point of water

c) Kelvin Scale (Absolute Scale)

    Zero K = coldest possible temp = no moleculecular motion
    273 K = freezing pt. of water = 0 ⁰C =32 ⁰F

    373 K = boiling pt of water = 100 ⁰Celsius = 212 ⁰F

** 1 degree of Celsius = 1 degree Kelvin * * (i.o.w., the size of a degree Celsius is equal to that of the size of one degree Kelvin)
 
 

⁰C = K - 273, K = ⁰C + 273

⁰C = 5/9(F - 32); F = 9/5C + 32

C. Evolution of Earth's Atmosphere

Earth's atmosphere has changed significantly since the Earth was first formed!

1) Primordial Atmosphere - derived from the solar nebula (existed short time frame)

2) Evolutionary Atmosphere - gases derrived from the earth's interior - outgassing; "purple sulfur bacteria" produced organic materials from inorganic
    elements (3.6 billion yrs ago)

3) Living Atmosphere - first photosynthesis by "blue-green algae" (3.3 billion yrs ago)

4) Modern Atmosphere - abundance of life as a consequence of high levels of oxygen

Defn - Atmosphere - a thin envelope of gases that surrounds the earth. It is held to the earth by the force of gravity, and it moves with the earth as the earth rotates.
 

• Total mass of the atmosphere = 5.1 EE 21 grams

• Total mass of the earth = 6.0 EE 27 grams

• Mass of the atmosphere ~ one millionth mass of the earth

(1) Composition of the Atmosphere

Natural Air = clean air + pollutants

Clean Air = dry air + water vapor
 

Dry Air (in % by volume):

Nitrogen (N2) 78.08%

Oxygen (O₂) 20.95%

Argon (Ar) 0.93%

Trace Gases:  He, Ne, Kr, Xe, Rn, H2
 
 

Water Vapor - a variable constituent of clean air

• ~ 4% in warm, moist tropical air

• ~ 0% in cold, dry,polar air

Variable constituents of dry air (trace amounts):

Ozone (O₃), carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx, ), carbon dioxide (CO₂)

Pressure (weight of the overlying atmosphere):

• pressure (force per unit area) of the atmosphere decreases w/height above the ground
• pressure is often used as a vertical coordinate in meteorological applications

Density - [mass per unit volume (m/v)] of the atmosphere also decreases with height above the ground.
 

Height Above the Ground     Fraction of the Atmosphere Below

5.5 km (3.4 mi)                                             1/2

11 km (6.8 mi)                                              3/4

16.5 km (10.2 mi)                                         9/10

32 km (19.9 mi)                                         99/100
 

Temperature - the atmos. can be classified by layers based on the average vertical temperature profile.
 
 

Troposphere:

The layer of the atmosphere nearest the earth to ~ 10 km) where temperature generally decreases with height.

• the location of most of the atmosphere's moisture and consequently weather.

• upper boundary- tropopause

The layer above the troposphere (~ 10-50 km) where temperature generally increases w/height

• very stable (warmer air over cooler air) > thus vertical motions are weak

• has a low moisture content > few clouds

• upper boundary is the stratopause

• most of the atmosphere's ozone (O₃) is found here (@ ~ 25 km)

• ozone in the stratosphere is destroyed when it absorbs ultraviolet radiation from the sun (O3 molecule becomes atomic (O) and molecular (O2) oxygen)

• some of the absorbed energy warms the stratosphere the ozone layer protects biological life below from dangerous UV radiation
• top of the stratosphere is the stratopause

• the layer above the stratosphere (~ 50-80 km) where temperature decreases w/height

• upper boundary is the mesopause

• the layer above the mesosphere (above ~ 80 km) where temperature increases w/height again

• O₂ (molecular oxygen) is photodissociated here

• O₂ absorbs UV radiation from the sun, and in the process, heating of the atmosphere occurs

• because of the low density of molecules and atoms, a small amount of UV energy absorption produces a large temperature increase

Vertical Structure By Composition:

"homosphere" (0-80 km)- a well mixed region where the composition is uniform (78% N2, 21% O2, etc.) due to turbulent mixing

"heterosphere"(80 km - gases are layered by atomic weight - less mixing here

Vertical Structure By Function:

"ionosphere" (mainly in the thermosphere) - an electrified region within the upper atmosphere where large concentrations of ions and free electrons exist - it filters harmful wavelengths of solar radiaton