Basic Properties of the
Atmosphere
Essential Points
1. Heat, Temperature and Temperature
Scales
2. The Electromagnetic Spectrum
3. Composition of the Atmosphere
4. Layers in the atmosphere are defined by
temperature profiles
5. How pressure varies in the atmosphere
6. Principal weather instruments
7. Earth’s radiation budget
Heat and Temperature
• Temperature: Average energy of
molecules or atoms in a material
• Heat: Total energy of molecules or atoms
in a material
• Can have large amount of heat but low
temperatures
• Can have high temperatures but little heat
1. Heat, Temperature and Temperature Scales
Heat and Temperature
• The Arctic Ocean has a large amount of
heat (because of large mass) even though
the temperature is low.
• Air in an oven at 500 F has high
temperature but little heat.
• However, touch anything solid in the oven,
and you’ll get burned. Same temperature,
much larger amount of heat.
1. Heat, Temperature and Temperature Scales
Heat and Temperature
• The earth’s outermost atmosphere is
extremely “hot” but its heat content is
negligible
• The surface of the moon can reach 250 F
in sunlight and -200 F in shadow, but the
vacuum around the Apollo astronauts
contained no heat.
• It takes time for things to warm up and
cool off.
1. Heat, Temperature and Temperature Scales
Temperature Scales
• Fahrenheit
– Water Freezes at 32 F
– Water Boils at 212 F
• Centigrade or Celsius
– Water Freezes at 0 C
– Water Boils at 100 C
• Two scales exactly equal at -40
1. Heat, Temperature and Temperature Scales
Converting C to F – In Your Head
• Double the Centigrade
• Subtract the first Digit
• Add 32
1. Heat, Temperature and Temperature Scales
Converting F to C – In Your Head
• Subtract 32
• Add the first Digit
• Divide by two
1. Heat, Temperature and Temperature Scales
Absolute Temperature
• Once atoms stop moving, that’s as cold as
it can get
• Absolute Zero = -273 C = -459 F
• Kelvin scale uses Celsius degrees and
starts at absolute zero
• Most formulas involving temperature use
the Kelvin Scale
1. Heat, Temperature and Temperature Scales
Electromagnetic Radiation
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Radio: cm to km wavelength
Microwaves: 0.1 mm to cm
Infrared:
0.001 to 0.1 mm
Visible light 0.0004 – 0.0007 mm
Ultraviolet 10-9 – 4 x 10-7 m
X-rays 10-13 – 10-9 m
Gamma Rays
10-15 –10-11 m
2. The Electromagnetic Spectrum
Composition of the Atmosphere
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Nitrogen
78.08%
Oxygen
20.95%
Argon
0.93% (9300 ppm)
Carbon Dioxide 0.035% (350 ppm)
Neon
18 ppm
Helium
5.2 ppm
Methane
1.4 ppm
Ozone
0.07 ppm
3. Composition of the Atmosphere
Other Components of the
Atmosphere
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Water Droplets
Ice Crystals
Sulfuric Acid Aerosols
Volcanic Ash
Windblown Dust
Sea Salt
Human Pollutants
3. Composition of the Atmosphere
Structure of the Atmosphere
• Defined by Temperature Profiles
• Troposphere
– Where Weather Happens
• Stratosphere
– Ozone Layer
• Mesosphere
• Thermosphere
– Ionosphere
4. Layers in the atmosphere are defined by temperature profiles
Troposphere
• Heating of the Surface creates warm air at
surface
• Warm air rises, but air expands as it rises
and cools as it expands (Adiabatic cooling)
• Heating + Adiabatic Cooling = Warm air at
surface, cooler air above
• Buoyancy = Cool air at surface, warmer air
above
• Two opposing tendencies = constant
turnover
4. Layers in the atmosphere are defined by temperature profiles
Stratosphere
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Altitude 11-50 km
Temperature increases with altitude
-60 C at base to 0 C at top
Reason: absorption of solar energy to
make ozone at upper levels (ozone layer)
• Ozone (O3) is effective at absorbing solar
ultraviolet radiation
4. Layers in the atmosphere are defined by temperature profiles
Mesosphere
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50 – 80 km altitude
Temperature decreases with altitude
0 C at base, -95 C at top
Top is coldest region of atmosphere
4. Layers in the atmosphere are defined by temperature profiles
Thermosphere
• 80 km and above
• Temperature increases with altitude as
atoms accelerated by solar radiation
• -95 C at base to 100 C at 120 km
• Heat content negligible
• Traces of atmosphere to 1000 km
• Formerly called Ionosphere
4. Layers in the atmosphere are defined by temperature profiles
Why is the Mesosphere so Cold?
• Stratosphere warmed because of ozone
layer
• Thermosphere warmed by atoms being
accelerated by sunlight
• Mesosphere is sandwiched between two
warmer layers
4. Layers in the atmosphere are defined by temperature profiles
Air Pressure
• By lucky coincidence, earth’s atmospheric
pressure is approximately neat round
numbers in metric terms
• 14.7 pounds per square inch (1 kg/cm2)
• Pressure of ten meters of water
• Approximately one bar or 100 kPa
• Weather reports use millibars (mb)
• One mb = pressure of one cm water
5. How pressure varies in the atmosphere
Pressure and Altitude
• Average at sea level 1013 mb
• 500 mb at 5 km (upper limit of permanent
human settlement)
• 280 mb at 10 km
• 56 mb at 20 km
• 1 mb at 50 km
• 0.00056 mb at 100 km
• Roughly drops by half each 5 km of
altitude
5. How pressure varies in the atmosphere
Pressure
and
Altitude
5. How pressure varies in the
atmosphere
Composition and Altitude
• Up to about 80 km, atmospheric
composition is uniform (troposphere,
stratosphere, mesosphere)
• This zone is called the homosphere
• Above 80 km light atoms rise
• This zone is sometimes called the
heterosphere
5. How pressure varies in the atmosphere
Mean Free Path
• Below 80 km, an atom accelerated by
solar radiation will very soon hit another
atom
• Energy gets evenly distributed
• Above 80 km atoms rarely hit other atoms
• Light atoms get accelerated more and fly
higher
• Few atoms escape entirely
5. How pressure varies in the atmosphere
Planets and Atmospheres
• At top of atmosphere, an atom behaves
like any ballistic object
• Velocity increases with temperature
• If velocity exceeds escape velocity, atom
or molecule escapes
• Earth escape velocity 11 km/sec.
• Moon escape velocity 2.4 km/sec
5. How pressure varies in the atmosphere
Atmospheric Measurements
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Temperature
Pressure
Humidity
Wind Velocity and Direction
6. Principal weather instruments
Weather Instruments
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Temperature:
Thermometer
Pressure:
Barometer
Humidity:
Hygrometer
Wind Velocity and Direction:
Anemometer and Wind Vane
6. Principal weather instruments
Thermometers
• Fluid
– Mercury
– Alcohol
– Use expansion of fluid
• Bimetallic
– Differential expansion of different metals
• Electronic
– Electrical resistance change with temperature
6. Principal weather instruments
Barometers
• Mercury
– Air pressure will support 10 meters of water
– Mercury is 13 times denser
– Air pressure will support 76 cm of mercury
• Aneroid
– Air pressure deforms an evacuated chamber
6. Principal weather instruments
Hygrometers
• Filament
– Hair expands and contracts with humidity
• Sling Psychrometer
– Measures cooling by evaporation
– Two thermometers
– Wet bulb and Dry bulb
• Electrical
– Chemicals change resistance as they absorb
moisture
6. Principal weather instruments
Sounding
• Balloons carry radiosondes
– Thermometer
– Barometer
– Hygrometer
– Transmitter
• Typically reach 30 km before balloon
breaks
6. Principal weather instruments
Radar
• Detect precipitation types and amounts
• Doppler radar measures velocity of winds
6. Principal weather instruments
Satellite Studies
• Visual imagery
• Infrared imagery
• Laser spectroscopy
6. Principal weather instruments
Earth’s Radiation Budget
• What comes in must go out
• Direct Reflectance (Short Wave)
– 31%
• Infrared Re-emission (Long Wave)
– 69%
7. Earth’s radiation budget
How Heat Moves
• Radiation
• Conduction
• Convection
7. Earth’s radiation budget
Albedo
Albedo = % incident energy reflected by a
body
• Fresh snow: 75 – 95%
• Old snow: 40 – 60%
• Desert:
25 – 30%
• Deciduous forest, grassland: 15 – 20%
• Conifer forest:
5 – 15%
• Ocean:
3 – 5%
• Camera light meters set to 18%
7. Earth’s radiation budget
Global Albedo
7. Earth’s radiation budget