Basic Properties of the Atmosphere

Essential Points1. 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

• 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

• Ultraviolet10-9 – 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• 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

• 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

• 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

• 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

• Temperature

• Pressure

• Humidity

• Wind Velocity and Direction

6. Principal weather instruments

Weather Instruments

• 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

AlbedoAlbedo = % 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