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Water in the Atmosphere
Essential Points1. Water is part of the atmosphere2. Partial pressure, vapor pressure and
humidity3. Condensation and Clouds4. Why it Rains5. Hail, Snow and other forms of water
Water in the Atmosphere
• Water vapor makes up 0-4% of the atmosphere
• Most variable component• Residence time of a water molecule in the
atmosphere = 10 days
1. Water is part of the atmosphere
Partial Pressure
• The pressure that would be exerted by each gas in the atmosphere if all the other gases were removed
• Proportional to the number of molecules present
2. Partial pressure, vapor pressure and humidity
Partial PressureGas Molecular
WeightPartial
PressureNitrogen 28 78%
Oxygen 32 21%
Argon 40 0.9%
Water Vapor 18 0-4%
Carbon Dioxide
44 350 ppm
2. Partial pressure, vapor pressure and humidity
Vapor Pressure
2. Partial pressure, vapor pressure and humidity
Vapor Pressure• The pressure of gas necessary to keep a
liquid or solid from evaporating• The pressure of a substance in the
atmosphere in equilibrium with a solid or liquid
• If partial pressure > vapor pressure, material condenses
• If partial pressure < vapor pressure, material evaporates
2. Partial pressure, vapor pressure and humidity
Vapor Pressure of Water0 C 32 F 6 mb
10 C 50 F 12 mb
20 C 68 F 23 mb
30 C 86 F 42 mb
40 C 104 F 74 mb
50 C 122 F 123 mb
2. Partial pressure, vapor pressure and humidity
Vapor Pressure and Humidity• The higher the vapor pressure, the more
moisture the air can hold• Vapor pressure increases with temperature• If partial pressure (water vapor actually in
air) stays constant, then –– Humidity decreases as it gets hotter– Humidity increases as it gets cooler
• Cooler temperatures can actually be less pleasant
2. Partial pressure, vapor pressure and humidity
Boiling Water
• When the vapor pressure of water = atmospheric pressure, water boils
• Since air pressure drops with altitude, so does the boiling point of water
2. Partial pressure, vapor pressure and humidity
Boiling Point of Water0 km 100 C 212 F5 km 83C 181 F10 km 73 C 163 F15 km 55 C 131 F20 km 40 C 104 F25 km 26 C 79 F30 km 14 C 57 F
2. Partial pressure, vapor pressure and humidity
Partial Pressure and Vapor Pressure
• Partial pressure = Pressure water vapor actually exerts
• Vapor pressure = Pressure water vapor could exert if the atmosphere were saturated
• Most of the time the partial pressure is less than the vapor pressure
2. Partial pressure, vapor pressure and humidity
Adiabatic Expansion
• Adiabatic = No heat transfer• Air warms or cools solely by expansion or
compression• Why? Expansion does work, energy comes
from internal heat• Dry air: 10 C/1000m (5.5 F/1000 feet)• Moist air: 6 C/1000m (3.3 F/1000 ft)
3. Condensation and Clouds
Lapse Rate and Stability• If air cools faster with altitude than
10C/km, it is unstable• If air cools 6-10 C/km with altitude, it is
stable for dry air but not moist air• If air cools less than 6 C/km with altitude, it
is stable• If air is warmer at high altitude than at the
surface, it is inverted and extremely stable
3. Condensation and Clouds
“Inverted?”• If warm air rises, why is it an “inversion” to
have warm air above cooler air?• Because normally, heating at the surface
keeps air at the surface warmer• That’s why the troposphere is “tropo”• Warm air above is more stable, but also
less common
1. Water is part of the atmosphere
Cloud Formation• Most cloud droplets require nuclei to form• Most nuclei are hygroscopic (attract water)• It is much easier to form water droplets
than ice crystals
3. Condensation and Clouds
Why Don’t Clouds Fall Down?
It’s a matter of Terminal Velocity – the faster things fall, the more air resistance they meet
• Skydiver – 50 m/sec • Mouse – 5 - 10 m/sec• Cloud droplet – 1 cm/sec
– The tiniest air motions keep such small particles suspended
3. Condensation and Clouds
Orographic Effects
• Orographic Lifting– Clouds– Precipitation
• Rain Shadows• Katabatic Winds• Chinook or Foehn Winds
3. Condensation and Clouds
Orographic Effects
3. Condensation and Clouds
Rain Forest, Washington
3. Condensation and Clouds
Rain Shadow, Nevada
3. Condensation and Clouds
Why Does it Rain?• Cloud droplets are about 1/100 mm in
diameter• It takes over a million cloud droplets to
make one raindrop• Coalescence of cloud droplets? Too Slow
4. Why it Rains
The Bergeron Process• Molecules are less tightly bound in liquids
than in solids• Therefore vapor pressure of water is
greater than that of ice• When ice and water droplets are present,
water vapor can be saturated with respect to water droplets but supersaturated with respect to ice
• Therefore water can evaporate from droplets and freeze onto ice.
4. Why it Rains
Freezing Rain
4. Why it Rains
Hail• Often associated with violent storms• Typically 1 cm or less• Can be 10 cm – capable of causing major
damage• Stones typically show concentric structure• Probably form by repeated trips through
supercooled regions
5. Hail, Snow and other forms of water
5. Hail, Snow and other forms of water
5. Hail, Snow and other forms of water
5. Hail, Snow and other forms of water
1. Water is part of the atmosphere
1. Water is part of the atmosphere
1. Water is part of the atmosphere
Record Hailstone, SD 2010
1. Water is part of the atmosphere
Fog
• Radiation: Surface cools at night by radiation
• Advection: Warm air carried over cool surface
• Upslope: Small-scale orographic clouds• Steam: Warm water evaporates into cold
air
5. Hail, Snow and other forms of water
Advection Fog, Labrador
5. Hail, Snow and other forms of water
Upslope Fog
1. Water is part of the atmosphere
Steam Fog
1. Water is part of the atmosphere
Frost
5. Hail, Snow and other forms of water
Ground Frost
5. Hail, Snow and other forms of water
Ice Shove
5. Hail, Snow and other forms of water
“Candle Ice”
5. Hail, Snow and other forms of water
Pancake Ice
5. Hail, Snow and other forms of water
Essential Points1. Water is part of the atmosphere2. Partial pressure, vapor pressure and
humidity3. Condensation and Clouds4. Why it Rains5. Hail, Snow and other forms of water
