Water in the Atmosphere
Essential Points
1. Water is part of the atmosphere
2. Partial pressure, vapor pressure and
humidity
3. Condensation and Clouds
4. Why it Rains
5. 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 Pressure
Gas
Nitrogen
Molecular
Weight
28
Partial
Pressure
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 Water
0C
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 Water
0 km
5 km
10 km
15 km
20 km
25 km
30 km
100 C
83C
73 C
55 C
40 C
26 C
14 C
212 F
181 F
163 F
131 F
104 F
79 F
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 Points
1. Water is part of the atmosphere
2. Partial pressure, vapor pressure and
humidity
3. Condensation and Clouds
4. Why it Rains
5. Hail, Snow and other forms of water