Lecture 8.Water vapor and humidity.
Ways to describe humidity. Absolute humidity. Specific humidity and mixing ratio. Actual and saturation vapor pressure. Relative humidity.
Reading: Ch. 4, 78–83.
Humidity
• The amount of
water vapor in
the air
Different ways to describe humidity
• Absolute humidity
• Specific humidity
• Mixing ratio
• Vapor pressure
• Relative humidity
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Absolute humidity
• The density of water vapor:
Specific humidity and mixing ratio
• Specific humidity =
Mass of water vapor
Mass of air
• Mixing ratio =
Mass of water vapor
Mass of dry air
Vapor pressure
• The part of the total atmospheric pressure
which is exerted by water vapor molecules
• Another way of stating this: the
partial pressure of water vapor
Example : If the parcel’s total pressure is
1000 mb, and the parcel consists
of 99% dry air and 1% water vapor,
the partial pressure of water vapor
will be 10 mb
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Evaporation, condensation, and saturation
More on Vapor Pressure
• Actual vapor
pressure :
• Saturation vapor pressure :
Describes the actual amount of water vapor in the air
Describes the amount of water vapor in the air when the air is saturated
A very important fact:
• Warm air has a greater capacity for water
vapor (and thus a higher saturation
pressure) than does cold air
Examples :
 “Sweating” on a pop can
 Eyeglasses fogging up
 “Seeing your breath” in
cold weather
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Relative humidity
•
Amount of water vapor in the air
Amount of water vapor the air can hold
•
Another way to express the same thing:
Actual vapor pressure
Saturation vapor pressure
Important facts about relative humidity
• It is dependent on, but does not describe
the actual amount of water vapor in the air
• It is dependent on, but does not describe
the amount of water vapor the air can hold
• It is always described as a percentage
• Relative humidity very strongly depends
on temperature (because saturation does!)
Cold polar air has a lower water vapor content, but higher relative humidity than desert air
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