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Absolute Humidity
Absolute Humidity
The actual amount of water vapor that is in the air.
Variables that represent absolute humidity:
Dewpoint temperature (Td)
Vapor pressure (e)
As water vapor increases, Td and e will also increase. They are directly related.
Dewpoint temperature (Td): The temperature to which the air would have to be cooled, without changing the pressure or moisture content, for saturation to occur.
A low Td indicates a small amount of water vapor in the air (left parcel).
A high Td indicates a large amount of water vapor in the air (right parcel).
Td will almost never be higher than T!
Relative Humidity (RH)
Relative Humidity (RH)
The percent ratio of the amount of water vapor actually in the air (e) to the amount of water vapor that would be in the air, at a given temperature, if the air was saturated (es).
RH and es are inversely related. If water vapor is held constant:
RH will decrease as es increases.
RH will increase as es decreases.
RH and water vapor are directly related. If temperature is held constant:
RH will increase as e increases.
RH will decrease as e decreases.
Saturation
Saturation
Saturation is achieved when RH = 100%. This is also true when:
Td = T
e = es
Dew Point Depression (Tdd) = T – Td = 0
If we achieve saturation, condensation occurs!
Ex: Water forming on the side of a glass. This is what we’ll look at in experiment #1 today!
Ways to Achieve Saturation (RH=100%)
Ways to Achieve Saturation (RH=100%)
Decrease temperature without changing water vapor.
Example:
Td is held constant at 20°C.
Temperature decreases until Td = T.
Result: RH=100%
2. Add water vapor without changing temperature.
Example:
T is held constant at 20°C.
Td increases until Td = T.
Result: RH=100%
3. Add water vapor AND decrease the temperature.
Example:
T decreases from 30°C to 20°C.
Td increases from 10°C to 20°C.
Td = T, result: RH=100%
Heat Index
Heat Index
An index that combines temperature and relative humidity to determine an apparent temperature
How hot one feels.
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