HUMIDITY
ABSOLUTE HUMIDITY:
Absolute humidity (expressed as grams of water vapor per cubic meter
volume of air) is a measure of the actual amount of water vapor
(moisture) in the air, regardless of the air's temperature. The higher the
amount (weight) of water vapor per kilogram, the higher the absolute
humidity.
RELATIVE HUMIDITY:
Relative humidity (RH) (expressed as a percent) also measures water
vapor, but RELATIVE to the temperature of the air.
In other words, it is a measure of the actual amount of water vapor in
the air compared to the total amount of vapor that can exist in the air
at its current temperature.
WARM AIR CAN HOLD MORE WATER VAPOR THAN COLD AIR, so with
the same amount of absolute/specific humidity, cooler air will have a
HIGHER relative humidity, and warmer air a LOWER relative humidity.
Humidity
• Capacity of air is primarily a function of
temperature
• Relative Humidity (RH) =
(actual water vapor content)
(max. water vapor capacity of the air)
x 100
• Heated air becomes lower in RH because
denominator gets larger
• Cooled air becomes higher in RH
Saturation vs
Air Temperature
The actual amount of
Water air can hold changes
With air temperature
Air at 104 F can hold 3 times
As much water as 68 F air !
(47 grams vs only 15 grams)
Air at 68 F can hold 4 times
As much water as air at 0 F
(15 grams vs only 4 grams)
47 grams
15 grams
32 F
104 F
68 F
4 grams
Saturation and Dew Point
• Saturated v.
unsaturated air
• Dew-point
temperature
– temperature to which
air must be cooled to
reach saturation
(100% RH)
• water on outside of
drinking glass
• ice on your car
window
• dew and fog
After Saturation Occurs the Air
Must Release Extra Water as Fluid
Water forms on the outside of a cold glass as the cold
Air surrounding the glass chills the air to the Dew
Point Temperature
The resulting water
is not from the glass,
the water is from
condensation of
moisture in the air
around the glass
Relative Humidity and Temp.
RH fluctuates over a day or season.
Measuring Relative Humidity
Sling psychrometer
Hair hygrometer
Which environment has higher water
vapor content?