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) x 100 (max. water vapor capacity of the air)

• Heated air becomes lower in RH because denominator gets larger

• Cooled air becomes higher in RH

Saturation vsAir Temperature

The actual amount ofWater air can hold changesWith air temperature

Air at 104 F can hold 3 timesAs much water as 68 F air !(47 grams vs only 15 grams)

Air at 68 F can hold 4 timesAs much water as air at 0 F(15 grams vs only 4 grams)

32 F

68 F

104 F

4 grams

15 grams

47 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 AirMust Release Extra Water as Fluid

Water forms on the outside of a cold glass as the coldAir 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?