Humidity and Relative Humidity

1. U.S. CURRENT RELATIVE HUMIDITY
[CLICK ON MAP]

2. ABSOLUTE HUMIDITY:
• A measure of the actual amount of water vapor
(moisture) in the air.
• Expressed as grams of water vapor per cubic
meter volume of air
• The higher the amount (weight) of water vapor per
kilogram, the higher the absolute humidity.
RELATIVE HUMIDITY (RH):
• Measures (expressed as a percent) water vapor
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.

3. HUMIDITY
• Humidity depends on the
air temperature:
• Warmer Temperature –
• Less Dense: larger
capacity
• More room for water
vapor to fit into the air.
• Cooler Temperature –
• More Dense: smaller
capacity
• Less room for water
vapor to fit into the air.

4. RELATIVE HUMIDITY
• The percentage of water vapor that is actually in
the air compared to the amount of water vapor the
air can hold.
• Relative Humidity (RH) =
(actual water vapor content) x 100
(max. water vapor capacity of the air)
• Air can hold a max of 22g/ft3 at 25°C
• If that air contains 11g/ft3 than the R.H. = 50%

5. 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)
32 F
68 F
104 F
4 grams
15 grams
47 grams

6. DEW POINT
The Dew Point refers to the temperature at which air is completely
saturated with water vapor.
PSYCHROMETERS
• Tools used to measure relative humidity (RH).
• Two thermometers – Wet Bulb and Dry Bulb
• Dry Bulb refers to the actually air temperature.
• Wet Bulb refers to the dew point.

7. PSYCHROMETERS
• As the psychrometer swings, the moisture of the wet
bulb’s cloth evaporates into the air and has a
cooling effect.
• The difference between the dry and wet bulb can
be calculated and translated into the RH.

8. BRAIN POP - HUMIDITY

11. WHICH WEIGH’S MORE?
A. HUMID
B. DRY

13. RELATIVE HUMIDITY AND TEMP.
RH fluctuates over a day or season.