What is humidity speaks to explain humidity, what is absolute and relative humidity in air.
Explain humidity, absolute and relative.
Absolute humidity is the term used to describe how much water vapor is actually in the air. It is expressed as the weight of water vapor per unit of volume. The unit of volume is given in pounds or grains (1/7000th of a pound) of moisture per cubic foot of air.
Relative humidity is the ratio showing how much water vapor is in the air compared to how much water vapor the air could hold at the same temperature and barometric pressure. It is referred to as a percentage. Therefore if the air holds half as much moisture as it possibly could, the relative humidity is 50%. If the air holds all the moisture it can then the relative humidity is 100%.
Measuring humidity, absolute and relative.
To measure both absolute and relative humidity a psychrometer is used. Sling psychrometers are still used but battery fan powered psychrometers are now used which regulate the speed of air being sensed by both the dry bulb and wet bulb thermometers. A psychrometric chart or data prepared in the form of tables is used.
To explain humidity, one thermometer is the dry bulb indicator while the second which is equipped with a wick over the sensing portion of the thermostat is the wet bulb.
With the wet bulb the evaporation cools the bulb of the thermometer. The amount of water vapor in the air controls the amount of evaporation. The motor water vapor in the air the less will be evaporated from the wick. Reducing the rate or amount of evaporation from the wick means there will be less difference in temperature readings between dry bulb and the wet bulb thermometers. This indicates a higher relative humidity.
Example of Table of Relative Humidity Dry bulb 75 Wet bulb 59 RH 40% Dry bulb 75 Wet bulb 61 RH 45% Dry bulb 75 Wet bulb 62 RH 50% Dry bulb 75 Wet bulb 64 RH 55%
Dry bulb 83 Wet bulb 66 RH 40% Dry bulb 83 Wet bulb 68 RH 45% Dry bulb 83 Wet bulb 69 RH 50% Dry bulb 83 Wet bulb 71 RH 55%
There are complete tables available for all dry and wet bulb temperatures.
To summarize and explain humidity, if the dry bulb reading is 75F and the wet bulb reading is 65F the relative humidity is 60%.
The absolute humidity for the same conditions is 5.6 grains of moisture per cubic ft of air and the dew point is 59F. If both the dry and wet bulb read the same the relative humidity is 100%, the absolute humidity is 9.4 grains per cubic foot and the dew point is 75F.
Return from Explain Humidity to Homepage