Terms and Definitions

Relative Humidity

The true definition of relative humidity (RH), is the ratio of the amount of water vapor actually in the air compared to the maximum amount of water vapor the air can hold at that particular temperature (and pressure). It is the ratio of the air's water vapor content to its capacity:

Relative Humidity = A / B

A = Amount of water vapour in the air

B = Amount of water vapor the air can hold

Relative humidity is given as a percent. So, air with a 50 percent relative humidity actually contains one-half the amount of water vapor it could hold. Air with 100 percent relative humidity is said to be saturated because it is filled to capacity with vapor. If we increase or decrease the amount of water vapor in the air, the relative humidity will change.

In many places, the air's total vapor content varies only slightly during an entire day, and so it is the changing air temperature that primarily regulates the full variation in relative humidity. As the air cools during the night, the relative humidity increases. Normally, the highest relative humidity occurs in the morning, during the coolest part of the day. As the air warms during the day, the relative humidity decreases, with the lowest values usually occurring during the warmest part of the afternoon. However, the relative humidity combined warm temperatures create the humiture or heat index which feels warmer than the actual temperature.

IP Definitions Specification for degrees of protection provided by enclosures (PDF, 102KB)

IP Ratings (PDF, 51KB)