What relative humidity means

The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. To see how this works, let's use the chart below, which is adapted from Meteorology Today by C. Donald Ahrens, published by West Publishing.

Air temperature in degrees C   Water vapor air can hold
                               at this temperature.
30 degrees                     30 grams per cubic meter of air 
20 degrees                     17 grams per cubic meter of air 
10 degrees                      9 grams per cubic meter of air

These numbers, which apply to air at sea level pressure, are based on measurements over the years. They are basic physical facts.

Now, let's see how dew point and relative humidity work. Imagine, that at 3 p.m. you measure the air's temperature at 30 degrees and you measure its humidity at 9 grams per cubic meter of air. What would happen if this air cooled to 10 degrees with no water vapor being added or taken away? As it cools to 10 degrees the air becomes saturated; that is, it can't hold any more water vapor than 9 grams per cubic meter. Cool the air even a tiny bit more and its water vapor will begin condensing to form clouds, fog or dew - depending on whether the air is high above the ground, just above the ground, or right at the ground. Back at 3 p.m., when we made the measurements, we could say that the air's dew point is 10 degrees C. That is, if this particular air were cooled to 10 degrees at ground level, its humidity would begin condensing to form dew.

How about relative humidity? At 3 p.m. the air has 9 grams of water vapor per cubic meter of air. We divide 9 by 30 and multiply by 100 to get a relative humidity of 30% In other words, the air actually has 30% of the water vapor it could hold at its current temperature. Cool the air to 20 degrees. Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53% (rounded off). Finally, when the air cools to 10 degrees, we divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapor it can hold at its new temperature.