An Introduction to Humidity

The U.S. Geological Survey estimates the Earth to have approximately 3,100 cubic miles of water in the air, mostly as water vapor, but also as clouds or precipitation at any one time.

Humidity is the condition of the atmosphere in relation to the water vapor it contains and is a fairly complicated subject to fully deal with, however a few brief notes will help you to understand enough of the subject for this particular purpose.

Water vapor is always present in the air in varying amounts, the amount that the air can hold depends on the temperature of the air. The higher the temperature the more water vapor the air can hold. Dew-Point is the temperature at which air containing a certain amount of water vapor becomes saturated; any further reduction in temperature would result in condensation.

What relative humidity means

The warmer air is, the more water vapor it can “hold.” Dew-Point is the measure of the amount of water vapor that is actually in the air. Relative humidity is the 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 the 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 to be 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 the air 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 just 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 was 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 can 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 can hold at the 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 as much vapor as it can hold at its new temperature.

Why humidity can be less than 100% When Raining

Humidity is a measure of the amount of water vapor in the air and not the total amount of vapor and liquid. For clouds to form and rain to start, the air has to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it’s falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.