Humidity is the condition of the atmosphere in relation to the water vapor it contains. Humidity is a complicated subject to explain fully; hopefully the following notes will help you understand the subject for our purpose of Humidity 101.

At any one time, it is estimated that the Earth has about 3,100 cubic miles of water in the air, mostly as water vapor, but also as clouds or precipitation.

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

The warmer air is, the more water vapor it can retain. 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 retain at the temperature it happens to be when you measure it. To understand how this works, use this chart from Meteorology Today by C. Donald Ahrens. These numbers apply to air at sea level pressure.

How does dew-point and relative humidity work? Imagine that a 3pm 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 cannot retain any more water vapor than 9 grams per cubic meter. Cool the air 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 ground level. Back at 3pm, when we made our 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 3pm 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 retain 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 retain at this new temperature, and multiply by 100 to get a relative humidity of 53%. 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 retain at this new temperature.

Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity. 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 in the air it is falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.

Most of us who have not studied physics or chemistry find it hard to believe that humid air is lighter then dry air. How can the air become lighter if we add water vapor to it?

Scientists have known this fact for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics.

To understand why humid air is less dense than dry air, we need to turn to one of the laws of nature discovered by the Italian physicist Amadeo Avogadro in 1812. He found that a fixed volume of gas, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container.

Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have an atomic weight of 28. Another 21% of the air is oxygen, with each molecule having an atomic weight of 32. The remaining 1% is a mixture of other gases, which we won't worry about. Molecules are free to move in and out of our cubic foot of air. What Amadeo Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave - remember, the total number of molecules in our cubic foot of air stays the same. The water molecules that replace the nitrogen or oxygen have an atomic weight of 18. This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.

Wait a minute, you might say, "I know water is heavier that air." True, liquid water is heavier, or more dense, than air. However, the water that makes the air humid is not liquid. It is water vapor, which is a gas that is lighter than nitrogen or oxygen.

Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.

Because of the temperature gradient, humidity decreases rapidly with altitude. The extremely low humidity at 12,000m is responsible for the uncomfortably low humidity in commercial aircraft. The ground level outdoor humidity is constantly changing. During the day, water vaporizes from forests, fields and lawns at about 1mm/day and about same from lakes, rivers and the ocean. As moist air rises to cooler air levels, clouds are formed. Also, there is always a humidity gradient between sunny and shadowy spots on the ground. Even a slight wind is effective in transferring moisture to a cooler spot where condensation can occur. Traces of atmospheric dust or other matter, such as the leaves of some plants, are capable of inducing from saturated air condensation that then drips to the ground.

During the daytime cycle, the water content of air increases while the sun shines. During the night the temperature drops and dew forms and recycles moisture to the soil. In coastal areas, the humidity can approach 100% at night on a regular basis.

Indoor climate differs in several fundamental ways from the outdoor climate because the air inside a building is confined in a comparatively small volume. In fact, it is often insufficient to maintain human moisture below a noticeable level. As a result, the quality of indoor air undergoes tremendous variations in a short period of time. This is reflected in indoor humidity trends. The moisture content of a closed room increases rapidly because each occupant continuously adds moisture to the air in the form of perspiration and with every breath day and night at a rate of several liters of water per day. This water readily condenses on cold walls and windows since there is no soil or surface to absorb it. Thus, if buildings are not carefully ventilated, the indoor habitat becomes saturated with moisture, causing not only air quality problems, but eventually structural damage.