Jeffrey Hovis, a science and operations officer with the National Ocean and Atmospheric Administration's National Weather Service in Charleston, WV, explains.
Update: In the original version of this article, the author referred to air holding water. As several readers pointed out, this was a technically inaccurate simplification. Following is a more detailed explanation.
The air that we breathe is made up of numerous gases, including water vapor. The term humidity generally refers to the amount of this water vapor in the atmosphere. Each atmospheric gas has its own vapor pressure, a measure of the number of molecules present at a given temperature. The vapor pressure of water thus measures the amount of water vapor in the air. The saturation vapor pressure is the vapor pressure when liquid water begins to condense. Relative humidity is determined by using the actual vapor pressure divided by the saturation vapor pressure (see below).
Meteorologists also use dewpoint temperature as a measure of the amount of water vapor in the atmosphere. This is the temperature at which the atmosphere becomes saturated and dew starts to form. It is also defined as the temperature at which the vapor pressure equals the saturation vapor pressure. Thus, as the temperature approaches and equals the dewpoint temperature, the vapor pressure and the saturation vapor pressure become the same. When this occurs, dew starts to form.
This phenomenon is illustrated by a glass of cool iced tea on a warm muggy day. The air around the glass cools and water condenses on the outside of the glass. The temperature at which the water begins to condense is the dewpoint temperature.
Original Story: There are a few meanings of the term humidity. All deal with the amount of moisture in the air, but differ slightly.
Relative humidity, a term often used by the local TV and radio media, is a measure of the actual amount of moisture in the air compared to the total amount of moisture that the air can hold. Warm air can hold more water than cool air. But if the air (warm or cool) is holding half as much moisture as it can hold when saturated, the relative humidity is 50 percent.
Meteorologists, in contrast, use dewpoint temperature as a measure of the moisture content of air. This is the temperature below which the air can no longer hold the moisture in vapor form and liquid water or dew will form. This phenomenon is illustrated by a glass of cool iced tea on a warm muggy day. The air around the glass cools and causes water to form on the outside of the glass. The temperature at which the water forms is the dewpoint temperature.
Humidity is most often used to describe how a person feels, often in conjunction with heat. If it is hot and humid, a person will usually feel much more uncomfortable. The reason for this is that the body tries to cool itself through evaporation of moisture on the skin. But when the air is humid, evaporating the moisture becomes more difficult and the actual cooling effect is much smaller.
The reader also inquired: "If humidity is the moisture content of the air, why is the west coast less humid than the eastern seaboard, considering they are both close to large water sources?
One of the reasons the west coast is less humid than the eastern seaboard has to do with the temperature of the large water source located nearby. In the Atlantic and Pacific oceans, the currents in the northern hemisphere flow clockwise. In the Pacific Ocean, this flow results in cool, even cold, water flowing from the northern Pacific southward along the west coast. In contrast, the flow in the Atlantic Ocean results in warm water flowing north from the equator along the eastern seaboard. The water in the Gulf of Mexico also becomes quite warm, especially in the summer.
Winds along the west coast quite often blow west to east which brings cool air from the cool water onshore. On the other hand, southerly winds often prevail along the eastern seaboard and these winds act to bring the warm air from the Gulf of Mexico and Atlantic Ocean northward into the area. Because warm air can hold more moisture than cool air, it feels more humid and the dewpoint temperatures are frequently higher along the eastern seaboard than they are along the west coast in the summer months.