Water vapor is the gaseous form of water that exists in the atmosphere along with nitrogen, oxygen and other minor gases such as argon and carbon dioxide.

The amount of water vapor varies considerably from time to time and from place to place. Most of the time it is the third most abundant gas behind nitrogen and oxygen, which together make up more than 99 percent of the dry atmosphere.

Humidity is the measure of the amount of water vapor mixed or dissolved in the atmosphere; the higher the humidity, the more water vapor.

There are several different but equivalent measurements of humidity. Specific humidity is the actual amount in a given mass of air. Values range from near 0 to more that 8 percent, although it is rarely more than half of that.

Relative humidity (RH) is the actual absolute humidity compared with the amount that air can dissolve at a given temperature. It is a more common measure since it is an indicator of how easily water will evaporate into the atmosphere.

At a temperature of 86 degrees Fahrenheit and 70 percent RH, water vapor comprises 2 percent of atmospheric gases, and in each breath of air there is 7 percent less oxygen than there is at 50 percent RH at the same temperature.

There is not only less oxygen at higher humidity, but the air is also less dense. Both factors make it harder to breathe when the humidity is high. The feeling that the air is heavier at higher RH is because each breath is less effective, not because the lungs must work harder.

Water vapor has other important and related roles in the atmosphere that make it one of the most important constituents. Because it varies so greatly, it is not usually included in the list of atmospheric gases, which is unintentionally misleading.

Water vapor transports moisture and latent heat within the lower atmosphere to cause weather. Latent heat accounts for nearly 90 percent of heat that is transferred from surface to atmosphere as water evaporates, is carried into the atmosphere and condenses as clouds.

Water vapor is a greenhouse gas that is more effective than both carbon dioxide and methane in absorbing long-wave infrared radiation emitted from Earth's surface. Its role is not restricted to absorbing and radiating energy, however. It also includes the formation of clouds and aerosols, both of which reflect sunlight and outgoing infrared (IR) radiation from Earth's surface.

Because it absorbs and emits IR radiation with a distinctive signature of wavelength bands, sensors aboard weather satellites can detect and measure water vapor in the lower atmosphere.

These water vapor maps are often more valuable in weather forecasting than the IR cloud images and Doppler images shown on television, because they show the potential for precipitation and air mass interaction.