The chemical formula for water vapor is H2O. Water is a unique compound because it has the ability to exist on the earth's surface in all three forms: liquid, solid (ice), and gas (water vapor).
Water vapor is the gas phase of water, and is invisible. The measure of the amount of water vapor suspended in the air is what is known as humidity. When water vapor condenses, it forms droplets, which turn into fog and clouds. If the droplets become large enough, they will fall to the earth as rain. At temperatures less than 32 degrees Fahrenheit, these droplets turn into sleet, ice, hail or snow.
Water vapor accounts for approximately 90 percent of the Earth's greenhouse effect. It keeps the temperature warm enough to support life.