Between 0 and 4 percent of the Earth's atmosphere is made up of water vapor. The amount of water vapor present in the atmosphere, also known as humidity, is the most variable characteristic of the atmosphere and is largely dependent on air temperature.Continue Reading
At 86 degrees Fahrenheit, a volume of air can hold up to 4 percent water vapor. At minus 40 F, air can hold no more than 0.2 percent water vapor.
Water vapor enters the atmosphere by way of evaporation from the Earth's surface. When a volume of air holds the maximum amount of water vapor possible for a given temperature, it is said to be saturated and has a relative humidity of 100 percent.Learn more about Atmosphere