The Earth's atmosphere contains mostly nitrogen and oxygen.  About 78 percent is nitrogen, and about 21 percent is oxygen. It also contains small quantities of other gases, some of which are carbon dioxide, water vapor, ozone, helium and hydrogen.

Water vapor in the atmosphere makes up about 0.001 percent of the Earth's total water supply. Water vapor is not distributed evenly throughout the Earth's atmosphere. In some places, up to 5% of the atmosphere can consist of water vapor. However, there are areas on Earth where there is hardly any water vapor in the air at all.

A large amount of the water vapor on Earth forms when water evaporates from bodies of water. Moisture near or on the ground - for example, from dew or after a rainfall - can also evaporate and form water vapor. Plants and animals produce water when they metabolize nutrients. This water may also become vapor.

Water evaporates as a consequence of solar radiation.

The concentration of water in the air is known as the humidity. Because warm air can hold more water than cold air, humidity depends on temperature.

Often on weather reports, humidity is reported as percentage. This is the relative humidity - a comparison between the amount of vapor there is in a specific region of the atmosphere and the greatest amount of water vapor that the air can possibly hold.

Air that is completely saturated with water has a relative humidity of 100 percent and is said to be at dew point.  If the air becomes any cooler, the water vapor will condense.

Absolute humidity, which is often measured in grams per cubic meter, is a measure of the mass of water vapor that a specific volume of air contains.

When water vapor condenses, it releases heat. The movement of water vapor through the atmosphere is one of the ways in which heat is redistributed between the hot tropics, where evaporation is greatest, and the cooler, temperate regions to the north and south, where condensation may occur.

In addition to gases, the atmosphere is a full of inanimate matter such as dust, soot, volcanic ash, salt crystals from sea spray and fine sand grains raised by storms in the deserts.

Two to three thousand tons of cosmic dust falls from space and into the Earth's atmosphere each day.

The estimated weight of the atmosphere is about five quadrillion metric tons.

About half of this weight is within 5 kilometers of the Earth's surface. This is because the atmosphere becomes less dense as altitude increases, although its composition remains the same, except for water vapor and some impurities.

Changes in temperature also cause changes in the density of the atmosphere.  Warm air is less dense than cold air, because heat from solar radiation causes warm air to expand. Therefore, warm air rises and cold air sinks. The rising and falling of air in the atmosphere affects the weather.

The Earth's atmosphere is full of life. An enormous number of plants and animals, including human beings, live in the densest bottom layer. It is a zone full of flowering, breathing and growing creatures.

There is also an abundance of life above this layer.

Up to a height of about six miles, the air is full of suspended bacteria, fungal spores and pollen grains.

Between 2500 and 4500 feet, a cube of air the size of a lump of sugar contains an average of about 70 microorganisms, including mold spores, yeast and bacteria.