In this topic you will learn about water in Earth's atmosphere.

Water in the air is called water vapor. Water vapor is water in the form of a gas. The amount of water vapor in the air is referred to as humidity. Droplets of water in clouds, fog, or rain are not considered humidity.

Water vapor enters the air mainly by evaporation. More than two-thirds of Earth is covered with water. The changing of liquid water from oceans, lakes, and rivers into a gas is called evaporation. Evaporation takes place when water molecules near the surface of the water acquire enough energy from the Sun to escape into the air in gaseous form. Plants also contain large amounts of water. Plants release water to the atmosphere in a process called transpiration. This is the second largest source of water in the atmosphere.

As Earth's water evaporates, water vapor in the atmosphere condenses. Evaporation and condensation are opposites, and they happen at the same time. Relative humidity is the comparison of condensation and evaporation. For example, 50% relative humidity means that the number of water molecules condensing each second is half of the number of water molecules evaporating each second. Condensation equals evaporation when the relative humidity is 100%. The amount of water vapor in a given volume of air at 100% humidity depends on the temperature. This is because the warmer the water is, the greater the evaporation. This means that the air must have a greater amount of water in it for condensation to equal evaporation.

Condensation is the changing of a gas to a liquid. In the atmosphere, condensation is usually the result of air being cooled. Air can be cooled when it is pushed upward over mountains by winds. When cooler and warmer air masses meet, the lighter, warmer air is pushed higher into the atmosphere where it cools. Air contacting warm surfaces on Earth gets warm. The warm air expands, rises, and cools.

Water vapor in the air can condense into droplets to form clouds. Clouds of tiny ice crystals will form if the temperature falls below the freezing point of water.

Sweating is one way our bodies release wastes. We don't always feel the sweat because we sweat gradually and it evaporates. As sweat evaporates, the water droplets absorb heat from the surface of the skin, which cools the skin. Sweat helps your body control surface temperature.