Homepage > The Atmosphere > Water in the Atmosphere

Water in the Atmosphere

As the sun heats the land and oceans, the amount of water in the atmosphere changes. Water is always moving between the atmosphere and Earth's surface. The movement of water between the atmosphere and Earth's surface is called the water cycle. Click on the link below to view an animated diagram of the water cycle.

The WATER CYCLE

Water vapor enters the air by evaporation from the oceans and other bodies of water. Evaporation is the process by which water molecules in liquid water escape into the air as water vapor. Water vapor is also added to the air by living things. Water enters the roots of plants, rises to the leaves, and is released as water vapor. As part of the water cycle, some of the water vapor in the atmosphere condenses to form clouds. Rain and snow fall from the clouds toward the surface. The water then runs off the surface or moves through the ground, back into the lakes, streams, and eventually the oceans.

Humidity

Humidity is a measure of the amount of water vapor in the air. Air's ability to hold water vapor depends on its temperature. Warm air can hold more water vapor than cool air.

Weather reports usually refer to the water vapor in the air as relative humidity. Relative humidity is the percentage of water vapor that is actually in the air compared to the maximum amount of water vapor the air can hold at a particular temperature.

Psychrometer

Relative humidity can be measured with an instrument called a psychrometer. A psychrometer has two thermometers, a wet-bulb thermometer and a dry-bulb thermometer. If the relative humidity is high, the water on the wet bulb evaporates slowly, and the wet-bulb temperature does not change much. If the relative humidity is low, the water on the wet bulb evaporates rapidly, and the wet-bulb temperature drops. The relative humidity can be found by comparing the temperatures of the wet-bulb and dry-bulb thermometers. Below is a picture of a sling psychrometer.

Clouds

Clouds form when water vapor in the air condenses to form liquid water or ice crystals. Molecules of water vapor in the air become liquid water in the process of condensation. Below you will see three main types of clouds based on their shape: cirrus, cumulus, and stratus. Clouds are further classified by their altitude. Drag your mouse over each picture to learn more about each type of cloud.

Cirrus Clouds

Cumulus Clouds

Stratus Clouds

Precipitation

Precipitation is any form of water that falls from clouds and reaches Earth's surface. Common types of precipitation include rain, sleet, freezing rain, snow, and hail.

Rain

Drops of water are called rain if they are at least 0.5 mm in diameter. Smaller drops are called drizzle, and even smaller drops are called mist.

Sleet

Sometimes raindrops fall through a layer of air that is below 0 degrees Celsius, the freezing point of water. As they fall, the raindrops freeze into solid particles of ice. Ice particles smaller than 5 mm in diameter are called sleet.

Freezing Rain

Sometimes raindrops falling through cold air near the ground do not freeze in the air. Instead, they freeze when they touch a cold surface. This kind of precipitation is called freezing rain.

Snow

Often water vapor in a cloud is converted directly into ice crystals called snowflakes. Snowflakes have an endless number of different shapes and patterns, all with six sides or branches. Snowflakes often join together into larger clumps of snow in which the six-sided crystals are hard to see.

Hail

Round pellets of ice larger than 5 mm in diameter are called hailstones. Hail forms only inside cumulonimbus clouds during thunderstorms. A hailstone starts as an ice pellet inside a cold region of a cloud. Strong updrafts carry the hailstone up through the cold region many times. Each time the hailstone goes through the cold region, a new layer of ice forms around it. Eventually the hailstone becomes heavy enough to fall to the ground.