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Interactives
WEATHER: What forces affect our weather?
Introduction
Atmosphere
Water Cycle
Powerful Storms
Ice and Snow
Forecasting
Changing Climate
Resources


If you live in the United States, there are 40 trillion gallons of water above your head on an average day. Each day, about four trillion gallons of this water fall to Earth as precipitation, such as rain, snow, or hail. Some of the water that falls to Earth soaks into the ground and provides runoff to rivers, lakes, and oceans. The remainder—more than 2.5 trillion gallons—returns to the atmosphere through evaporation, and the process begins again.

This continuous process of precipitation and evaporation is called the water cycle, or hydrologic cycle. It's been going on ever since oceans were formed on this planet 3.8 billion years ago. Earth was formed 4.6 billion years ago, but water was not present at the start. At some point, possibly because of the heating of hydrogen and oxygen as Earth developed, water vapor began to form in the atmosphere. Oceans formed and the cycle began.

Seventy percent of Earth is covered by water. In the atmosphere, rivers, oceans, groundwater, and elsewhere on Earth there are a total of 326 million cubic miles of water (more than 326,000,000 trillion gallons). Less than one percent of that water is present in rivers, lakes, and groundwater: we use these sources for our drinking water. Most of it—97 percent—is in the oceans. The oceans distribute heat around the planet, keeping heat and cold circulating by way of surface currents.

Clouds

Of the 326 million cubic miles of water on our planet, 3,100 cubic miles are found in the atmosphere. As water evaporates from the oceans, it enters the atmosphere and collects on small particles in the air as droplets or ice (a process called condensation) and forms clouds. When enough water or ice collects in a cloud, it rains. If the temperature is low enough, it snows.

There are many kinds of clouds. Each signals a different kind of weather. Cirrus clouds, for example, are high up in the troposphere. Winds in the upper troposphere make these clouds look wispy and thin. Though they are composed of ice, they are usually associated with pleasant weather. Stratus clouds, which form in lower parts of the troposphere, consist of water droplets and cover most of the sky with an even, gray color similar to a fog. These clouds (as well as some cumulus, nimbostratus, and other cloud types) can signal light rain. Cumulonimbus clouds are tall, dense clouds shaped like a block or anvil. They signal thunderstorms and also spawn tornadoes, as well as other violent weather effects such as hail and lightning.

How can ocean cycles affect the weather? Find out about warm and cool ocean currents in "El Niño: Warming up the Waters."

 "Weather" is inspired by programs from Planet Earth.

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