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


In the darkest regions of deep space, the temperature is a chilly -450° Fahrenheit. Closer to our Sun, temperatures reach thousands of degrees Fahrenheit. What makes Earth's climate so moderate? Separating Earth from the extreme and inhospitable climate of space is a 500-mile-thick cocoon of gases called the atmosphere.

All planets have an atmosphere, a layer of gases that surrounds them. The Sun's atmosphere is made up of hydrogen, while Earth's is made up primarily of nitrogen and oxygen. Carbon dioxide, ozone, and other gases are also present. These gases keep our planet warm and protect us from the direct effects of the Sun's radiation. Without this regulation, Earth could not sustain life.

The structure of the atmosphere

The atmosphere is made up of several layers: the troposphere, stratosphere, mesosphere, ionosphere, and exosphere. Closest to Earth is the troposphere. Most of the clouds you see in the sky are found in the troposphere, and this is the layer of the atmosphere we associate with weather. Extending up to 10 miles above Earth's surface, the troposphere contains a variety of gases: water vapor, carbon dioxide, methane, nitrous oxide, and others. These gases help retain heat, a portion of which is then radiated back to warm the surface of Earth.

Above the troposphere is the stratosphere, which includes the ozone layer. The stratosphere extends from about 10 to 30 miles above the surface of Earth. Ozone molecules, which are concentrated in this layer, absorb ultraviolet radiation from the Sun and protect us from its harmful effects.

Thirty to 50 miles above the surface is the mesosphere, the coldest part of the atmosphere. Above the mesosphere, in a layer called the ionosphere (also called the thermosphere), things start to heat up. Temperatures in the ionosphere, which extends about 50 to 180 miles from the surface of Earth, can reach up to several thousand degrees Fahrenheit. Beyond the ionosphere is the exosphere, which extends to roughly 500 miles above the surface of Earth. This is the outermost layer of the atmosphere, the transition zone into space.

The greenhouse effect

The gases in the atmosphere that help retain heat are called greenhouse gases. These gases, primarily carbon dioxide (CO2), absorb heat instead of allowing it to escape into space. This "greenhouse effect" makes the planet a hospitable place.

However, greenhouse gases can have negative effects, too. Human activity has increased the amount of CO2 in the atmosphere. Since the 1800s, industrialized societies have burned fossil fuels such as coal, oil, and natural gas; these processes all give off CO2. During the past 25 years, the amount of CO2 in the atmosphere has increased by about 8 percent. With more CO2 in the atmosphere, more heat is absorbed and retained, causing global temperatures to rise.

Some scientists project that by the next century, CO2 levels in the atmosphere could be twice what they are today, causing a global temperature increase of about 3 degrees. Three degrees may not seem like much, but even a few degrees can have serious consequences. Tropical diseases could increase, since mosquitoes and other disease-carrying insects thrive in a warmer climate. Sea levels could rise, and coastal cities such as New Orleans and Washington, D.C., could be battered by storm surges. Prosperous farmland could dry up and agricultural regions could shift, wreaking havoc on the global economy.

It is possible that the recent warming trend is due more to natural cycles of cooling and warming than to human activity. Global climate change occurs on a scale of tens or hundreds of thousands of years, but scientists have only begun to study these effects in the last 150 years. Still, most scientists agree that just as climate affects our lives, we can affect the climate. Just how much we can influence it remains to be seen.

Is our atmosphere threatened by human activity? Find out more in "A Hole in the Sky: Understanding Ozone Depletion."

 "Weather" is inspired by programs from Planet Earth.

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