Every time you take a breath, you are inhaling
Earth’s atmosphere. You cannot see, smell, or taste Earth’s atmosphere. It is
the air all around you. Other planets also have an atmosphere. An atmosphere is
a blanket of gases that wraps around a planet or any other object in space.
EARTH’S ATMOSPHERE IS AIR
Earth’s atmosphere is made up of a mix of gases called
air. Air contains more nitrogen than any other gas. Nitrogen makes up 78
percent of the air. Oxygen, the gas that is most important for keeping you
alive, makes up 21 percent. Earth is the only planet to have so much oxygen in
its atmosphere. Water vapor and other gases are also present in small amounts
in Earth’s atmosphere.
The pull of gravity holds the atmosphere in
place. Without gravity, the air in Earth’s atmosphere would float off into
space. Gravity is the force that also keeps you from floating away from Earth.
THE WEIGHT OF AIR
Air has weight. You cannot feel the weight of
air, but all the air in the atmosphere presses downward. This weight is called
atmospheric pressure. Atmospheric pressure depends on how much gas is in the
atmosphere. The higher you go, the less air there is and the lower the
atmospheric pressure gets. The atmosphere is heaviest and the atmospheric
pressure highest close to Earth.
Differences in air temperature close to Earth form
areas of high and low pressure. Warm air is light and rises upward. It makes
low-pressure areas. Cold air is heavy and sinks. It makes high-pressure areas.
WEATHER AND THE ATMOSPHERE
Air in the atmosphere is always moving. You can
feel air blowing on your face. You can see air scattering autumn leaves and
making tree branches sway. Moving air is called wind. The wind blows where
areas of high and low atmospheric pressure meet. As warm air rises, cold air
rushes in to take its place.
Big areas of high and low atmospheric pressure
cause storms. Thunderstorms often occur where big areas of high and low
pressure come together. Huge thunderclouds form in these places.
Water vapor in the atmosphere makes clouds.
Water vapor is a gas. As the gas cools, it turns to liquid water. The water
falls to Earth as rain or snow.
A LAYER CAKE OF AIR
Earth’s atmosphere extends about 6,000 miles (9,600
kilometers) above the surface of Earth, where we live. You can think of the
atmosphere as having several layers. Most of our weather comes from winds,
temperature changes, and water vapor in the layer nearest Earth’s surface. This
layer is called the troposphere. Most of the clouds you see in the sky are
floating in the troposphere.
The stratosphere is the layer above the
troposphere. Jet airplanes fly in the stratosphere because there are few clouds
up so high and the ride is usually less bumpy. Earth’s ozone layer is in the
stratosphere. The ozone layer absorbs, or soaks up, harmful rays from the Sun.
These harmful rays would probably destroy life if they reached Earth’s surface.
The atmosphere gets thinner and thinner in the next
layers up, the mesosphere and thermosphere. The top layer of Earth’s atmosphere
is the exosphere. The atmosphere ends here, about 6,000 miles (9,600
kilometers) above Earth’s surface. The thin air here gradually merges with
outer space.
ATMOSPHERES ON OTHER PLANETS
Any planet that has gas around it has an
atmosphere. Mercury, the planet closest to the Sun has almost no atmosphere.
Pluto, the planet farthest from the Sun, is so cold that sometimes its
atmosphere freezes. The gases in Pluto’s atmosphere turn to ice.
There are colorful bands of clouds in the
atmospheres of some planets. A gas called methane makes the atmospheres of
Neptune and Uranus look smooth and blue. Jupiter has a swirling storm in its
atmosphere called the Great Red Spot that may be like a giant hurricane.
