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The colorful clouds near the top of this picture are Polar Stratospheric Clouds (PSCs). The PSCs are flying high above the darker cirrus clouds at the bottom of the picture. This photo was taken from a jet airplane.
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Image courtesy of NASA (Paul Newman, GSFC).

Polar Stratospheric Clouds

The stratosphere is a layer of Earth's atmosphere. We live in the troposphere, the layer below the stratosphere. There are many different kinds of clouds in the troposphere. However, there is only one kind of cloud in the stratosphere! Those clouds are called Polar Stratospheric Clouds (PSCs).

PSCs only form when it is really, really cold. They are mostly seen in the winter near the North or South Pole. They form way high up in the stratosphere where the temperature is colder than -78° C (-108° F). PSCs are about 15 to 25 km (about 9 to 16 miles) above the ground.

We live in the troposphere. There is lots of water vapor in the air in the troposphere. That's why we have lots of clouds here. The clouds are made of tiny water droplets or small ice crystals. The stratosphere is different. It is very dry. There are not many clouds there. Some PSC clouds are made of water ice crystals, like "normal" clouds. Other PSCs have nitric acid and sulfuric acid in them!

PSCs are usually very colorful. Ice crystals in the clouds break sunlight up into different colors. People often see PSCs around sunrise or sunset.

PSCs can cause trouble. They help create the ozone hole. The ozone layer protects us from ultraviolet radiation in sunlight.

PSCs may help us too. They might warn us about global warming. Global warming heats air near the ground in the troposphere. However, global warming cools down air in the stratosphere. Remember, PSCs need really cold temperatures. Maybe there will be more PSCs in the future if the stratosphere gets colder. Scientists are watching carefully to see.

Last modified February 27, 2009 by Randy Russell.

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