The bright, colorful clouds towards the top of this image are Polar Stratospheric Clouds (PSCs). Notice that the PSCs are far above the darker cirrus clouds at the bottom of the picture, which was taken from a jet aircraft. Click on image for full size (91 Kb) Image courtesy of NASA (Paul Newman, GSFC).

The stratosphere layer of Earth's atmosphere is very dry and nearly cloud-free. There is just one type of cloud found in the stratosphere - Polar Stratospheric Clouds (PSCs). Also called nacreous clouds, PSCs form in very cold temperatures at high altitudes over the polar regions in winter.

PSCs form in the stratosphere at altitudes between 15 and 25 km (about 50,000 to 80,000 feet), far above even the relatively high-flying cirrus clouds in the troposphere below. The temperature must be around -78° C (-108° F) for PSCs to form, so these clouds only occur in the chilly lower stratosphere... in winter... near the North and South Poles!

Clouds of many types are common in the moist troposphere, the lowest layer of Earth's atmosphere. Those clouds are made of frozen crystals or small droplets of water. By contrast, the stratosphere (which is directly above the troposphere) is very dry; clouds are rare there. There are two distinct types of Polar Stratospheric Clouds: Type I clouds are composed of nitric acid and water (and sometimes also sulfuric acid), while Type II clouds contain only water ice.

Because of their high altitudes, PSCs are often illuminated by sunlight long after sunset or well before sunrise, when other clouds are dark and shadowed. The droplets and crystals in PSCs diffract sunlight into rainbow hues, so these clouds are often seen as brightly-colored wisps during morning or evening twilight.

PSCs have a darker side, however. The clouds strongly promote various chemical reactions that destroy ozone in the stratosphere. PSCs play a major role in depleting the ozone layer, Earth's ultraviolet-blocking "sunscreen". An increase in occurrence of PSCs could also be an indicator of global warming. Warming of the troposphere (down low, where we live) is expected to be accompanied by cooling of the stratosphere. Colder conditions in the stratosphere could mean that the ultra-frigid temperatures needed for PSC formation may occur more often.

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. Click on image for full size (91 Kb) Image courtesy of NASA (Paul Newman, GSFC).

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 form at very high altitudes, between 15 and 25 km (about 50,000 to 80,000 feet). PSCs only form at very cold temperatures around -78° C (-108° F). Sometimes, in winter near the North or South Pole, temperatures in the lower stratosphere get that cold. That's when PSCs can form.

There is lots of water vapor in the air in the troposphere. That's why we have lots of clouds down here in the troposphere. The clouds are made of tiny water droplets or small water ice crystals. On the other hand, the stratosphere above us is very, very dry. Clouds are rare in the stratosphere. Some PSCs are made up of water ice crystals. Other PSCs have nitric acid and sulfuric acid in them!

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

Although they are beautiful, PSCs are also troublemakers. Earth's ozone layer helps protects us from ultraviolet radiation in sunlight. PSCs help out certain chemical reactions that destroy ozone. PSCs are part of the cause of the ozone hole.

PSCs may help warn us about global warming. Global warming is heating up the troposphere (down low, where we live), but it is cooling the stratosphere. That could mean that PSCs, which need really cold temperatures, could form more often. Scientists are watching carefully to see if there are more PSCs than there used to be.

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. Click on image for full size (91 Kb) Image courtesy of NASA (Paul Newman, GSFC).

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.