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KFOX14 Meteorologist Brad Montgomery 

In looking up at the sky we often see what are called contrails. These are very straight or almost straight looking clouds that can be stretched across the sky for at times miles and miles. But what causes these clouds? In most cases the cause is commercial aircraft at high altitudes.

At high altitudes between 30 thousand up to about 45 thousand feet the air is very cold, as cold as -20 to -40 degrees Fahrenheit. At these temperatures the exhaust, which contains moisture, can freeze. When this freezes a cloud can form behind the plane as it travels through the sky.

These clouds are most common when there is already some sort of moisture in the atmosphere, usually in the form of high clouds such as you see in the picture. The higher moisture allows the condensation trails or contrails left behind the planes to last longer or even expand in coverage.

And with thousands of planes taking off and landing in the U.S. alone every day, they can make a lot of contrails with their exhaust. As a result contrails would be the majority of the manmade clouds you see in the sky.