It form at very high altitudes where temperature are very cold.

Explanation:

Paul Shepson, professor of atmospheric chemistry at Purdue University's School of Science, explains.

In the earths atmosphere, pressure, which is related to the number of molecules per unit volume, decreases exponentially with altitude. Thus, if a parcel of air from the surface rises (because of wind flowing up the side of a mountain, for example), it undergoes an expansion, from higher to lower pressure. When you allow air to expand, it cools. This phenomenon is familiar to everyone--stick your finger on the valve of a car tire, and let some air escape. It is not cool inside the tire, but as the air comes out it expands and thus cools.