The USGS Water Science School
Cirrus clouds exist very high up in the atmosphere, and are, thus, made of ice crystals. But, people make their own high-altitude clouds all over the world every day. They are the "contrails", or condensation trails caused by airplanes.
If the exhaust from the airplane contains water vapor, and if the air is very cold (which it is at high altitudes), then the water vapor in the exhaust will condense out into what is essentially a cirrus cloud. As this picture shows, contrails start out as thin ribbons but with upper-atmosphere winds, they can dissipate and essentially form much larger sets of cirrus clouds.
Credit: Betsy Kellenberger