Why Don't All Planes Leave Smoke Trails?

Ever wonder why some planes leave trails of smoke and others don't? Is it just that certain commercial jetliners are trying out some geometric skywriting while others can't be bothered?

Actually the answer is much more complicated. First of all, those long trails you see sometimes coming off the back of planes, or from their wings, isn't smoke at all. It's more like what you see when you exhale on a cold winter day and see your breath.

Contrails, as the lines are called, are formed when the hot, humid air coming out of the jet exhaust mixes with low vapor pressure in cold temperatures. I'm talking cold like the kind of cold that usually exists 26,000 feet up or higher. This is completely different from the real smoke planes use when they sky-write.

As you've probably noticed, some contrails will remain in the air for quite some time, while others dissipate more quickly. It all depends on how humid the air is and how windy it is up there. If the atmosphere is near saturation, the contrail may hang around for hours. But if the atmosphere is dry, they usually only last a few minutes, spreading out, thinning, and then, poof! They're gone. Unless, of course, they're captured by a talented photographer...

Photo credits (all used under Creative Commons license.):

Photo 1, Photo 2, Photo 3, Photo 4, Photo 5, Photo 6