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A lingering concern If conditions are right, newly formed contrails will begin feeding off surrounding water vapor. Like vaporous cancers, they start growing and spreading. In time, they can expand horizontally to such an extent that they become indistinguishable from cirrus clouds, those thin, diaphanous sheets often seen way up high. These artificial cirrus clouds can last for many hours, and the amount of sky they end up covering can be astonishing: one study showed that contrails from just six aircraft expanded to shroud some 7,700 square miles.
Persistent (non-spreading) contrails look like long white lines that remain visible after the airplane has disappeared. This shows that the air where the airplane is flying is quite humid, and there is a large amount of water vapor available to form a contrail. Persistent contrails can be further divided into two classes: those that spread and those that don't. Persistent contrails look like long, narrow white pencil-lines across the sky. Persistent spreading contrails look like long, broad, fuzzy white lines. This is the type most likely to affect climate because they cover a larger area and last longer than short-lived or persistent contrails.
How Long do Contrails Last?
Q) How long do contrails last?
A) Anywhere from less than a second, up to several hours.
This is an oft-asked question. The answer is reasonably straightforward, but misunderstanding is common. To understand why a contrail can last as little as a fraction of a second, or as long as several hours, you need to understand what a contrail is, and how it forms.
Here’s another question, which has the same answer:
Q) How long does a snowflake last?
Why is this basically the same question? Because contrails are generally made of ice crystals. Jet exhaust contains a lot of water vapor (the chemical reaction actually produces more water than there was originally jet fuel), and when this gets shot out of the back of the engine at 2000MPH, it hits the frigid air (typically colder than -40 degrees), and the water vapor condenses and freezes, very quickly, into tiny ice crystals, just like snowflakes.
So why do these ice crystals sometimes stay around for a long time, and sometimes vanish in seconds? The temperature is well below freezing, so they can’t melt, can they? This is puzzling, because it involves something that most people know nothing about: “sublimation“.
Sublimation is when a substance (in this case, water), goes directly from being solid to being vapor (a gas), without actually melting into a liquid. It’s like evaporation, except instead of a liquid evaporating, it’s a solid (ice). If the air is dry (i.e. there is little water vapor in it), then the ice crystals will quickly sublimate into vapor, and the trail will vanish quickly.
However, if the air already has a lot of water vapor in it, then the ice will sublimate slower, and the trail will last longer.
If the air has so much water vapor in it already that it can’t hold any more (i.e. it’s “supersaturated”), then the ice crystals can’t sublimate, and so the contrail will stay around for a long time. The ice crystals might even attract water from the air, if there is enough, and the contrail will get thicker. Winds might make the contrail spread out to even cover the whole sky.
The above is a simplification, as other factors like temperature, pressure, and sunlight have an effect. But it explains the basic reasons why some trails last only a few seconds, and some can last for hours, and spread out to cover the sky.
Finally, there is one more way of asking the question:
Q) How long do clouds last?
This is the same question because contrails are clouds. Contrails are physically very similar to cirrus clouds (except they are long and thin), and so they act almost exactly the same. You see cirrus clouds that last for hours, so why not contrails?
...when your breath causes vapor, why under no condition does it persist, linger, and turn into a small cirrus cloud?
I grew up on AF Bases with planes flying in and out all the time, never seen a contrail last more than a minute.