Do contrails last long enough to explain chemtrails?

Question

I often hear the claim that "chemtrails" have to exist, because normal contrails don't linger very long. Is there a definitive scientific explanation for why normal contrails sometimes do, in fact stick around and sometimes even seed very large clouds?

An ideal explanation would also clearly state why one wouldn't be able to identify the correct conditions from the ground.

Answer 1

The longevity of a contrail is dependent on the weather conditions which indeed might be difficult to spot from the ground. I don't know about "unable" though. As far as I know the 'controversy' about contrails has never had any substantial evidence.

As for the time a contrail is visible, the most described factor is Humidity¹:

When the ambient relative humidity is high, the resulting ice-crystal plume may last for several hours.

Here is an article explaining how the trails have been used by sailors for forecasting, and has a couple of links to other articles explaining the phenomenon.

^{(1) Encyclopedia Britannica}

Answer 2

The conditions required for contrail persistence are ice-supersaturation. This means that the air is cold and humid enough that the water vapor will directly "condense" out of the air in the form of ice (the process is called "deposition"). Ice supersaturation is around 60-70% of the normally reported relative humidity, which applies to water¹.

However, and this leads to the reason why you can't tell from the ground, deposition in most conditions only happens on to ice. i.e. it requires some kind of seed, or nuclei, for the ice to form on. And for ice, this requires a specific molecular framework.

Now actual condensation into liquid is less fussy about what types of nuclei are required. So once the relative humidity in the air goes over 100%, the water condenses out as liquid, and you'll get a cloud.

So you can have a cloudless sky that still has regions (usually in layers) that are ice-supersaturated, but not water supersaturated. When a plane flies through those regions, the water in the exhaust bumps up the humidity over 100%, liquid water droplets condense out. If it's below -40°C/-40°F (yes they are the same) then these droplets will freeze and become ice crystal seeds.

So a contrail starts as a bunch of seed crystals. If the air is ice-supersaturated, these crystals will grow (increasing in mass by a factor of 1,000 or more), and hence can persist for hours, and even spread out to cover the sky.

^{(1) [FAA, Contrail Microphysics, 2010.][1] http://www.faa.gov/about/office_org/headquarters_offices/apl/research/science_integrated_modeling/accri/media/Contrail%20Microphysics.pdf}