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# Comment: I've been watching aircraft

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### I've been watching aircraft

I've been watching aircraft since commercial flight was piston/propellor craft and I can assure you that contrails that persist or grow are nothing new.

Whether a contrail quickly disappears, hangs around for minutes, for hours, or grows into a solid overcast, is solely a matter of the relative humidity of the air the aircraft flew through.

In an undergraduate meteorology survey class I took, back in the late '60s, one of the weather phenomena discussed (which contributed to making forecasting a tough problem) was supersaturated air masses. They'd be clear as crystal until disturbed, then turn into a cloud, fog bank, or even a storm.

You had positive feedback both in triggering condensation and then in the condensation being more reflective of solar heat than the underlying ground, changing the thermal energy input. This leads to what later mathematical theory calls a "butterfly catastrophe", where a tiny input grows to an enormous change of outcome.

The poster-child example given was a small airport in Alaska: It often starts the day with crystal clear, supersaturated, calm air. This lasts until the first small plane lands, disturbing the air with the prop wash, airfoil turbulence and wingtip vortices, and exhaust heat, moisture, and minor dust from burned oil - effectively "seeding" the air mass. This causes the water to suddenly condense into a thick fog in the aircraft's wake. The fog quickly spreads out in all directions, becoming a thick fog bank completely blanketing the airport and the surrounding countryside for several hours, until solar heating "burned it off" by raising the air temperature above the dewpoint.

So you get to land no more than ONE plane in the morning on such days, after which the airport is shut for half a day. Further, the fog bank reflects the sunlight and delays the time when the air becomes stable and the airport can operate normally. So landing that one plane in the morning is frowned upon. Taking off in the morning also triggers the fog. So if you manage to get airborne (without socking in the airport just by taxing to the runway), you don't have a place to land until midafternoon.

That means: For each job "created or saved" about five were destroyed.