What does "supersaturation with respect to ice" mean?

Question

And why is this important in relation to the appearance and life-time of contrails?

We know what contrails are, right?

Answer 1

This is one of the very interesting fact about water:

In order to change from gas to liquid, or liquid to solid, it needs to dissipate energy in form of heat.

Reversly, when it goes from solid to liquid, or liquid to gas, it has to absorb energy in form of heat. The latter is the reason we perspire in order to cool down our skin.

When water vapour rises in the atmosphere and cools down, it has to give away heat to condense. The problem is: those molecules are so small that they have virtually no mass. Therefore they can't transfer that heat. If it then meets anything like a dust corn, it will become a water droplet. The same happens for water droplets becoming ice. In fact, it is said that water can be undercooled to minus 40 degrees Celsius before it freezes!

This answers an interesting question: When the temperature is below freezing point, what are clouds made of: ice or water? The answer is: Both. Some molecules had a chance to freeze and some didn't. It is when a water droplet meets an iced one that snow starts forming and it is that random process tha makes that no two snowcorns are alike.

What happens in contrails is that the water from the combustion cools down immediately to form droplets of ice or water that you see as a cloud. But the 'polluting' factor that is needed for water to condense is found exclusively in the combustion of the engine because at high altitude the air is much purer for other type of particles like industrial smoke, desert sand, etc.

Note that contrails can also be observed very briefly at e.g. the wingtips of an aircraft when near the ground. That is because the air is first compressed under the wing, then released at the wingtip and cooled down by adiabatic effect. The latter is what makes your refrigerator to work.

Even with my ultralight aircraft, I can sometimes see tiny trails of damp from the wing tips. But they are immediately gone as they mix with the rest of the air.

Answer 2

cannot add more solute when the temperture is cold.