Icing conditions

Ron Garret writes:

Except for boots, the safest policy is to just turn it all on if there's
any possibility of ice.

And how do I recognize a possibility of ice? Should I assume that any
time I enter a cloud? Is there a safe external temperature above
which ice is never an issue? Logically I'd assume that ice would form
at zero degrees Celsius, but I don't know what margin above that to
provide. At the same time, I don't want to wear things out (even
though they never wear out in the sim) by running them when there's no
real chance of ice.

If ice starts to collect on places like the wings, what's the first
thing I would notice in the way the aircraft behaves? I do have a
light that shines on the wing so I can look at it, but I need to know
what the symptoms are of ice build-up as well.

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