JimM wrote:

Header says it all really but what does the temperature have to drop to
before you get a 'frost'

Cheers
Jim

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Remove BRAIN before replying

Freezing point ;-))

Sorry - I guess the point is that you can get very strong temperature
gradients close to surfaces. The temperature at the surface will depend
on heat loss by radiation plus heat loss/gain by conduction from the
surrounding air. The steepness of the temperature gradient will be
influenced by many factors. For example:

Wind speed - moderate winds will often prevent frost by ensuring "warm"
air is always moving across the surface. Commercial fruit growers use
large "fans" to generate gentle breezes across orchards to prevent late
frosts damaging blossom.

Cloud cover - radiative cooling rates will depend on what the surface is
"looking" at, a clear "cold" sky or warm clouds.

Mist or fog - odd as it sounds, fine water droplets freezing in the air
release heat, preventing the temperature dropping too low. Again, this
is used to practical effect in California where commercial citrus
growers spray water into the air above the trees to prevent late frosts
damaging blossom.

The angle of the surface - on a night with a clear, dark sky, a vertical
surface (which doesn't "see" the sky) radiates slighty less heat than a
horizontal surface. This is why you will sometimes find ice on the car
windscreen/rear window but water on the side windows.

Of course, all this becomes incidental when general air temperatures
drop below freezing point.
--
Larry Stoter