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After a clear, cold night, fence posts and car tops are
decorated with ice crystals, and the grass is frosty.
Where did the water come from? How cold does it have to be for dew or
frost to form?
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There is water vapor in the air, even when it is very cold.
At night the ground and objects on it are cooled by radiation
(as already discussed). The grass cools most quickly,
because it is insulated from the ground. When the air touches
these cold objects, it is cooled further and may
go below the "dew point temperature" -- now there is more
water in the air than it can hold. So this water is
deposited on the cool object. If the temperature is
above freezing, we get dew; but if the temperature is
below freezing, the water is deposited directly on the
growing ice crystals without ever forming a liquid.
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