Everybody knows that water freezes at 32 deg F.
Then how come frost (which is just ice that has come from
the water vapor in the air) can sometimes be seen in the morning on nights when
the temperature never got below 32 deg F?
Similarly, backpackers and desert dwellers occasionally report a film of
ice on a puddle or bucket of water on not-quite-freezing nights.
The answer has to do with the nature of heat transfer. Heat can move by conduction (movement through
a solid, or a fluid at rest) by convection (movement between a solid and a
flowing fluid) or by radiation (direct exchange of energy via electromagnetic waves). The temperature of a surface depends on the
temperatures of the surroundings plus the effects of all three modes of heat
transfer.
