Re: heat of fusion and the water cycle... how do they fit together?

This refers to the Latent Heat of Fusion
for water.  Latent Heat is the term used to identify the heat
energy stored and released by chemical bonding between
atoms and molecules upon a change of state of the matter
(in this case when water freezes or melts to go between liquid water
and solid ice, about 80 calories/gram).

In terms of temperature and energy, liquid water molecules
have a higher internal energy compared to their ice crystal counterparts.
When water freezes it gives up this energy upon changing state.
This heat energy is what we call latent heat.  It is why
farmers spray their crops with a film of water to protect their plants
from freezing in cold weather.  The freezing water gives up latent heat
and warms the plant.  The ice then acts as a thermal insulator
(can you say igloo) to protect the plant against the subzero
temperatures.

In summer, rising hot air causes water vapor to reach cooler temperatures
aloft, condensing into water droplets (clouds).  The release of
latent heat in this change of state helps power thunderstorms.