The role of water in the energy budget of the earth-atmosphere system is profound.
The ability of water to change phase - from vapor to liquid to solid - gives it a unique
role in Nature, for water substance can absorb energy from one location, hold it internally as
"latent heat", and then release it at a totally different location. The hydrologic cycle is
therefore one of the most important energy transport processes for moving energy about in the
earth-atmosphere system in Nature's attempt to achieve energy equilibrium conditions.
The role of water in the development of intense atmospheric convection and storminess
is a good example of a positive feedback process in Nature. Water vapor condensing
to liquid droplets in a cloud releases latent heat to the surrounding air, thus increasing
the temperature of the air. When air warms, its molecules move further away from each other
leading to a general expansion of the volume of air. Increase of the volume of a fixed mass
of air molecules means that the air density decreases. The air is thus lighter (less heavy)
and is more buoyant owing to the release of latent heat. This increase in buoyancy then acts
to cause the air to rise in the atmosphere, thus enhancing convection. As the air in a
convective cloud rises, it sucks new air in behind it. As illustrated in the diagram above,
this essentially means that convection near Earth's surface must draw new air from surrounding
areas into the core of the convective storm. This fetching inward of fresh, humid air near Earth's
surface supplies a constant source of water vapor to continue the condensation process in the
core of the convective storm. Thus, as a positive feedback process, one sees that the condensation
of water vapor in a convective storms establishes a chain of events which ultimately draws
in fresh supplies of humid air to stimulate even more condensation of water vapor. A convective storm
may thus become self sustaining, drawing the energy for its growth and development from
the latent energy contained within the water vapor in humid air masses. The growth of self sustaining convective
storms can continue until the storm leeches most of the water vapor from the surrounding region.
Subsequent precipitation from the storm returns water in the liquid phase back to Earth's surface
so that the process can repeat another day.
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