Latent Heat
"When water vapor condenses, latent heat is released!">

We want to understand how the energy budget of the earth is balanced by the presence of water, or more specifically, how the hydrologic cycle is necessary to the maintenance of the earth's energy balance.

We know that radiation is the primary mechanism for the transferring energy between the atmosphere and the earth. Conduction plays a small role since conduction only operates effectively in a very thin layer of air near the earth called the molecular boundary layer. The air moves (or convects) and transfers the heat throughout the rest of the troposphere,. However, since the heat is already in the air, there is no further transfer from ground to air.

The result is excess heat in the earth itself. Fortunately, this build-up of energy does not go into raising the temperature. Instead it goes to evaporating water. As water evaporates, what was once liquid and convective motions (in this case, convection refers to vertical motions) air rises, and cools adiabatically. As air cools, the air will eventually reach saturation where further cooling will result in condensation of the water. Just as heat is needed to evaporate water, that heat is released to the atmosphere when water vapor condenses. This heat is known as latent heat since it was hidden within the water molecules. If clouds have enough vertical development, rain forms and the rain is brought to the surface by the earth's gravitational attraction. This final step is important since it brings the water back to the surface of the earth.

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