Geoscience Reference
In-Depth Information
2
Water Vapor in
the Atmosphere
Introduction
Hydrometeorologists are commonly concerned with quantifying the amount of
water in the atmosphere in vapor, liquid, and solid form and with seeking to
describe the way energy and water move vertically in the atmosphere toward and
away from the ground. In this chapter we consider the basic definitions and
important concepts needed for this.
Latent heat
The molecules that make up ice are held rigidly together in close proximity by
intermolecular forces. In liquid water the molecules are also close together but,
because they are at a higher temperature, they move around and their average
separation is therefore somewhat greater. In water vapor, molecular separation is
very much larger: molecules in water vapor are typically separated by about ten
molecular diameters. As water molecules move farther apart, the forces that bind
them reduce quickly with distance and at ten molecular diameters these forces are
much smaller than when the molecules are in near contact. Viewed in this way, the
transition from ice to liquid water and then to water vapor can be viewed as a
temperature related increase in the separation of molecules in the face of the
attractive intermolecular forces acting between them.
To move against a force requires work, and therefore energy has to be given to
separate water molecules to give changes in phase. The amount of energy needed
is directly related to the number of molecules present and therefore to the mass of
water that changes phase. The amount of energy needed for ice-to-liquid water
transition is called the
latent heat of fusion
and is 0.333 MJ kg
−1
. Because the change
