Coupling to the Global Energy Cycle

From the fact that energy is required to cause liquid water to change to a vapor, it is readily apparent that any consideration of global changes in water substance will become entangled with considerations of the global energy budget (Figure 4) of the earth. For example, a major portion of the intense solar radiation received by the earth in the tropics is used to evaporate water, using about 2.4 x 10⁶ J for each kg of water evaporated. This kg of water (1 liter, or approximately 1 quart) may move toward the mid-latitudes and be re-condensed to liquid in a cloud over California, for instance. During the condensation process, the 2.4 x 10⁶ J/kg is released as sensible heat, so the net effect is that the water vapor has served as a vehicle for moving heat from the point of evaporation to the point of condensation. To provide an appreciation for the magnitude of energy carried as latent heat by this one kg of water, 2.4 x 10⁶ J of heat energy is equivalent to the kinetic energy of a 1,000 kg object moving at 71 m/s (e.g., a 2,200 lb automobile moving at 158 miles per hour!) or the potential energy that would have to be supplied to raise this 1,000 kg object a vertical distance of 24.5 m (e.g., raising an automobile from the ground to the top of an 8 story building!).

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