AWH 6-2

Chapter 6, Water Vapor 6-2 6.2 The Hydrologic Cycle The hydrologic cycle (see Figure 6-1) involves the continuous circulation of water in the Earth-atmosphere system. Water vapor plays a critical role in the cycle. Figure 6-1. The Hydrologic Cycle 6.2.1 Evaporation Evaporation is the phase transition by which a liquid is changed to a vapor (gas). In meteorology, the substance meteorologists are concerned about the most is water, and the primary source is the ocean. On average, about 120 cm (47 in) is evaporated into the atmosphere from the ocean each year. For evaporation to take place, energy is required. The energy can come from any source: the Sun, the atmosphere, the Earth, or objects on the Earth, such as humans. Humans experience evaporation in their body. When the body heats up due to the air temperature or through exercise, the body sweats, secreting water onto the skin. The purpose is to cause the body to use its heat to evaporate the liquid, thereby removing heat and cooling the body. The same effect can be seen when stepping out of a shower or swimming pool. The coolness felt is from the removal of bodily heat used to evaporate the water on the skin. 6.2.2 Transpiration Transpiration is the evaporation of water from plants. In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant, only one percent is used in the growth process of the plant. The remaining 99 percent is passed into the atmosphere. 6.2.3 Sublimation Sublimation is the phase transition by which a solid is changed into vapor (a gas) without passing through the liquid phase. In the atmosphere, sublimation of water occurs when ice and snow (solids) change into water vapor (a gas).