A large fraction of the net radiation at the bottom of the atmosphere (Section 2.8) is absorbed by the oceans which cover about 71 per cent of the Earth's surface, and that absorbed energy is mostly used in evaporating water. Some 6*1013 tonnes are evaporated from the land each year, but about six times as much from the oceans (Chapter 6). So evaporation is an important part of the movement of energy, the theme of this second part of the book.
Water can exist in any of three 'states'—either as an invisible gas (i.e. water vapour), a liquid (e.g. cloud droplets, raindrops, dew, groundwater, river, lake or sea) or as a solid, such as snow, frost, hail or ice. (Steam which you can see from a kettle is not water vapour but consists of liquid droplets formed from the vapour; it is a miniature cloud.) The difference between the three states lies in the tightness of packing the water molecules together; they can be fitted together either stacked closely into a regular structure as a solid, irregularly packed together as a liquid, or loose as independent vapour molecules.
Water is the only common substance found in all three states. It is unusual in other ways too. It has an exceptionally high specific heat, it expands on freezing, it is most dense at about 4 K above freezing point (which is extra-ordinary) and the amount of heat needed to vaporise liquid water is large. It takes considerable energy to heat a cupful of water from freezing point to boiling point, but six times as much energy to evaporate that amount of water. These distinctive properties arise from the particular arrangement of the two hydrogen atoms and one oxygen atom in each water molecule (Note 4.A).
With three possible states there are six possible transformations of a substance from one form to another (i.e. changes of state), shown in Figure 4.1. There may be a change from a gas to liquid (called condensation, from a liquid to solid (freezing), or from a solid to gas (sublimation). Also from solid to liquid (melting), and gas to solid. The last is also called sublimation (which can be confusing), or t
deposition in the case of clouds. The combination of simultaneous melting and sublimation from ice, such as occurs on a glacier, is sometimes called ablation. Finally, the change from liquid to gas is evaporation, the topic of the present chapter.
Any change of state involves the loosening and reforming of bonds between adjacent molecules, as the result of thermal vibrations. The energy involved is described in units of 'kilojoules', defined in Note 1.J. If about 2,830 kilojoules are added to a kilogram of ice at 0°C, the resulting molecular motion disrupts the bonds completely and water vapour is produced, i.e. sublimation occurs (Figure 4.1). Evaporation requires only about 2,460 kJ/kg, because a liquid has weaker bonds to break than a solid has. More precisely, 2,490 kJ/kg are needed for evaporation at 0°C, or 2,430 at 30°C.
Heat added for evaporation makes no difference to the water's temperature, because the energy is used in breaking bonds not in altering molecular velocities (Section 3.1). After evaporation, the vapour contains the energy absorbed during the change of state. It is contained as latent heat, meaning hidden heat, as first pointed out by Joseph Black (1728-99). Such heat cannot be felt, or measured with a thermometer.
The rate of latent-heat absorption at an evaporating surface is L.E (W/m2) where L (kJ/ kg) is the latent heat required to evaporate a kilogram of water, and E is the rate of evaporation in units of kilograms per square metre per second. Thus, breaking sufficient bonds to melt ice (at 0°C) requires about 340 kJ/kg (i.e. 2,830 -2,490), whilst freezing liberates an equal amount (Note 4.B). Likewise, the amount of heat taken in during evaporation exactly equals the amount released when the opposite process of condensation occurs. We shall see that such liberation of heat is important in the atmosphere (Section 4.7 and Chapter 7).
Transport of latent heat occurs when water evaporates in one place and condenses in another. For instance, consider Figure 4.2. Evaporation from the ocean incorporates latent heat into the moistened air, and that same heat is later released downwind if the vapour forms cloud (Chapter 8). This is a major factor in conveying heat toward the poles (Chapter 5).
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