17.5: Transfer and Storage of Heat by Water

Essential to Know

• Water has a high heat capacity. More heat is needed to raise the temperature of water by 1°C than is required to do the same for almost any other substance.
• Large amounts of heat can be absorbed and stored in the waters of the mixed layer (approximately the upper 100 m) of the oceans without causing major water temperature changes.
• Heat can be transported with water masses and released at another location by radiation, conduction, and evaporation. This is a mechanism by which heat is transported from the tropics to higher latitudes.
• Water has a high latent heat of fusion. More heat is needed to melt ice than is needed to melt almost any other substance.
• Ice in the polar regions acts as a thermostat because of water’s high latent heat of fusion. Large amounts of heat can be lost or gained seasonally by the conversion of ice to water, or vice versa, without changing the temperature of the ocean surface water.
• Water has a high latent heat of vaporization. More heat is needed to vaporize or evaporate water than is needed to vaporize any other substance.
• Water evaporated from the ocean surface contains large quantities of heat as latent heat of vaporization. This heat energy is transported with the air mass and then released to the atmosphere when the water vapor condenses. This is another mechanism by which heat is transported from the tropics to higher latitudes.

Understanding the Concept

Water has a very high heat capacity (Chap. 5, Fig. 5-10). This means that it takes much more heat energy to increase the temperature of water by 1°C than it does to cause the same temperature change in other substances, such as rocks or air. Consequently, in regions where solar radiation is intense, the surface layer of the oceans can absorb large quantities of heat while the water temperature undergoes little change.

Most of the solar heat absorbed by the oceans is initially absorbed in the upper few meters of water because solar energy is absorbed rapidly and does not penetrate far into the water column. The heat energy is distributed rapidly by turbulence throughout the ocean’s upper mixed layer, stirred primarily by winds (Chap. 8). Thus, solar heat is distributed and stored in large volumes of water. In contrast, solar heat reaching the land is transferred downward only slowly by conduction (a very inefficient heat transfer mechanism) and is mostly radiated back into the atmosphere and space at longer wavelengths.

Because large quantities of heat can be stored in the surface layers of the oceans, heat can be transported with ocean currents. As the heated water travels, the heat energy can be released to the atmosphere. Thus, the high heat capacity of water facilitates the transfer of heat from tropical regions to colder regions near the poles (Chap. 7). The mild climate of parts of Europe in comparison with the climate of eastern North America at similar latitudes is partially the result of this mechanism. Heat is transported from the tropics into European seas by the warm Gulf Stream water and then released to the atmosphere (Chap. 8).

Water also has a high latent heat of fusion and a high latent heat of vaporization. This means that a large amount of heat energy is needed to convert ice to water, or water to water vapor (Chap. 5, Fig. 5-10). Heat energy added to ice to form water, or to water to form water vapor, is released when water freezes, or water vapor condenses. Thus, heat can be stored and transferred from one location to another if the conversions between ice and water, or between water and water vapor, occur at different times or places. For example, heat from the ocean surface can be used to vaporize or evaporate water. Water vapor can be transported through the atmosphere for thousands of kilometers until it recondenses and releases its heat energy to the atmosphere. Also, icebergs can be transported to lower latitudes by currents, where they melt as they absorb heat from the sun or surrounding water.

The high latent heat of fusion is important in polar regions. If water had a lower latent heat of fusion, the extent of the Arctic Ocean ice and the Antarctic ice sheet would vary more between summer and winter than they do at present. The presence of ice moderates the climate in these regions. As heat is lost in winter and gained in summer, most of the loss or gain goes to the freezing or melting of ice, and the ocean water temperature remains at the freezing point. Thus, the heat stored in the water by the melting of ice in summer is returned to the atmosphere during the winter as the water refreezes. In this way, the polar ice acts virtually as a thermostat, preventing air temperatures from becoming much higher in summer and much lower in winter.

The high latent heat of vaporization is important globally because it allows large amounts of heat energy to be transferred from the oceans to the atmosphere. Once in the atmosphere, the heat is redistributed by atmospheric circulation. Water need not boil for water molecules to be transferred from the liquid to the vapor phase (Chap. 5). At any temperature, water molecules are transferred from ocean to atmosphere by evaporation at the sea surface, but the rate of evaporation generally increases with increasing water temperature. When a water molecule evaporates from the oceans, it carries with it its latent heat of vaporization, which is more than 1000 times the amount of heat needed to raise the temperature of the same amount of water vapor by 1°C. Once evaporated, the water carries its latent heat through the atmosphere until the air cools, causing water vapor to condense to liquid water (clouds and rain).

When water molecules condense, they release their latent heat of vaporization to the surrounding air molecules. Because the heat capacity of all atmospheric gases is less than that of water, one water molecule can release enough heat to significantly increase the temperature of the air around it. Thus, evaporation and condensation of small amounts of water can transfer large amounts of heat energy from the oceans to the atmosphere. This mechanism is responsible for transporting heat from the equator toward the poles, thus moderating climate differences that otherwise would be extreme with increasing latitude (Chap. 7). In addition, this mechanism is responsible for the more moderate climate of coastal areas than of inland areas of the same latitude. It is also the driving energy of air movements in the atmosphere, including storms and hurricanes (Chap. 7).