Water and heat storage - (n.)

How does water react to heating or cooling? How much energy can it store? It turns out that water is much more efficient in storing heat than any other common substance on Earth. To increase the temperature of fluid water by one degree Centigrade takes one calorie of heat energy for each gram of water. To do the same for a rock, for comparison, takes 0.2 calories per gram, and for petroleum requires 0.5 calories per gram. To change a gram of ice into water (without increasing the temperature) takes 80 calories. Conversely, when water freezes, it releases this amount of heat into the environment. To change a gram of water into vapor takes 580 calories. This means that evaporation is the best cooling mechanism around! Our body takes advantage of this fact when cooling itself by sweating: the evaporation of the sweat extracts heat from the surroundings, lowering the temperature of the skin. The heat expended in the phase change of water to vapor is not lost, but is contained in the vapor in a latent form. When the vapor condenses, the heat is freed for warming the surrounding air. This release of heat during condensation is the ultimate driving force for hurricanes.

The phase changes of water combined with its unique heat-related properties are intimately involved in all aspects of climate and weather. Water transfers and stores heat on an immense scale, and thereby evens out the temperature differences between day and night, summer and winter, and tropics and polar areas. Consider the temperature differences between day and night: they are largest in the water-starved desert, whereas in areas where water is abundant evaporation during the day tends to lower the temperature, and condensation during the night (the familiar dew) tends to raise it. During the day, clouds protect the ground from the heat of the sun, and at night, from the black coldness of space. Seasonal differences are similarly tempered by the great heat reserves of water, hence the mildness of coastal climates. Likewise, geographic differences are moderated by the transfer of heat through water motion and water phase changes. Most of this transfer is by water vapor in the air. Warm winds blowing over warm ocean regions pick up the water, cooling the sea surface. In cold regions, the air becomes unable to hold the vapor. The vapor condenses and gives off its heat of evaporation. If it freezes to snow, it also gives off its heat of fusion, to the surrounding air. Thus, a blizzard brings warmth: a somewhat unexpected conclusion, perhaps, to those who have been surprised by a snowstorm during a hike in the mountains.