The Importance of Water

Introduction
Water vapor is the main greenhouse gas. The reason why the doubling of carbon dioxide results in an estimated warming by 4 – 9°F is the amplifying power of water vapor. The carbon dioxide alone would only account for a modest fraction of this increase. Water vapor is a gas that is ubiquitous in the atmosphere, but greatly varies in concentration, unlike the other greenhouse gases. Water concentration depends on the temperature of the air (which sets the capacity of the air to hold water) and on the history of a given parcel of air (that is, whether it had a chance to take up water according to its capacity).

Water Vapor & Deserts
Dry air is not good at intercepting infrared radiation. For example, the air moving over coastal mountains into the rain shadow desert on the other side loses its water in the mountains and arrives dry in the desert. This dry air does little to keep heat radiation from escaping into space. Also, a lack of clouds over the desert means that the path to space is rather open. Thus, upon the setting of the Sun, the heat of the ground is rapidly dissipated and it gets chilly quickly. In a sense, the poetic notion that “the cold of space is invading the desert at night” is not so far from the truth.


The saturation of air with water vapor depends on both its temperature (on the x-axis) and water vapor pressure (on the y- axis). The water vapor pressure is a measure of the amount of vapor in the air. At higher temperatures, much more water has to be present in air before clouds can form.
Air coming down a mountainside into the desert heats up because it is being compressed. The lack of water vapor makes the descending air heat up a lot quicker than one might expect. Heating of falling dry air follows the rule of 1°C for every 100 m. For example, a vertical drop of air from 1500 meters high up in the mountains results in a roughly 27°F temperature rise. This is what makes deserts, like those east of San Diego, hot and dry.

The temperature of the air up in the mountains is lower than that at sea level because the air coming from the coast expands and cools on rising. For example, if air saturated with water vapor rises from near sea level at a temperature of 70°F and then climbs up on top of the mountains, it cools at the rate of about 0.6°C for every 100 m, because as the water condenses it heats the air. So, this rising air cools by about 16°F — much less than it warms on the other side, when falling into the desert. Thus, by the time it reaches the desert floor, its temperature is up to 81°F, according to this rough calculation. This is not exactly a nice cool sea breeze anymore. Also, the Sun can readily heat the dry air to well over 90°F. In this manner, the lack of water makes for large day-to-night contrasts in temperature, in the deserts of San Diego and elsewhere.

Water and Heat
On a global scale, falling winds are associated with the great desert belts of the Earth, centered between 25 and 30 degrees of latitude. These winds are dry, having been stripped of moisture in the updrafts of the tropics, where rain is the rule. Because of the great importance of water as a greenhouse gas, the availability of water and its temperature is crucial to the workings of climate in any given region. Also, great amounts of energy can be taken up (or released) by water when it changes “phase,” that is, when it changes its state between gas, fluid and solid. This property of water, and others, makes the hydrosphere a planetary “air conditioner” of great efficiency.

Even without change of phase, water can do amazing things with heat storage and transport. 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. By comparison to create a one degree centigrade increase in a rock takes 0.2 calories per gram and for petroleum requires 0.5 calories per gram.

But the change of water from one state to another is the process to focus on, when contemplating the heat budget of Earth. 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, and our body takes advantage of this fact when cooling itself by sweating. The heat expended in turning water into vapor is not lost, but is contained in the vapor as latent heat. When the vapor condenses, this heat is freed for warming the surrounding air (See glossary for more on “latent heat”).

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, tropics and polar areas. Besides moist winds, ocean currents transfer heat from the warm tropics to the cold polar regions. A familiar example is the relatively mild climate of Norway, which depends upon the warm waters brought north along the Norwegian coast through an extension of the Gulf Stream System. Excepting the poles, the most severe climates are in the interior of continents, far from the sea with its benign influence on seasonal contrast.