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Absorbed solar energy at the surface is partitioned into sensible and latent heat, i.e., it is utilized for warming the air, water and soils or for evaporating water or melting snow and ice where available. Desert areas lack soil moisture much of the year, so nearly all absorbed energy goes into sensible heating of soils and air.


Adding water vapor at lower levels has the same effect as heating at these levels. Air rich in water vapor is lighter or less dense than dry air. It may be a little hard to understand that when you add water vapor, the air actually becomes lighter. That is because a given volume of air contains a set number of molecules of the mixture of the various atmospheric constituents. Thus the lighter water (H2O) molecules displace heavier nitrogen (N2) or oxygen (O2) molecules resulting in a lighter mixture. Water vapor also contains the potential for latent heat release within the lower portion of the atmosphere as the air expands and cools during ascent.

Solar radiation in the form of heat is transmitted over 150 million km of empty space from our sun to earth to keep our planet a hospitable place fit for life. This energy, with the help of greenhouse gases, keeps temperatures at a comfortable level in most parts of the world and is the source of the general circulation of the atmosphere and oceans. One greenhouse gas, carbon dioxide (CO2), is present in only very small amounts, but is still of utmost importance. Not only does it help determine the temperature of the earth but it is also importance in the exchange of O2 and CO2 between plants and animals. Another greenhouse gas or substance of immense importance is water (H2O), which is present in its three basic states and is well distributed over much of the surface of the earth. Earth has thus often been referred to as the water planet. Many factors thus combine to give earth a pleasant climate where life has adjusted well to these environmental conditions.

Energy from the sun is mainly absorbed at the earth's surface, the so-called active surface. Temperatures here are typically the warmest, and are also the most variable, in both the atmosphere and the ocean. The temperature distribution in the ocean is simpler than in the atmosphere since water is an incompressible fluid and thus does not exhibit the large changes in temperature due to pressure changes that we find in air. Warm water typically is less dense and will be situated above cold water unless large salinity differences exist. Salinity differences makes it possible over some limited areas to have colder fresh water overlying warmer more saline water.

Over much of the earth's surface, however, we have the very stable conditions of warm less saline water overlying colder more saline water. This may particularly be the case where rainfall greatly exceeds evaporation, such as over much of the tropical western Pacific, or where fresh or brackish river water flows out over the highly saline sea waters, such as near the mouth of the Amazon River.

Within the Mediterranean Sea evaporation greatly exceeds precipitation and the salinity is higher than in the Atlantic in spite of large river inflows from the Nile and the Black Sea. This higher salinity water flows out into the Atlantic across the sill at Gibraltar and it can be traced for a considerable distance as it slides underneath the Atlantic water masses of same temperature. The hydrologic cycle in the atmosphere with its evaporation and precipitation and runoff from the continents thus affects ocean circulations in important ways.

Sunlight and heat penetrate only a very limited distance into water and keep the surface waters warm. Nearly all the heating takes place in the upper 10 meters or so of the ocean. Conduction takes care of some of this vertical heat transfer. Winds and waves mix the water in the boundary layer of the ocean, reducing the vertical temperature gradient within this mixed layer. An occasional storm or hurricane may supply added mixing. Depending on the time of the year, this may result in increased storage of warm water in a deeper layer than otherwise would be the case just like a circulation pump in a swimming pool will result in increased heat storage. This added heat storage by mixing by storms is best accomplished during the heating cycle of spring and summer. Heat stored in the oceans thus is a source of added evaporation and rainfall and kinetic energy in general.

During the diurnal and annual cooling cycles, water parcels cooled by radiational heat loss or evaporation or sensible heat loss will gently sink down and find the level of their new density and be replaced by water from below. Down deeper (and at variable depths seasonally and across the globe) we find the thermocline where temperatures drop off with depth rather rapidly. Below the thermocline, temperatures drop off at a slower rate again, and down still deeper the temperatures are rather uniform and close to freezing.

This cold, dark region contains the majority of the water in the oceans. Much of this deep water has its origin in the Southern Ocean near Antarctica and in the North Atlantic at high latitudes. Here the ocean is nearly isothermal and well mixed, and the normal vertical stratification is lacking. This lack of static stability means that vertical motions of water masses may occur with greater ease. The cold surface waters will be free to sink and spread equatorward, while warmer or less saline surface waters spread in laterally to replace the sinking water.

In the case of fresh water, we have the peculiarity that water has a maximum density at about 4 C. Thus water colder than 4 C (and ice, of course) is less dense and will stay at the surface during the autumn and early winter cooling cycle. The fact that ice floats on water is a peculiarity that is not shared with many substances in nature. Just imagine what earth would be like if the oceans filled up with ice from the bottom!

On the horizontal scale across the earth, sea surface temperatures vary from the freezing or melting point of salt water, near minus 2 degrees Celsius along the margins of the ice at high latitudes, to 30 C over a large region of the equatorial western Pacific and Indian Ocean. Sea surface temperatures are largely a function of the radiation balance. There is some modification of sea surface temperatures due to evaporative cooling and sensible heat transfers from water to air. These latter effects can be large in winter near the east coasts of continents. Along the east coasts of Asia and North America, the very cold air from the continent blows out over open water and is heated and moistened from below. The waters give up large amounts of sensible and latent heat to the atmosphere and are cooled in the process. Heavy snow squalls often occur and additional latent heat exchanges take place as heat is added to the air when snow flakes form, and taken away from the ocean, as the snow drops into the water and melts.

Water vapor content of air is an important variant that affects both the rate of evaporation and radiative heat losses. High dewpoint temperatures found over warm waters of the tropics help maintain the high sea surface temperatures by limiting both evaporation and the rate of outgoing radiative heat losses. The 30 C (86 F) sea surface temperatures seem to be an upper limit as to how warm the large scale deep ocean can get within the radiation climate here on earth. There are, however, some enclosed ocean areas with light winds and poor mixing in both the atmosphere and the ocean where we can develop a shallow warm surface layer where water temperatures may exceed 30 C by a degree or two. Such areas include the Red Sea, Persian Gulf, parts of the Gulf of Mexico and the Sea of Cortez (Gulf of California) during summer and early fall.

Over the open ocean, in the absence of a strong advection of cold or warm air, the air temperature is usually within a degree of the sea surface temperature with the relative humidity close to 85 to 90 percent. A typical report from the warm, near equatorial central and western Pacific would thus have an air temperature of 30 C (86 F) and a dewpoint temperature of 26 C (79 F). Evaporative cooling would be only moderately intense in this environment and excessive rainfall would likely mean slightly lower salinities in this region. Atmospheric pressure would also be relatively low, say at 1005 mb versus about 1012 mb in the eastern Pacific, as a heat induced trough of low pressure would be present. A trough of low pressure will generally result in light and variable winds with attendant high humidities in a deep layer of the atmosphere with no subsidence inversion. This is the conditions under which heavy convective cloud clusters may form and there is tendency for tropical storms to form within this region.

The strongest evaporation over the oceans occurs where winds blow from higher toward lower latitudes, such as to the east and equatorward of the subtropical high pressure centers. Rapid evaporation also occurs over the middle and higher latitudes in the fall and winter months when the vertical exchange of heat and moisture between ocean and atmosphere is most intense. That is, within the large cyclonic circulations that are the migratory winter storms, and also where the cold air flows off the continents out over the relatively warm ocean. The violent snow squalls of the Great Lakes of North America or over the Sea of Japan are manifestations of the very rapid evaporation and subsequent precipitation as cold air moves out over open water.

In contrast, evaporation over land areas is more closely tied to the solar radiation cycle, assuming soil and vegetation moisture is available. Even in desert areas with little or no soil moisture do we find that heat storage in soils and rocks helps bring about a minor time lag in the period of the most intense evaporation.

There are parts of the oceans where evaporation is quite low, such as within the southerly or southwesterly air flow to the west and northwest of subtropical high pressure cells of the northern hemisphere. Here air masses with dewpoint temperatures as high or higher than the sea surface temperature flow northward over progressively cooler waters. Under these conditions there is little or no evaporation and thus no evaporative cooling of the surface waters. There may even be a slight warming, latent as well as sensible heating, of the surface waters, as moist and warm air flows northward over progressively colder waters. Much of the latent heat of condensation will be released within the air mass itself as fog develops. Some condensation of moisture may however also take place onto the cold water surface causing warming.

Latent heat of condensation occurs profusely on ice and snow and is a very important factor in causing very rapid melting, such as of hail stones falling through humid air on the way to the ground. Very rapid melting with resultant flooding may also occur, for instance, when warm and moist air ahead of a cold front blows across snow cover, or over the ice fields and glaciers of Greenland and Iceland, or Alaska. The demise of glaciers and ice fields at the end of the ice ages is therefore to a large extent caused by high humidities and not just by high air temperatures, though the two tend to go together.