When the surface of water cools by radiation to a cooler air it gains in specific gravity and sinks and warmer water comes up to take its place and in turn be cooled and sink; thus a circulation is established which continues in fresh water until every part of the body of water has fallen to 39° and in salt water to 28°. At these temperatures the two waters reach their maximum density. With the further cooling of salt water particles of ice form and rise to the top, as already described. With the cooling of fresh water below 39° the law that holds good for all higher temperatures is reversed and expansion of volume begins, which continues until 32° is reached. Therefore, fresh water of any temperature between 39° and 32° may float upon water that is considerably warmer; in fact, it has less specific gravity at 32° than at 46°. At 32° that which was a liquid becomes a solid and still further suddenly expands its volume.
The Cold of Ocean Bottoms. Few have any idea of the enormous volume of cold water that lies upon the surface of the earth, three fourths of which is covered with oceans whose depths average two miles and in many places are five miles. Below one mile in depth these oceans are always at about the freezing point of salt water, which is 28°, except in the tropics, where it is but little warmer, varying between 34° and 36°.
How Temperatures of Inclosed Seas Differ from Those of Oceans. We will take the Red Sea as an example. It is 180 miles wide and extends in a nearly north and south direction for 1450 miles, about one half of it lying within the tropics. Evaporation takes place at a rapid rate, but only the surface water of the Indian Ocean on the south is able to enter to take the place of that which is lost, for a bar or sill at the entrance, extending from the bottom to within twelve hundred feet of the surface, separates the deep water of the sea from that of the outside ocean. Its surface temperatures vary about as the Indian Ocean, being 85° in summer and 70° in winter. Both bodies of water decrease in temperature at about the same rate down to the level of the sill, where the temperature remains constant the year through at 70°. Here a marked difference occurs, for the sea, which has a depth of 7200 feet, maintains the same temperature of 70° all the way down to the bottom; while the ocean continues to decrease in temperature down to a depth of about six thousand feet, where a temperature of 34° to 36° prevails throughout the year. A similar condition exists with relation to the Mediterranean and the Atlantic Ocean. At the top of the sill, which is 1140 feet below the surface, the temperature of both bodies is 55°, and this degree of heat is maintained all the way down to the bottom of the Mediterranean, while in the Atlantic Ocean, at the same depth as the bottom of the Mediterranean, the temperature is only 35°.
How the Temperature of Water Changes with Latitude, Season, and Depth. It is impossible to name a given temperature as prevailing over bodies of water at all places on the same parallel of latitude, because ocean currents soon move water heated in one latitude to a higher or a lower position. At the equator the surface temperature is between 82° and 84°; it changes less than one degree between day and night, and not over five degrees between winter and summer; and below twenty-four hundred feet there is no difference between the seasons, the daily variation ceasing at less than a hundred feet. Below six thousand feet the temperature is always near the freezing point of fresh water.
In the middle latitudes the surface variation is from 50° in winter to 68° in summer.
At latitude 70° N. the surface temperature has but a small daily variation, and a yearly range of from 35° for winter to 45° for summer; at a depth of twenty-four hundred feet it remains steady at 32°.
From this level there is a gradual decrease to a depth of six thousand feet, where a constant temperature of 28° exists, and below this there is no change. The temperature of Lake Superior decreases down to a depth of two hundred forty feet, where a temperature of 39° continues throughout the year, as it does downward for the remainder of the distance to the bottom, which has an average depth of nine hundred feet.
Direction of Wind Affects Shore Temperature of Water. Onshore winds skim off the warm surface water and drive it shoreward, where it banks up, and, pressing downward, causes the colder water beneath to flow back seaward. In like manner, offshore winds blow off the top water near the shore and send it out to sea, and colder water rises to take its place.
Great Heat of the Earth’s Interior. We are ignorant of the conditions of matter under the heating effect of the enormous pressure that exists near the center of the earth, but it is probable that pressure prevents it from changing from a solid to a liquid or a gaseous form. The surface of the solid earth rises to a much higher temperature as the solar rays fall upon it than does a water surface, or the air immediately above, because it is a poor reflector, a poor conductor, and a poor radiator, and when dry does not get any cooling effect from evaporation. Solar heat ceases to be apparent at a depth that varies with the latitude and the conditions of the soil with regard to moisture and specific heat, but everywhere at less than fifty feet.
At the poles and for some distance away the earth is covered with ice or snow the entire year and is frozen to a considerable depth. In the interior of Siberia and some parts of Alaska only a thin stratum of soil thaws out under the heat of summer. Beginning at about fifty feet, there is an increase of temperature downward, but it is not the same for all places, varying from a degree for forty feet to a degree for one hundred feet. Taking the average of the increase with depth, water would boil at ninety-five hundred feet and the hardest rock be molten at thirty miles. At a depth of 3490 feet near Berlin, the temperature was found to be 116°, while it was only 108° at the same depth at Wheeling, West Virginia, and in both places there is no change from day to night or from winter to summer.