The oceans, like the atmosphere, have a layered structure defined according to their vertical temperature profile. Water temperature is generally highest at the surface and decreases with depth because the principal heat sources are solar insolation and heat supplied by the overlying atmosphere. Below this warm layer, temperatures drop rapidly in a zone known as the thermocline. Very cold water extends from the thermocline to the deep ocean floor, where temperatures range from 0°C to 5°C. About 90 percent of the total volume of the oceans is contained in the deep water below the thermocline. Figure 5.31 shows representative temperature profiles for tropical, midlatitude (temperate), and polar oceans.