Hence somewhere around 30 degrees north and south of the equator, we find the world's major desert zones. Figure 3.4 illustrates these and other global climate zones resulting from Hadley driven temperature and precipitation. The average depth of the oceans is 3.8 km, but the ocean reaches a depth of 11 km in the Marinas trench off the island of Guam, a major subduction zone in the Pacific Ocean. The oceans receive their heat from the sun, mainly in the equatorial region and this heat is distributed around the Planet by ocean currents. In addition to heating land and water, absorption of EMR also causes water to evaporate, driving the hydrologic cycle.
Water vapour enters the atmosphere and circulates with the air.
Warm air rises and water vapour rises with it. The farther the air-water combination travels from the surface, the cooler it becomes. Cooling causes water vapour to condense and to precipitate as rain, snow or ice which falls back to Earth. In time, return flow to the oceans from streams and rivers balances the net loss from ocean evaporation, but this is not a rapid process. The huge volume of the world's oceans relative to the input of water from rivers means that it would take 40,000 years to completely exchange all the water in the oceans.