The fundamental driving force for earth's climate systems is the uneven heating of the earth's surface by the sun. The earth acts as a giant heat engine.
Weather redistributes the incoming heat around the globe.
Global atmospheric circulation and ocean currents play important roles in redistributing this uneven heating. Precipitation occurs most frequently in situations where warm moist air rises and cools. Examples include the intertropical convergence zone, windward sides of mountains and frontal zones. Dry conditions occur where cool dry air warms up, sucking up moisture from the surroundings. Typically, positive feedback mechanisms reinforce strong trade wind flows in the Pacific and the development of marked sea surface temperature variation, with warm surface waters in the western basin.