What causes the water cycle?

---> The sun warms the oceans and the earth --->
---> water evaporates and turns into clouds --->
---> clouds drop rain onto the land --->
---> water runs off into the ocean --->
---> The water cycle starts again. --->
---> The sun warms the oceans and the earth --->
etc, et cetera

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The SUN is the biggest engine, which drives all the forces of nature.

Precipitation is one of such amazing process by nature, through which water returns to the earth. Initially water is evaporated from oceans and lands, where oceans contribute 65-70% of total precipitation as evaporation. Once, water is evaporated by the energy of the sun, water turns into water vapour and gets condensed in the atmosphere to form clouds, these clouds release water in the form of precipitation in the form of rain or snow (depending on local weather conditions).

Rain is then intercepted by trees, building cover etc. When the rain water reaches the ground, soil absorbs the water until its saturation point is reached, water from soil then flows as sub-surface water or ground water to oceans.

Water on the surface forms channels, channels joins to form rivulets, then to rivers and great rivers, all flows towards the ocean. Water again then gets evaporated from oceans and lands to form clouds.

Although ocean contributes most of the rain, water through evapo-transpiration also contributes a significant amount. But basically the SUN is the most important source of all force in nature.