Although water is widespread on Earth much of it is not readily useable by humans. Of all the water on Earth, more than 97% is salt water held in the oceans. The remaining 3% constitutes the fresh water supply on the planet. The majority of fresh water is held in ice (glaciers and polar ice caps) and a large proportion also lies too far underground to be exploitable. The amount of fresh water directly available is less than 1% of the total water in the Earth System. While the atmosphere does not hold a large proportion of Earth’s fresh water at any one time, huge amounts of water are continually cycling through this reservoir on very rapid timescales. The cycling of water through the atmosphere is essential to renew the fresh water resources on which all life depends.