ABSTRACT
The hydrologic cycle consists of the circulation of water from land and ocean to atmosphere, then condensation, normally into cloud, followed by precipitation back either to the oceans, or to the land, where the water either evaporates or flows back to sea and evaporates there. The atmosphere holds very little of the world’s water, and the time any molecule spends as atmospheric humidity is only a few days, on average. A consequence of a greater rainfall and smaller evaporation in the northern hemisphere, compared with the southern, is that, on average, winds must carry moisture northwards across the equator, and oceans flow southwards across it, to maintain the continuity of the hydrologic cycle. A measure of atmospheric moisture more widely known than either vapour pressure or dewpoint is relative humidity, or simply RH.
