Another purpose of the hydrologic cycle is to heat/cool the surface of the Earth, reducing temperature fluctuations and, thus, enhancing the possibility of
life.
Rain is produced under both cool and warm atmospheric conditions (winter and summer rains).
Average global terrestrial precipitation is about 800 mm per year.
For the past 120,000 years, humans have evolved around the middle of the climatic spectrum
[800 mm of annual precipitation].
The 800-mm isohyet: Health and hope.
