The Physics
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Volume of Earth's Annual Precipitation

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Bibliographic Entry Result
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Pidwirny, M. Global Distribution of Precipitation. Fundamentals of Physical Geography, 2nd Edition. 17 April 2008. "The average annual precipitation of the entire surface of our planet is estimated to be about 1050 millimeters per year or approximately 88 millimeters per month." 5.36 × 1014 m3
Climate. Encyclopædia Britannica. Encyclopædia Britannica Online. 2008. "Because Earth's average annual rainfall is about 100 cm (39 inches), the average time that the water spends in the atmosphere, between its evaporation from the surface and its return as precipitation, is about 1/40 of a year, or about nine days." 5.1 × 1014 m3
Legates, David R., Cort J. Willmott. Mean seasonal and spatial variability in gauge-corrected, global precipitation. International Journal of Climatology 10(1990): 111-127. "Annual average global precipitation is approximately 1123 mm (gauge corrections considered), which is consistent with other reported values. (Chonka-PTT)" 5.73 × 1014 m3
Peixoto, J.P. and M.A. Kettani. "The Control of the Water Cycle." Scientific American. Vol. 228, No. 4 (April 1973): 46-61. 4.23 × 1014 m3
Discovering Science: A Temperate Earth (Course S103). London: Open University (1998): 103. "Because 1000 mm = 1 m, the total volume is (1 m) × (5.1 × 1014 m2), which is 5.1 × 1014 m3." 5.1 × 1014 m3

Precipitation is the product of any condensation in the atmospheric water vapor that reaches the surface of the Earth. It can take on many forms such as rain, snow, or hail although the overwhelming majority occurs as rain. Precipitation is part of the hydrologic cycle and serves to bring water back down to the Earth. Precipitate forms as moist, warm air rises and cools. During the cooling process, the water vapor collects on condensation nuclei like dust particles which results in the formation of clouds. When the water droplets fuse to create large drops too heavy to be sustained in the air by air currents, they begin to fall as precipitation in a process known as coalescence.

The volume of global precipitation is calculated by taking the product of the Earth's surface area and its average annual rainfall…

Surface Area of Earth = 4 * pi * (Radius of Earth)2 = 5.1 × 1014 m2

Volume of Water = Depth × Surface Area = (1 m) × (5.1 × 1014 m2) = 5.1 × 1014 cubic meters

Global precipitation is unevenly distributed due to a variety of factors including the pattern of global winds, the changing latitude of the location, and the presence of mountains. This is clearly shown by the disparity in the record high and low annual rainfalls — the record high belongs to Cherrapunji, India at 26,470 mm from 1960-1961 while the record low goes to Atacama, Chile at 0.5 mm. The pattern of global winds partially explains why the equatorial belt consistently experiences more precipitation — the trade winds from both hemispheres contribute to a larger push of air upwards. Latitude has an inverse relationship with precipitation. Areas near the poles tend to experience less precipitation because the cold air cannot support the same amount of moisture that warm air can. Equatorial areas however, experience more solar heating which encourages the process of convection and subsequently precipitation. Mountainous regions are often classified as either windward (wet) or leeward (dry) side with respect to the mountain. The classification results from a phenomenon known as orographic uplift — air masses are forced to rise as it passes over increasing elevation.

Therefore the next time you complain about the enormous rainfall in the region where you live, think about how much precipitation the Earth receives — enough to fill nearly half a million Empire State Buildings.

Vernon Wu -- 2008