Tony Williams, age 14, of Charlotte, North Carolina, for his question:
What is meant by the hydraulic cycle?
Our period of history has been called the Age of Industry and the Age of Science, the Iron Age and the Space Age. In Andy's opinion, it also happens to be the Age of Fancy Language. Experts in some circles strain themselves to use flossy terms instead of simple words. We ordinary folk are impressed no end until we refer to a dictionary. The fancy language fails to awe us when we find the simple words for precipitation and dialogue, hydraulic and confrontation. ~ ~r
Hydro is borrowed from the Greek word for water, and hydraulic means a system of water in motion. Certain man made hydraulic pumps use moving water and hydraulic cement hardens under water. So far, our Age of Industry has not produced methods to keep such systems operating in perpetual motion. However, nature uses perpetual motion to cycle a stupendous supply of water on a planetary scale. This global hydraulic cycle, alias the water cycle, circulates moving water to remodel the surface of the earth and supply its living things.
The sun bestows a fairly steady quota of radiant energy on the globe. Some of this energy is converted to heat at the surface of the land and sea. The shape of the globe, the tilt of its axis, plus its geographical ups and downs cause this solar energy to be distributed unevenly. The weathery air gets heat from the warm surface and its warmed gases become thirsty. About one third of the earth's total solar energy is used by the air to evaporate moisture from the oceans and the moist land. This moisture becomes invisible vapor.
Masses of warm air, loaded with gaseous water, expand and cool aloft. As they cool, they can hold less vapor. The surplus condenses in liquid cloud droplets and fine crystals of ice. As local and global winds waft the clouds along they grow heavier. Finally gravity pulls them back to earth as rains and snows. About a quarter of the global downpour falls on land, providing enough fresh rain water to supply every person with 22,000 gallons every day. However, a large portion is consumed by plants and another portion sinks down to replenish the reservoir of ground water. A large portion also runs into rivers that carry it to the sea. Meantime nature's hydraulic cycle is completing this perpetual circulation of water on a global scale.
The amount of water involved in the system is immense. Every day, a billion tons of precipitation falls here and there. Every year, the downpour is enough to douse 40 inches of rain on the entire globe. Every year, the earth's hydraulic cycle involves about 100,000 cubic miles of water in this system of perpetual motion. This is the amount of water evaporated to gaseous vapor, lifted and shifted by the atmosphere, changed back to liquid and finally dumped back on land and sea. And without this never ending circulation of water, all life on earth would perish from thirst.
The cloudy atmosphere always holds enough moisture to dump an inch of water on the entire surface of the earth. However, such a world wide shower never occurs. On a global scale, the water evaporates and precipitates unevenly. The world average precipitation is 40 inches per year. But an arid desert may get merely an inch or less a year while the wettest region may get 450 inches.