Jul 7, 2012

saturday med.itations: Water in the Sky

Clouds are masses of droplets of water so tiny that it takes a microscope to see them. They are so light in weight that they float in the air. Even with all the water droplets, a cloud is still more than 99 percent pure air. It takes a million or more of these droplets to make a raindrop.

In the temperate zones raindrops start as ice crystals. It is many degrees below zero where raindrops form. A raindrop won’t form until it has a tiny speck of something – usually dust – around which the water in the tiny droplets can gather to form an ice crystal. In a process hard to imagine, the freezing water droplets release heat into the air. Since warm air rises, the air around the developing ice crystals becomes warmer and also begins to rise, taking the tiny bits of ice along with it. As the crystals move upward, they bump into other tiny droplets that join the growing ice crystals.

The masses of frozen droplets become larger and heavier. Eventually the air cannot hold them up any longer, and they begin to fall. By the time the clumps of ice crystals reach the ground, however, they have melted into raindrops. Once in a awhile, in storm clouds called thunderheads, the hot summer air is already rising and the frozen water continues to rise until it grows into hailstorms that are so large that they don’t melt before they reach the ground.

Through a process called “cloud seeding,” pilots drop tiny yellow crystals of a chemical called silver iodide into certain kinds of clouds. The crystals of this chemical are constructed very much like water crystals, or ice. When the yellow-colored crystals drop into the clouds, the tiny water droplets begin to attach themselves to the silver iodide to form ice crystals, which then makes rain.

-source: Nature Quest

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