A good rain demands celebration. Water is the most precious substance on earth, and yet it falls from abundance, freely, from the sky. Water is also a restless substance. Its molecules unlock easily, allowing it to change in shape and form, to be evaporated into vapor or condensed into droplets or frozen into ice. When it appears as raindrops it is just one step in a continuous process of evaporation, condensation, and precipitation called the hydrological cycle. That cycle and the way water behaves in general on our planet are apparently unique. No other body on the solar system combines the necessary ingredients to maintain water in its life-giving forms. Smaller planets and moons lack the gravitational attraction to keep vapor and water on the surface from escaping into space. Hotter planets cannot keep it from boiling away; colder planets can only keep it locked up in ice caps.
Raindrops are formed in a cloud when thousands of tiny water droplets, each condensed around a microscopic bit of dust or other particle that can be served as a condensation nucleus, join together into units large enough and heavy enough to fall toward the ground without evaporating. An average raindrop is 15 million times larger than a droplet in a cloud, and contains trillions of molecules of H20. When it falls from a moving rain cloud, the first drops pass through warm, dry air that evaporates all but the largest of them. As those few large drops strike the ground, they cool and humidify the air, reducing the evaporation of other drops and paving the way for a downpour. In extremely hot and dry conditions, all the raindrops might evaporate, creating visible curtains of rain in the air called fallstreaks, or “virga,” which trail from beneath a cloud but disappear before they reach the ground.
In some parts of the world, when it rains it pours. The wettest spot on earth is Mount Waialeale, Hawaii, where during a recent 32 year period it rained an average of 460 inches a year. At another extreme, not a drop of rain fell on Arica, Chile, for more than fourteen consecutive years from October 1902 to January 1918.