National Geographic : 1952 Aug
Much is also locked up in the world's extensive regions of perma frost, in Alaska, northern Canada, and Asia, where the ground is per manently frozen in some places to depths of as much as 1,300 feet.* There is always some moisture in the air everywhere, even over des erts. Air containing no moisture at all would be irritating to breathe; it would dry up the membranes of the nose, throat, and lungs. Clouds form when moisture in the air condenses into tiny droplets of water or ice crystals. The drop lets form around "hygroscopic," or water-attracting, particles of dust, smoke, salt from the sea, or gas particles always present in the air. At first the droplets or ice crys tals are tiny, scarcely a hundredth of a millimeter in diameter. When the larger droplets or crystals are about two-tenths of a millimeter in diameter, gravity generally pulls upon them with enough force to make them start to fall. Raindrops and snowflakes often grow as they fall, by joining with others; by the time they reach the earth they may be 8,000,000 times the size of the original vapor drop lets. They cannot grow larger than this, because in falling they tend to lose their teardrop shape, "sau cering" out and splitting up. The rainiest place in the world is Mount Waialeale, Kauai Island, Territory of Hawaii. Here in a 28 year period official records show an annual average of 489 inches of rain (page 280). Rivals for the honor are Cherrapunji, Assam, in the east ern Himalayas, and Buena Vista, Colombia. In contrast, the place with the minimum recorded rainfall is the city of Iquique, in northern Chile, which has averaged .04 inch of rain a year during the last 40 years; some years no rain fell there at all. * See "Our Hometown Planet, Earth," by F. Barrows Colton, NATIONAL GEO GRAPHIC MAGAZINE, January, 1952. 273 Hall, U. S. Soil Conservation Service Living Plants Prevent Erosion Vegetation breaks the force of rains, prevents splash erosion, and retards runoff. Root structures bind the soil and keep it from washing. Only a thin crust of top soil (an average 7 inches) stands between Americans and starvation. An inch of this soil takes three to ten centuries to build; it can wash away in a single storm.