| RAIN | |||||||||||||
| Rain is condensed atmopheric water vapor. In other words, the large number of water droplets in the clouds form because, as warm, moist air rises, it eventually becomes so cold that the water vapor condenses. Collecting around bits of dust and other microscopic particles known as condensation nuclei, the droplets at first are so small that they literally float on air. But as they ride on air currents, the droplets collide, gradually forming larger and larger droplets. In time they may become big and heavy enough to fall as rain. However, at least half of all rain forms differently. Snowflakes or ice crystals, in the frigid upper regions of a cloud, melt as they pass through warmer air below and land on earth as rain.
The smallest raindrops, called drizzle, drift so lazily to earth that they seem to float. The largest, nearly ¼ inch in diameter, pelt down at 25 feet per second. | ||||||||||||
| FUN-FACT: Whatever the drop's size, none are teardrop-shaped. Although the largest drops are flattened on the bottom, most are round. | |||||||||||||
| SNOW | |||||||||||||
| When temperatures in a cloud are low enough, its moisture content may be released, not as rain, but as feather-light snowflakes. This happens because at very low temperatures, cloud droplets become supercooled, which means that they reamain liquid even though their temperature is below freezing. Under certain conditions the supercooled droplets evaporate and the vapor then freezes directly into very small ice crystals. As more vapor freezes on the first tiny crystals, they grow into snowflakes. Their shape depends on the temperature and the amount of moisture in the air. |  | ||||||||||||
| FUN-FACT: Snowflakes are about 90% air, which also means that the water in 10 inches of snowfall equals only 1 inch of rain. FUN-FACT: In the Earth's history 1,000,000,000,000,000,000,000,000,000,000,000,000 snowflakes are estimated to have fallen. HAIL |  The damage to this windshield was done by large hail, such as the two hailstones also in this picture. Hail is born high up in thunder clouds. Caught alternately in updrafts and downdrafts, pellets of ice enlarge as they are coated with additional layers of ice. Their final size depends on how long they are cycled through the drafts within the cloud. Most are the size of grapes when they fall to earth, but many grow as big as walnuts, and a few are as large as tennis balls. The damage from hailstones can be severe. They sometimes dent the roofs of cars and damage buildings, but their worst damage is to agriculture. Since they usually fall in summer, when thunderstorms are most common, they sometimes ruin entire fields of crops.
| FUN-FACT: One of the biggest hailstones on record was 5½ inches in diameter and weighed 1½ pounds. SLEET | Falling raindrops may pass through a layer of very cold air and freeze into solid pellets, or sleet, that actually bounce when they hit the ground.
| FUN-FACT: Some drops may become so cold that they freeze on impact and coat everything they touch with a glassy layer of ice. This is called freezing rain. DEW & FROST | 
As the air begins to cool after dark, it gradually approaches the dew point*. When the saturated air comes in contact with the slightly cooler surfaces of leaves and grass, the water vapor condenses and coats them with dew- just as it coats a glass of iced tea on a hot and humid summer day.
| Frost forms only when temperatures are below freezing. Then, when the saturated air comes in contact with a freezing cold surface, the water vapor does not condense into liquid, but instead freezes directly from its gas state into very small ice crystals. FUN-FACT: The crystals grow as more and more vapor freezes, sometimes forming delicate featherlike patterns. | |||||||
* The temperature at which the air becomes saturated with water vapor.
