| Water from the earth's oceans and lakes is constantly evaporating into the atmosphere. As the water vapor evaporates from the oceans and lakes, it creates clouds. As a cloud, water vapor can travel hundreds of miles from its point of evaporation until the right conditions (such as a low pressure cell or front) cause the moisture in the clouds to condense. Precipitation can take a number of forms: Rain, Sleet, Hail and Snow. Precipitation also appears as Dew or Frost.
Rain is water falling in liquid form and as such, it is a primary component of the water cycle. The water cycle describes the cycle in which moisture from the oceans evaporates, condenses into clouds, precipitates back to earth, and eventually returns to the oceans. Clouds contain huge quantities of water vapor. This vapor condenses around small dust particles and then these tiny drops of water are enlarged by additional condensation or by merging with other drops. Eventually, the water becomes too heavy to be supported by the atmosphere, and the raindrops begin falling from the cloud. Raindrops vary in size from about 0.02" (0.5 mm) to as much as 0.33" (8 mm) or larger.
Sleet is water falling as small, partially melted grains of ice and occurs only during the winter. As raindrops fall from clouds, they pass through layers of air at different temperatures. If they pass through a layer with a temperature below the freezing point, they turn into sleet. Snowflakes that have melted by passing through a warm layer will turn into sleet if they then pass through a freezing layer. Sleet often falls together with snow and rain, and may deposit an icy coating on exposed surfaces.
Hail is water falling as pellets composed of ice or of ice and snow, occurring at any time of the year, usually during the passage of a cold front or during a thunderstorm. When hail occurs, it can cause a great deal of damage and injury to crops, livestock, property, and airplanes. Small hailstones have a soft center and a single outer coat of ice. They are formed when the surfaces of snow clumps melt and refreeze or become coated with water droplets that subsequently freeze. Large hailstones usually have alternate hard and soft layers, and there is no consensus on precisely how these large stones form and grow. Hailstones are spherical or irregularly spherical and usually vary in diameter up to 1/2" (1.3 cm); in rare cases hailstones having diameters up to 5" (12.7 cm) have been observed. Unfortunately, on May 5, 1955, Fort Worth Texas was one of those rare cases, with hailstones reaching the size of softballs.
Snow is water falling as ice crystals. Snow is specifically formed by the sublimation of water vapor around a dust particle into solid crystals at temperatures below freezing (0° C / 32° F). Snowflakes form symmetrical hexagonal crystals, with no two snow crystals developing identically. Each snowflake differs from the others in size, lacy structure, and/or surface markings. Generally speaking, snow is ten times less dense than rain. For example, 10 inches of snow is roughly equivalent to 1 inch of rain.
Dew refers to a thin film of water that condenses on the surface of objects near the ground. Dew forms when radiational cooling of these objects during the nighttime hours also cools the shallow layer of overlying air in contact with them, causing the condensation of some water vapor. This condensation occurs because the capacity of air to hold water vapor decreases as the air is cooled. The temperature at which condensation begins, for a sample of air with a given water vapor content, is termed the dew point. If a dew point temperature below 32°F (0°C) is reached, the water vapor converts directly to frost.
Frost is ice formed by the condensation of atmospheric water vapor on a surface when the temperature of the surface is below 32°F (0°C). In the formation of frost, a water vapor is changed directly to a solid (sublimation). Frost often appears as a thin feathery deposit of ice. |
