Precipitation

Precipitation is the primary factor that controls the hydrologic cycle. It takes different forms, such as rain, snow, hail, sleet, drizzle, dew, and fog. It supplies most of the freshwater on the Earth. Most precipitation starts from space as snow, as the upper space is cooler. If the temperature of the surface closer to the ground is below 32 degrees F (0 degrees C), then the precipitation falls on the ground in the form of snow. If the ground and closer surface temperature is above 32 degrees F (0 degrees C), the precipitation takes the form of rain. When the air at the ground is below freezing, the raindrops can freeze while hitting the ground, and that is known as freezing rain.

When a dust particle in the atmosphere attracts a moisture drop, hail is formed. Drizzle consists of very small raindrops, 1/1000 of a normal raindrop size. Sleet is a type of precipitation between rain and snow, but very distinct from hail. Dew is another form of precipitation that can be seen in the early morning on colder days. Water vapor in the atmosphere condenses on the surface of exposed objects at a greater rate than that at which it can evaporate, developing dew. Fog as such is not precipitation, but is considered one because of its low-altitude occurrence. This consists of a cloud in contact with the ground, and produces water droplets when intercepted with vegetation or other exposed objects. If the precipitation evaporates before reaching the ground, it is then known as virga.

Dynamic and adiabatic cooling causes precipitation. Because of this process, condensation of water vapor occurs and then falls to the Earth as rain, snow, or other forms of precipitation. Vertical air motion is the leading factor of all rainfall from clouds after condensation. Rising air in the tropics and midlatitudes (40 to 60 degrees N and S latitudes) causes more precipitation and descending air patterns in the subtropics (20 to 30 degrees N and S latitudes) and in the poles causes less. Precipitation is classified into different categories based on the conditions that generate vertical air motion: convective, orographic, and cyclonic.

Convective precipitation is mostly seen in the tropics. Heating up of the air at ground level, then moving upward and mixing with the water vapor in the atmosphere, with dynamic cooling in space, causes precipitation to fall on Earth. This is called convective precipitation. Orographic precipitation occurs because of the interception of moisture-laden air or clouds by mountain ranges. Cyclonic precipitation is caused by movement of moist air masses from high-pressure regions to low-pressure regions. In the hydrologic cycle, the total volume of precipitation onto land is measured at 42,471 sq. mi. (110,000 sq. km), while the total volume of precipitation on the ocean surface is 176,834 sq. mi. (458,000 sq. km).

Precipitation has greater ecological, geographical, and regional impact because of its characteristics, such as relative amount, seasonal timing, and most importantly the size and intensity. Lowintensity and well-distributed seasonal precipitation is good for agriculture. High-intensity, longduration precipitation creates more problems than good. Precipitation is governed primarily by atmospheric water vapor, but its variation depends upon other climatic factors such as temperature, wind, and atmospheric pressure at different locations and season. The amount of water vapor is very high in the atmosphere closer to water bodies. Therefore, coastal areas always have heavier precipitation (high-intensity, long-duration) than inland areas. Thunderstorms, hurricanes, typhoons, cyclones, blizzards, and hailstorms are high-intensity precipitations with damaging ability.