Precipitation forms under three main types of conditions: convective uplift, frontal uplift, and orographic uplift. Convective uplift is common in the equatorial tropics and during the summer months. As the sun heats up the Earth's surface, the humid, warm air rises into the atmosphere, cools, forms tall, unstable cumulonimbus clouds, and then releases its moisture in a quick downpour.
Frontal uplifts are the result of cold and warm fronts colliding. This can take different forms. Fronts are transition zones between air masses of different temperatures. When warm air collides with cooler air along a warm front, the warm air gently rides up over the cooler air. If cold air collides with warm air along a cold front, the colder, denser air pushes up the warm air rapidly. Depending on the particular conditions, rains from frontal events can be a steady, soaking rain that last for several hours, or severe weather events that spawn quick downpours, high winds, and tornadoes. Orographic uplift, sometimes called "relief rain," occurs when a warm air mass meets a geographical barrier (usually a mountain range) that pushes the warm air up into the atmosphere, where it cools and condenses.
No matter what the type, the development of rain requires the presence of extremely small particles called condensation nuclei to form droplets. These particles, trapped in cloud formations, can be composed of anything from dust and soot to sea salt and phytoplankton, and at 0.02 mm. in diameter, are just 1/100th the size of a typical raindrop. Condensation nuclei give water vapor in the clouds something to coalesce around, thus increasing the size of the individual molecules of water vapor into droplets heavy enough to fall out of the clouds.
The speed with which raindrops fall depends primarily on their weight: for example, a 0.5 mm. raindrop would fall at around 6.5 ft. (2 m.) per second, while a five mm. drop would fall at 29.5 ft. (9 m.) per second. The average raindrop is around two mm. in diameter. The largest raindrop ever recorded was 10 mm. in diameter, but those are rare; drops larger than five mm. tend to be unstable, often breaking into smaller droplets as they fall. Raindrops are not shaped like teardrops: rather, they are spherical, or flattened on the top, with larger drops sometimes shaped liked parachutes.
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