Precipitation is any form of water, liquid or solid aqueous deposit, that fall from the clouds in the sky. It forms in the Earth's atmosphere and then drops onto Earth’s surface. Precipitation is an essential part of Earth’s water cycle because it connects the ocean, land, and atmosphere.    
How it works
Droplets of water suspended in the air, also known as water vapor, buildup in the Earth's atmosphere. From Earth, we see water vapor in the atmosphere as clouds and/or fog. Precipitation forms around the cloud condensation nuclei (CCN), or tiny pieces of material, such as dust, that collect in clouds. Eventually, the clouds can’t hold the water vapor any longer and this creates precipitation in the form of a liquid or a solid. Forms of precipitation “can only fall to the Earth's surface if they grow to a size that can overcome updrafts." 
Where it comes from
Not all clouds produce precipitation... In fact, most clouds do not. In those clouds, the water droplets and ice crystals are too small to overcome the natural updrafts in the atmosphere. This allows the small water droplets and ice crystals remain in the atmosphere until they are converted back into vapor. 
How it's measured
Certain kinds of precipitation, like rain and snow, have been measured for centuries using instruments as basic as containers with markings on them. Over time, very advanced, complex tools have been created to measure the precipitation fall. Today, these specialized devices provide information about the size of raindrops to how wind impacts the measurements taken. Weather radar systems on the ground send out a pulse of energy into the clouds and that provides the information of where and how much precipitation there will be. Other tools that aren’t stationed on the ground are the satellites orbiting above the Earth. These satellites give display a bigger picture of precipitation across the entire globe. 
Why it's measured
It is important to measure precipitation because each form affects life on Earth in different ways. When rain is measured for example, it is measured for where it occurs and how much it occurs. This is so that scientists can better understand the impact of precipitation on streams, rivers, surface runoff and groundwater. It has been noted that “frequent and detailed measurements help scientists make models of and determine changes in Earth’s water cycle.” 
Types of Precipitation
The most common forms of precipitation are rain, snow, hail, and sleet.
Rain is the most commonly known form of precipitation that falls to Earth’s surface. Rain is small droplets of water that fall from the clouds when they get too heavy for the clouds to hold. As these drops fall, they grow larger in size as they collect more water on their way down. Most people think of raindrops the way cartoon pictures portray it... looking like teardrops. However, real raindrops are spherical. Records state that “raindrops must have a diameter greater than 0.5 millimeters and smaller than 5 millimeters.” Raindrops that are too small aren't heavy enough to descend to Earth, and raindrops that are too heavy to be held together as a single drop separate. 
Snow requires temperatures on the ground to be 32-degrees Fahrenheit or colder, and any snow that falls on a ground warmer than 32 degrees melts on contact. Snow is precipitation that falls in the mid and high latitudes in the form of ice crystals. These ice crystals have a delicate, complex structure because they are formed individually in clouds. Snowflakes are so uniquely formed because ice crystals’ growth is the most rapid process. As the different patterned snowflakes fall to Earth’s surface, they cluster together depending on the temperature and humidity of the air. A “graupel” is when snow falls in the form of a ball instead of soft flakes, and this mostly happens “when snow is melted and precipitation forms around the snow crystal.” Snowfall tends to occur in the fall, winter, and spring months of the year because this is when atmospheric temperatures commonly drop below freezing. 
Hail descends to earth from the clouds as small rock-like forms of hard, solid ice. Even though these pieces are typically small, they’ve been recorded as being as large as 6 inches wide. They’ve also been measured at weighing more than a pound, but this depends on the storm. The creation of hailstones starts in the highest part of each cloud during cold thunderstorms. The stones form through the upward movement of air through the clouds which freezes the water droplets and stops them from falling as water. Before the pieces fall to earth, the frozen droplets collect more cold water which makes the forms larger. Snowfall is usually calm, but hail storms are quite the opposite.  Eventually the hailstones become too heavy for the clouds and fall. Because of the stones’ heaviness, scientists have estimated that the stones have reached speeds of more than 80 mph on their descend. 
Sleet starts as a relatively warm liquid when it first forms in above freezing temperate atmosphere. It later freezes on it’s descend to Earth.  Sleet is translucent slush that is typically smaller and wetter than hailstones.  This form of precipitation typically has a diameter smaller than 5 millimeters. 
Other closely related articles in this wiki include:
- Society, N. G., & Society, N. G. (2011, February 15). precipitation. Retrieved March 22, 2016, from http://education.nationalgeographic.org/encyclopedia/precipitation/
- Precipitation Measurement Missions | An international partnership to understand precipitation and its impact on humankind. Retrieved March 22, 2016, from http://pmm.nasa.gov/precipitation-measurement-missions
- Pidwirny, M. (2011, September 10). Precipitation and fog. Retrieved March 22, 2016, from http://www.eoearth.org/view/article/51cbeea97896bb431f69970e/
- Precipitation. Retrieved March 22, 2016, from https://eo.ucar.edu/basics/wx_2_b.html
Relevant online sources to this wiki article include:
- The home page of Penn State Brandywine, the home of the EARTH 100 wiki article writers!
- NASA Precipitation Measurement Mission has current information on what's happening in the world of precipitation.
- NASA Precipitation Education is a very helpful page for learners of all ages!
- A great video from NY Times on The Smell of Rain
- The National Geographic Rainforest site is insightful to rain as it is a type of precipitation mentioned above.
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