Precipitation is a vital part of the hydrologic cycle, and is defined as any water released from the clouds in the sky in the form of rain, freezing rain, sleet, snow, or hail. This is the primary method that water uses to move around in the hydrologic cycle. Any form of precipitation that falls is one component of the weather occurring in an area at any point in time. However, the trend of how much precipitation an area gets over an extended period of time contributes to the overall climate of that area. For a more in-depth explanation of the difference between weather and climate, click here.
The figure above shows precipitation trends worldwide. Precipitation doesn't fall evenly throughout the world, a country, or even a city. The different amounts of rain that fall at any point is significant to the formation of unique environments and ecosystems. As well, although many people think of arid, hot climates as being deserts, there are many different areas around the world that get significantly low amounts of precipitation and are thus deserts. Anything coloured green on the above map is classified as a desert - including Greenland and Antarctica.
All types of precipitation originate in clouds. Clouds contain water vapour and cloud droplets, which are simply small drops of condensed water that are too small to fall as precipitation. Despite being unable to fall as precipitation, these cloud droplets are what actually form visible clouds. Clouds themselves are continually evaporating and condensing, again moving water through the hydrologic cycle. Most of the condensed water in clouds doesn't actually fall as precipitation. The reason for this is because the speed at which the droplets fall isn't enough to overcome updraft speeds which keep clouds afloat.
For precipitation to form, tiny water droplets inside of clouds must condense on small particles of dust, salt, or smoke. These particles provide a core for precipitation to form over. As water continues to condense on the dust, a water droplet forms. If enough collisions of condensing water vapour particles occurs, a droplet is produced and falls from the cloud. This process can take a long time since millions of cloud droplets are required to produce a single raindrop.
Another mechanism for precipitation formation is known as the Bergeron-Findeisen process. In this process, a precipitation-sized drop is created through a process that causes ice crystals to grow quickly by taking water vapour from a cloud. Once these crystals have formed from the water vapour, they can melt and fall as rain or remain as they are and fall as snow.
Acid deposition is any type of precipitation that has a lower pH (and is therefore more acidic) than normal. However, the term almost always used for all of these is acid rain. This higher acidity can cause problems in ecosystems and the environment, and remains one of the major environmental concerns from fuel use despite the immense progress made to address it since the 1970s.
Acid rain is produced when water in the air combines with nitrogen oxides and sulfur dioxide, two types of pollutants, and then falls down the surface of the Earth. These pollutants may also collect on the Earth's surface and the rain may combine with it upon arrival to the Earth, so the term "acid deposition" is often preferred over acid rain.