Precipitation

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. 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.

Figure 1. A map of the Earth showing how much precipitation falls in different areas.

Figure 1 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. While many people think of arid, hot climates as being deserts, there are many different areas around the world that receive little to no precipitation annually and are thus classified as deserts. Anything coloured green on the above map is classified as a desert - including Greenland and Antarctica.

Formation

All types of precipitation originate in clouds. Clouds contain water vapour and cloud droplets, which are small drops of condensed water that are too small to fall as precipitation. Being unable to fall as precipitation, these cloud droplets remain aloft and are what form visible clouds. Clouds themselves are continually evaporating and condensing. Most of the condensed water in clouds doesn't actually fall as precipitation because the mass of the droplets isn't great enough to overcome the effect of circulating updraft winds which keep them afloat.

For precipitation to form, water vapour in the atmosphere must condense on small particles of dust, salt, or smoke known as cloud condensation nuclei. These particles provide a core on which a few molecules of water vapour condense in a process called nucleation. As water continues to condense on the dust, a water droplet forms. Once the water droplet accumulates enough mass to overcome the air currents that keep it aloft, it begins to fall to Earth. 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.

Figure 2. Mountainous terrain forces air upwards, resulting in precipitation.

Orographic Precipitation

Orographic precipitation refers to the process where moist air masses are forced upwards as they move over elevated topography such as mountain rages and the subsequent cooling of these air masses results in a precipitation event. This type of precipitation is responsible for the rain shadow phenomenon and is common in coastal regions.

Convective Precipitation

Convective precipitation occurs when humid air masses at surface heat up sufficiently to start rising. As these air masses rise and expand they begin to cool down and condense. The precipitation events are characterized by their short duration and high-intensity, and are common about the tropics due to the combination of high humidity and high temperatures.

Cyclonic Precipitation

Cyclonic precipitation is precipitation that forms as a result of the collision of a warm and cold air mass. When the two air masses collide, the warmer air is forced up over top of the colder air. Precipitation can then occur as a result of the general cooling of the warm air mass as it rises, and can also occur along the boundary where the warm and cold air meet.

Acid Deposition

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Acid deposition is any type of precipitation that has a pH of less than that of regular precipitation. The most commonly used term is acid rain, which usually has a pH around four. This higher acidity can cause problems across many ecosystems and the environment, and remains one of the major environmental concerns from fossil fuel use despite attempts made to address the issue since the 1970s.

Acid rain is produced when water in the air combines with nitrogen oxides and sulfur dioxide and then falls down the surface of the Earth. These pollutants may also collect on the Earth's surface and then combine with rain as it falls, so the term "acid deposition" is often preferred over acid rain.

For Further Reading

• Acid rain
• Evaporation
• Cloud
• Water
• Hydrologic cycle
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