Precipitation

Precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull.

This process is a vital component of the Earth's water cycle. It is responsible for depositing most of the fresh water on our planet. While much water falls over the oceans, much of it also falls over land. The globally averaged annual precipitation is about 1000 mm, but over land, it is only about 600 mm.

Understanding how this water moves is essential for studying different climates. For example, the Köppen climate classification system uses average annual rainfall to distinguish between various climate regimes.

Precipitation begins when a portion of the atmosphere becomes saturated. Saturation occurs when the air reaches 100% relative humidity. At this point, water vapor condenses into liquid or ice. This condensation often happens on condensation nuclei, which are tiny particles like dust, salt, or ice. There are two main ways to reach this saturated state. First, the air can be cooled to its dew point. The dew point is the specific temperature where air must be cooled to condense water. Second, more water vapor can be added to the air through processes like evaporation or transpiration from plants.

Scientists identify several mechanisms that cool the air to its dew point. Adiabatic cooling occurs when air rises and expands, which can happen due to convection or physical barriers like mountains. Conductive cooling happens when air touches a colder surface. Radiational cooling occurs through the emission of infrared radiation. Finally, evaporative cooling happens when moisture is added to the air through evaporation.

As these processes occur, moisture within the rising air condenses into clouds, such as nimbostratus or cumulonimbus clouds. Inside these clouds, smaller droplets undergo coalescence. This is when droplets collide and fuse to create larger drops. Once these drops are heavy enough to overcome air resistance, they descend toward Earth.

Precipitation can be divided into three main categories based on its state. The first category is liquid water, which includes rain and drizzle. The second category includes liquid water that freezes on contact with a surface, known as freezing rain or freezing drizzle. The third category consists of frozen forms, such as snow, ice pellets, hail, and graupel. Mixtures of these different types can often fall at the same time.

Different cloud types also play a role. For instance, stratus clouds are stable decks that form when cool air is trapped under warm air. In contrast, convective processes involve strong vertical motions that can cause the atmosphere to overturn within an hour, leading to intense, short periods of rain called showers.

Frozen precipitation takes many distinct forms. Snowflakes form when tiny supercooled cloud droplets freeze into ice crystals. These crystals grow into large snowflakes by consuming surrounding water droplets in a process called the Wegener–Bergeron–Findeisen process.

Ice pellets, sometimes called sleet in the US, are small, translucent balls of ice. They form when snowflakes melt in a warm layer of air and then re-freeze in a sub-freezing layer near the ground. Hail is a different phenomenon that grows within storm clouds. Strong updrafts push hailstones up into the cloud, where they collect more ice. This "wet growth" allows hailstones to reach diameters of 5 cm or more, sometimes weighing more than 30 grams.

Geography significantly influences where and how precipitation falls. In mountainous regions, orographic precipitation occurs when air is forced to rise up the windward side of the terrain. This can lead to heavy precipitation at high elevations. Conversely, the leeward side of mountains may experience dry, desert climates due to compressional heating.

Another specific type is lake-effect snowfall. This occurs when relatively warm water bodies provide moisture that is lifted into a cold cyclonic flow. This can result in locally heavy snowfall.

Most global precipitation, however, occurs within the tropics and is driven by convection.

To study these events, meteorologists use specific measurement tools. Liquid precipitation, like rain, is measured using a rain gauge in millimeters (mm) or inches. One millimeter of rain is equivalent to one liter of water per square meter. For solid precipitation, a snow gauge is used to measure height in centimeters. Scientists may also melt the snow to find its water equivalent in millimeters. This helps determine how much actual liquid was contained in the frozen mass.

Even beyond Earth, precipitation exists; on Saturn's moon Titan, methane falls as a slow-falling drizzle, creating puddles at the equator and poles.