- Key People:
- Vincent Joseph Schaefer
- Tor Harold Percival Bergeron
- Related Topics:
- Rain
- Snow
- Aerosol
- Hail
- Does it rain on other planets?
Precipitation
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What is precipitation in weather?
Precipitation includes all forms of liquid and solid water particles that fall from clouds and reach the ground. These particles include drizzle, rain, snow, snow pellets, ice crystals, and hail.
Precipitation is a key component of the water cycle, which is the continuous exchange of water between the atmosphere and the Earth's surface. It is an essential process for sustaining rivers, lakes, and life on Earth. While generally beneficial, heavy rainfall can cause soil erosion, landslides, and flooding.
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What are the main types of precipitation?
Precipitation includes all liquid and solid water particles that fall from clouds and reach the ground. These particles include drizzle, rain, snow, snow pellets, ice crystals, and hail. The size of the particle determines if it is precipitation versus a cloud particle.
The creation of precipitation requires water vapor to condense and form clouds. Then cloud particles must grow and combine until they are heavy enough to fall to the ground.
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How does precipitation form in the atmosphere?
Precipitation forms through condensation of atmospheric moisture, creating clouds where particles grow and combine until heavy enough to fall. This process involves water evaporating from the Earth's surface, rising as vapor, and condensing into clouds.
There are two main mechanisms for precipitation:
- Coalescence: Larger-than-average cloud droplets collide and merge with smaller ones, growing into raindrops. This occurs in tropical and subtropical clouds where temperatures remain above freezing.
- Ice-crystal process: In colder clouds, supercooled water droplets and ice crystals coexist. Ice crystals grow at the expense of water droplets and, as they grow, may join to form a snowflake. When these flakes fall into warmer air, they melt and form raindrops.
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How does temperature affect the type of precipitation?
Temperature significantly influences the type of precipitation that occurs.
In cold clouds, ice crystals grow through deposition of water vapor, leading to snow. The specific form of ice crystals depends on temperature and supersaturation. For instance, dendritic patterns appear at −14 °C, while hollow ice columns form at −40 °C.
Warmer temperatures can cause frozen precipitation to melt, resulting in rain. If a subfreezing air layer exists near the surface, rain may refreeze as sleet or freezing rain upon impact. Hail, often formed in cumulonimbus clouds with strong updrafts, exhibits alternating ice layers due to varying temperatures and liquid water concentrations within the cloud.
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Why is precipitation important for Earth’s water cycle?
Precipitation is a key component of the hydrologic cycle, facilitating the continuous exchange of water between the atmosphere and the Earth's surface. It is the primary source of fresh water, essential for sustaining rivers, lakes, and all forms of life on Earth.
Precipitation patterns also influence regional climates, with increased precipitation expected in polar regions and decreased precipitation in middle latitudes. These shifts can lead to an increased risk of both drought and flooding in various regions, highlighting its significant impact on global water resources.
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How do scientists measure and predict precipitation?
Scientists measure and predict precipitation using various instruments and techniques. Rain gauges directly measure precipitation, while Doppler weather radar provides data on the intensity and extent of rain and other precipitation types.
To predict precipitation, meteorologists use computer models that apply expert knowledge to measure and predict atmospheric conditions. These models use statistical relationships between past weather phenomena and developments to extrapolate weather elements.
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Precipitation, all liquid and solid water particles that fall from clouds and reach the ground. These particles include drizzle, rain, snow, snow pellets, ice crystals, and hail. (This article contains a brief treatment of precipitation. For more-extensive coverage, see climate: Precipitation.)
The essential difference between a precipitation particle and a cloud particle is one of size. An average raindrop has a mass equivalent to about one million cloud droplets. Because of their large size, precipitation particles have significant falling speeds and are able to survive the fall from the cloud to the ground.
The transition from a cloud containing only cloud droplets to one containing a mixture of cloud droplets and precipitation particles involves two basically different steps: the formation of incipient precipitation elements directly from the vapour state and the subsequent growth of those elements through aggregation and collision with cloud droplets. The initial precipitation elements may be either ice crystals or chemical-solution droplets.
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Development of precipitation through the growth of ice crystals depends on the fact that cloud droplets can freeze spontaneously at temperatures below about −40 °C, or −40 °F. (The reduction of cloud droplets to temperatures below the normal freezing point is termed supercooling.) Within supercooled clouds, ice crystals may form through sublimation of water vapour on certain atmospheric dust particles known as sublimation nuclei. In natural clouds, ice crystals form at temperatures colder than about −15 °C (+5 °F). The exact temperature of ice crystal formation depends largely on the physical-chemical nature of the sublimation nucleus.
Once ice crystals have formed within a supercooled cloud, they continue to grow as long as their temperature is colder than freezing. The rates of growth depend primarily upon the temperature and degree of vapour saturation of the ambient air. The crystals grow at the expense of the water droplets. In favourable conditions—e.g., in a large, rapidly growing cumulus cloud—an ice crystal will grow to a size of about 0.13 millimetre (0.005 inch) in three to five minutes after formation. At this size, the rate of growth through sublimation slows down, and further growth is largely through aggregation and collision with cloud droplets.
Small solution drops are also important as incipient precipitation particles. The atmosphere contains many small particles of soluble chemical substances. The two most common are sodium chloride swept up from the oceans and sulfate-bearing compounds formed through gaseous reactions in the atmosphere. Such particles, called condensation nuclei, collect water because of their hygroscopic nature and, at relative humidities above about 80 percent, exist as solution droplets. In tropical maritime air masses, the number of condensation nuclei is often very large. Clouds forming in such air may develop a number of large solution droplets long before the tops of the clouds reach temperatures favourable to the formation of ice crystals.
Regardless of whether the initial precipitation particle is an ice crystal or a droplet formed on a condensation nucleus, the bulk of the growth of the precipitation particle is through the mechanisms of collision and coalescence. Because of their larger size, the incipient precipitation elements fall faster than do cloud droplets. As a result, they collide with the droplets lying in their fall path. The rate of growth of a precipitation particle through collision and coalescence is governed by the relative sizes of the particle and the cloud droplets in the fall path that are actually hit by the precipitation particle and the fraction of these droplets that actually coalesce with the particle after collision.
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