Types of Precipitation
Rain
Rain is defined as the precipitation of liquid water drops larger than 0.5 mm in diameter (0.02 inch). When the drops are smaller, the precipitation is commonly referred to as drizzle. Raindrop concentrations typically range from 100 to 1,000 per cubic meter (3 to 30 per cubic foot); drizzle droplets are typically more numerous. Raindrops rarely have diameters larger than 4 mm because they break up as they grow in size. The concentration generally decreases as the diameter increases. Except when it is heavy, rain does not reduce visibility as much as drizzle does. Rain is classified by meteorologists based on its rate of fall. The hourly rates for light, moderate, and heavy rain are less than 2.5 mm, 2.8 to 7.6 mm, and more than 7.6 mm, respectively. Raindrops can form as a result of the collision of small water droplets or as a result of the melting of snowflakes and other ice particles as they fall into warm air near the ground. The annual rainfall exceeds 500 mm in most of Europe, South America, eastern North America, and central Africa, while it is less than 500 mm in most of Asia, excluding India, Tibet, and China, and is less than 250 mm in a long tongue extending from Arabia to northeast Mongolia. The central regions of Australia, most of northern and southwest Africa, parts of the intermontane region of the United States, and parts of the west-central and southeast coasts of South America all receive less than 250 mm of rain per year. Portions of Africa's western coast between the Equator and 10° N, a strip of India's western coast, parts of Assam, a coastal strip of Myanmar (Burma), windward mountain slopes in North and South America's temperate latitudes, and many isolated tropical stations receive more than 2,500 mm of rain per year.
Drizzle
Drizzle: A fairly uniform precipitation of very fine,
close-together drops of water falling from a cloud. Drizzle drops are typically
less than 0.5 mm in diameter. Because the drops appear to float, even minor air
movements are visible. Drizzle falls from a stratus layer, usually low and
sometimes touching the ground (fog). Drizzle can cause significant amounts of
precipitation (up to 1 mm/h), especially along coasts and in mountainous areas.
Due to partial evaporation, drops falling on the edge of a rain zone or during
light rainfall may be as small as drizzle drops. Raindrops differ from drizzle
drops in this situation because they are more dispersed.
Snow
Snow, the solid form of water that crystallizes in the
atmosphere and falls to the Earth, covers about 23 percent of the Earth's
surface permanently or temporarily. Snow falls at sea level poleward of
latitude 35° N and 35° S, though it generally falls only at higher latitudes on
the west coasts of continents. Snowfall occurs exclusively in mountain regions
near the equator, at elevations of 4,900 meters or higher. Snow cover has a
large impact on the climate as well as plant, animal, and human life. It
creates a cold climate by increasing the reflection of solar radiation and
interfering with heat conduction from the ground. Small
plants are protected from the effects of the coldest winter temperatures due to
the low heat conduction; however, the late disappearance of snow in the spring
delays plant growth. When the snow melts in the spring, the runoff feeds rivers
and provides water for irrigation and other human activities. Heavy snowfalls
can impede transportation in variable temperate climates, but they provide a
firm surface for travel in remote Arctic, Antarctic, and mountainous areas
using traditional dogsleds or snowshoes, or modern snowmobiles. Snowflakes are
formed by ice crystals that have a hexagonal pattern, which is often
beautifully intricate. Because of the low heat conduction, small
plants are protected from the effects of the coldest winter temperatures;
however, the late disappearance of snow in the spring delays plant growth. When
the snow melts in the spring, the runoff feeds rivers and serves as a source of
water for irrigation and other human activities. Heavy snowfalls can impede
travel in temperate climates, but they provide a firm surface for travel in
remote Arctic, Antarctic, and mountainous areas using traditional dogsleds or
snowshoes, or modern snowmobiles. Snowflakes are formed by hexagonal ice
crystals, which are often beautifully intricate.
Graupel
Snow crystals may collide with supercooled water droplets
under certain atmospheric conditions. These droplets, with a diameter of about
10 m, can exist as liquids at temperatures as low as 40 °C, well below the
normal freezing point. When a snow crystal comes into contact with supercooled
droplets, the liquid droplets freeze onto the crystal's surface. This crystal
growth process is known as accretion. Crystals with frozen droplets on their
surfaces are frequently referred to as rimed. When this process continues to
the point where the shape of the original snow crystal is no longer
discernible, the resulting crystal is known as graupel. Meteorologists
previously referred to Graupel as soft hail. However, graupel can be
distinguished from hail by the shape and strength of the pellet as well as the
circumstances under which it falls. Hail ice forms in hard, uniform layers and
typically falls only during thunderstorms. In wintry mix situations, Graupel
forms fragile, oblong shapes and falls in place of typical snowflakes, often in
tandem with ice pellets. Graupel is also brittle enough that it usually breaks
apart when touched.
Hail
Particles of ice precipitate (hailstones). These can be
transparent, partially or completely opaque. They are typically spheroidal,
conical, or irregular in shape, and range in diameter from 5 to 50 mm. The
particles may fall from a cloud or clump together in irregular lumps. Hail
always falls in the form of showers. They are most commonly seen during severe
thunderstorms. Hailstones typically form around a nucleus, which may or may not
be in the geometric center. The nucleus can range in size from a few
millimeters to a centimeter. The nucleus is spheroidal or conical in shape and
made of ice that is usually opaque but can be transparent. Even within a single
fall, hailstones can take on a wide range of shapes and sizes. For
example, a "onion skin" formation is made up of a nucleus surrounded
by alternating layers of opaque and transparent ice. Except for very large
hailstones, which have been found to have 20 or more layers, there are usually
no more than five layers. Some hailstones have no layers and are made entirely
of transparent or opaque ice. Hailstones have a density of 0.85 g/cm3 to 0.92
g/cm3, but they can have a lower density if they have large cavities filled
with air. Some hailstones are made up of spongy ice, which is a combination of
ice, water, and air.
Sleet
The conditions that cause freezing rain are similar to
those that cause sleet: Snow falls through a layer of warm air, melts, and
becomes raindrops before being intercepted by a layer of freezing cold air just
above the Earth's surface. Because the bottom layer of cold air is thin, melted
snow does not have time to refreeze as it falls through. It falls to the ground
as liquid water—rain—and then freezes when it comes into contact with a
freezing cold surface, such as a tree branch, a road, or a bridge. Hail
is made up of ice pellets as well, but hailstones are larger than sleet
pellets. Hailstones form when thunderstorm updrafts (which are more common in
the spring and summer than in the winter) quickly lift water droplets high in
the troposphere, where they freeze at extremely low temperatures and then fall.
Sleet is formed when falling snow melts and then refreezes before hitting the
ground.
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