Water that falls to the surface from the
atmosphere in liquid or fluid form. Whether it falls as rain,
drizzle, fog, snow, sleet, freezing rain, or hail, it is measured
as a liquid-water equivalent. The types and amounts of precipitation
in different parts of the world vary greatly, from
places that have never had any measurable precipitation to
places that regularly receive hundreds of centimeters of rain
per year. Precipitation is strongly seasonal in some places,
with dry and wet seasons, and distributed more regularly in
other climates.
Rain is liquid precipitation with droplets greater than
0.02 inches (0.5 mm) in diameter, whereas drizzle has
droplets of 0.008–0.02 inch (0.2–0.5 mm) in diameter. Fog is
a cloud whose base is at the surface, and it has smaller particles
that only truly become precipitation when wind drives
them against surfaces or the ground. Freezing rain and drizzle
both fall in liquid form but freeze upon hitting cold surfaces
on the ground, creating a frozen coating known as glaze.
Sleet consists of frozen ice pellets less than 0.2 inch (5 mm) in
diameter, and hail consists of larger transparent to opaque
particles that typically have diameters of 0.2–0.8 inch (5
mm–2 cm), but sometimes as large as golf balls or rarely even
grapefruits. Snow is frozen precipitation consisting of complex
hexagonal ice crystals that fall to the ground.
In tropical regions and temperate climates in warmer
parts of the year, most precipitation falls as rain and drizzle.
Heavy rain is defined as more than 0.16 inch (4 mm) of precipitation
per hour, moderate rain falls at 0.16–0.02 inch (4
mm–0.5 mm) per hour, and light rain (commonly called drizzle)
is less than 0.02 inch (0.5 mm) per hour. Frequent and
steady rains characterize some regions; others are characterized
by infrequent but intense downpours, including thunderstorms
that may shed hailstones. At high elevation, high
latitudes, and in midlatitudes in colder months, most precipitation
falls as frozen solid particles. Most frozen precipitation
falls as snow that typically has a water equivalent of onetenth
the amount of snow that falls (i.e., 10 cm of snow
equals 10 mm of rain). More freezing rain and sleet than
snow characterize some regions, particularly coastal regions
influenced by warm ocean currents.
Uplift within clouds or larger scale systems are generally
necessary to initiate the formation of water droplets that
become precipitation. Convection cells in thunderheads, air
forced over mountains, zones of convergence along fronts,
and cyclonic systems can all produce dramatic uplift and
induce precipitation. One of the biggest obstacles that must
be overcome for precipitation to form is that very small water
(or ice droplets) that are separated by very wide spaces must
coalesce to form particles large enough to fall as precipitation.
Additionally, the particles must overcome the forces of
evaporation as they rise or fall through unsaturated air in
order to make it to the ground. Rapid lateral and vertical
motions in clouds, leading to collisions between particles, aid
the coalescence of particles, and gravity then accelerates particles
to the ground with larger particles initially falling faster
than smaller ones since they are less affected by updrafts.
Large particles therefore tend to collide with and incorporate
smaller particles. Frozen particles form in the upper levels of
vertically extensive cloud systems and may alternately fall
into and rise out of lower levels, where they partially melt,
grow, and rise on updrafts. Such cycling can produce relatively
large particles that may fall as precipitation.
See also CLIMATE; CLOUDS; HURRICANE.
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