All forms of water falling from the sky on
the Earth's surface are called precipitation. Two types of
precipitation exist :
- The stratiform precipitations are of low
intensity but that last long because they cover a large area.
They occur in depression and in the hollows which are associated
with types stratus clouds.
- The convective precipitation which are
intensive and can be accompanied by thunderstorm
or hail but they do not last long. They occur by the convective
instability of the air that is associated with cumulus
or cumulonimbus clouds.
The precipitation can fall in three forms
depending on the temperature of the air :
- Liquid there is rain and drizzle ;
- icy found freezing rain and freezing drizzle ;
- And there is solid precipitation in the form of :
snow, snow pellets, snow grains, ice crystals, hail and
The unit of measurement of precipitation
is millimeters (mm).
1 mm = 1 litre of water / m2
1 mm = 10 tons of water / ha
DO PRECIPITATIONS FORM ?
- Rain and drizzle are formed
within clouds when water vapour condenses and creates droplets
of 0,002 to 0,02 mm. When they will reach a size of 0,10 mm
to 1 mm as they will meet with updrafts drops will be trained
and there will be precipitation. The drops of drizzle are
small, between 100 to 400 µm in diameter (1.µm.=.0,001.mm).
They fall only very low clouds. Raindrops are 0,50.mm
to about 3.mm. The drops will
be even larger than the updrafts are strong.
- Rain and freezing drizzle formed as rain
and drizzle but after the fall of the droplets before they
reach the ground in liquid form and as soon as they arrive
on a floor or an object where the temperature is below 32°F
(0°C) they freeze on contact. Freezing, the rain forms
transparent and brittle freeze.
- Snow forms when the air is quite humid
and the temperature is very low (below 5°F (-15°C))
in clouds where there is
the presence of of tiny corpuscles fluttering in the air (sand,
smoke, ash ,...). Gradually, as the crystals fall along the
cloud, the temperature must be below 32°F (0°C) so
that they do not melt and stick together to the ground forming
all around them the frozen branches called flakes. Greater
is the distance between the birth of the snowflake and the
soil, more there are chances that the snowflake becomes large.
It takes thousands of ice crystals to have a single snowflake.
Temperature and humidity are vary the form of the flakes.
Small flakes are formed at colder temperatures as the air
is not wet enough to they grow.
Between 21,2°F (-6°C) and 14°F
(-10°C) the growth of crystals is
in the axis of symmetry. There are snowflakes
of type needle (left) or column (to the right).
At 10,4°F (-12°C) the growth
of crystals occurs in the plane
of the hexagon, which gives in plate-like snowflakes.
Between 8,6°F (-13°C) and -0,4°F
(-18°C) the growth of crystals occurs in
here to see an animation of the formation of these crystals
the directions symmetry and form dendrites, which gives flakes
: Kenneth Libbrecht/Caltech.
Irregular particle (left), Button Cufflinks
(middle) and ice pellets (right)
- Hail is formed in cumulonimbus
clouds located between 1.000
km and 10.000 km altitude where
the temperature is very low, down to -40°F (-40°C).
So that the water droplets form at the base of cloud
are brought back to high altitude by rise then freeze. Then
by the weight the frozen droplets fall on the edges of the
clouds melting a bit. A comes and goes vertical repeats and
hail accumulates layers of opaque and clear ice. Hailstones
can reach the ground around 99,41 mph (160.km/h).
The diameter of a hailstone varies from 0,50 to 5 cm. With
this speed and a more or less large diameter hail can cause
serious damage. But tennis ball sized hail stones or even
19 cm in diameter have already fallen. In 1959, at the Kazakhstan,
it weighed a hailstone of 1,9 kg ! (All of these records have
been approved by WMO).
A large hailstone
a tennis ball
Hailstones are formed in cumulonimbus
Rainfall occur during a depression. The latter
has the main feature a clear dividing line between warm
and cold air mass called front, along which the temperature,
humidity, speed and wind direction changes abruptly. When
there is a front, warm air rises creating training extended
cloud layers and the occurrence of rainfall, characteristic
A video explaining the formation of
precipitation from the movement of air masses
The cold front is the surface of separation
between a warm and cold air mass of which the mass of cold
air advance to the warm air mass. Precipitation of short duration
but important occur when warm air, laden with moisture rises
up and creates clouds (cumulonimbus).
In summer, thunderstorms
and hail precipitation can take place according the importance
of cumulonimbus. In winter it's rain or snow precipitations
that take place according to air temperature. Precipitations
do not last long. Cold fronts generally move faster than warm
fronts, between 18,6 and 28 mph (30 and 45 km/h). Progressively
the front approaches the atmospheric pressure decreases logically,
the temperature decreases with the arrival of cirrus
and/or cirrostratus and/or
cumulonimbus in the sky. Then after the passage of the front
air pressure increases with low temperatures and some
cumulus. Cold fronts tend to follow closely the warm fronts
immobilized in depressions.
here to see an animation of the advanced cold front
There is formation of a warm front when a
mass of warm air moving forward to a cold air mass. As the
front approaches the atmospheric pressure decreases slightly,
the temperature increases and the wind changes direction.
In the sky will be observed, in order : cirrus,
and/or altocumulus, altostratus
and finally a nimbostratus
with rainfall. Then, when the warm front is passed it is possible
to see stratus. According
season, the precipitations will be either rain or drizzle
or hail or the sleet or snow. Precipitations last longer than
those of the cold front as they can be extended until 48 hours
after the passage of the warm front. Area precipitations can
make between 250 and 300 km long. Warm fronts move only 12,42
mph (20 km/h).
here to see an animation of advanced warm front
front is created when a cold front, faster joins a warm front
in the circulation of the disturbance. The more dense cold
air mass lifts the lighter warm air mass that eventually no
longer be in contact with the ground. It occurs when
regular rainfall and can be significant. The arrival of an
occluded front is marked by a decrease in atmospheric pressure
and a large nebulosity accented by cumulus
" fractus " at low altitude.
There are 2 types of occluded front :
- The case of occlusion to hot character,
The cup of warm occluded front
The case of occlusion in cold character, it is possible that
cumulonimbus are drowned in the
nimbostratus and cause significant precipitation.
The cup of cold occluded front
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