Geoscience Reference
In-Depth Information
E+HDSHGRUFXPXOLIRUPFORXGV
Common to
both cloud
types
D/D\HURUVWUDWLIRUPFORXGV
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Large
cloud
droplets
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Ice nuclei
Strong
updraughts
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Ice crystal
Coalescence
with smaller
drops
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Large
droplet
grows
slowly
by
coalescence
in weak
updraughts
Large
dendritic
crystal
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Soft hail
Large
liquid
water
concentrations
Weak
updraughts
Low
liquid water
concentrations
Large drop
breaks up
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Snowflake
melting
to
raindrop
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Freezing level
Continuous
growth by
coalescence
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Raindrop
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More unstable
drops
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Cloud base
Condensation level
Drizzle /
small raindrop
Collision -
coalesence
method
Collision -
coalesence
method
Bergeron -
Findeisen
method
Many large
raindrops
these processes operate. Cloud thickness and updraught
speed are largely dependent upon instability and conver-
gence in the atmosphere. Precipitation has been classified
in terms of the factor which gives rise to the upward
movement, so let us have a look at this in a little more
detail.
the United Kingdom about 20 per cent of the annual
rainfall is by convection, with the proportion increasing
towards the south and east. This convectional rain may
be the result of cold air moving over a warmer ground
surface or the result of strong surface heating; both
situations will give the steep lapse rates characteristic of
instability and convection.
Hail is a type of precipitation composed of spheres or
irregular lumps of ice and particularly associated with
convectional precipitation. It falls in narrow bands
associated with cumulonimbus clouds and so frequently
misses the observing stations. However, the destruction it
can produce is dramatic. Crops can be torn to shreds,
glasshouses ruined and even cars dented by the weight of
half a kilogram or more of ice falling from the skies.
Splitting open a large hailstone will show that it is
composed of alternating layers of clear and opaque
complex movements within the cloud, being swept up to
the higher, colder parts of the cloud several times. When
this happens, any moisture condensing on the stone will
freeze instantly, including any trapped air, producing
opaque ice. At lower levels in the cloud condensed water
takes a longer time to freeze. Air bubbles can escape,
leaving a layer of clear ice when it eventually freezes. The
alternating layers of clear and opaque ice indicate the
Origins of precipitation
Convectional precipitation
The spontaneous rising of moist air due to instability is
known as convection. We have seen that upward-growing
clouds are associated with convection. Since the
updraughts are usually strong, cooling of the air is rapid
and lots of water can be condensed quickly. Collisions and
coalescence are likely to be frequent, so the larger droplets
rapidly increase in size. Eventually, growing larger and
heavier, the droplets overcome the lift provided by the
updraught, and they start to fall through the cloud into
the clear air beneath. As the volume of water in these
big drops is large relative to their surface area, little
evaporation takes place in the non-saturated air below the
cloud. At the ground there will be a burst of heavy rain as
the shower passes.
Unstable air which favours convectional rain is most
frequently found in warm and humid areas, but even in
