Download 3Clouds and precipitation

EASC 11
Clouds and Precipitation
Fill in your note outline as you follow along with clouds
and precipitation….
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Water exists in all three states of matter in the
atmosphere – solid (hail, ice particles, snow),
liquid (rain, cloud droplets), gas (water vapour),
and changes between these states
evaporation: when liquid becomes gas
condensation: when gas becomes liquid
freezing: when liquid becomes solid
melting: when solid becomes liquid
sublimation: when solid becomes gas without
passing through liquid phase
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Water vapour is spread throughout the
troposphere by convection currents and
winds – there is little water vapour above
the troposphere because there are no
rising air currents to carry water vapour
Molecules of liquid water are always in
motion
Air’s capacity for holding water vapour
roughly doubles for every 11° C increase
in temperature
Saturated air has a relative humidity of
100%
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Specific humidity is the amount of water vapour
actually in the air
Relative humidity compares the actual amount of
water vapour in the air (specific humidity) with the
maximum amount of water vapour the air could
hold at that temperature
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Relative humidity is calculated as follows:
Relative humidity = specific humidity x 100%
maximum capacity
(@ that temp)
E.g.: if specific humidity = 11
maximum capacity = 22
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RH = 11/22 x 100% = 50%
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Instruments used to measure relative humidity
include a hygrometer and psychrometer
• Clouds are simply high mists, fogs, or hazes
• Clouds form when water vapour is cooled below
its dewpoint – water condenses on
“condensation nuclei”, which are tiny particles in
the air such as dust
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If there are no condensation nuclei, air
can be cooled below the dewpoint with
no condensation, and air is said to be
supersaturated
The shape of clouds show how air is
moving in stratiform (layered) clouds, air
movement is mainly horizontal; in
cumuliform clouds, air movement is
mainly vertical
At temperatures above 0°C clouds consist
of only liquid water; from 0° to -18°C
clouds consist of liquid water and ice
below -18°C clouds consist only of snow
and ice
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There are four cloud locations or “families”
high clouds
middle clouds
low clouds
Clouds with vertical development
There are four cloud structures
stratus – layers or sheets
cumulus – clouds with vertical development
cirrus – thin, feathery, tufted
“nimbus” clouds are literally rain clouds
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Fog is a cloud that cannot fly, and forms when a
surface layer of air cools below the dewpoint and
water vapour condenses at the surface of the
Earth
Radiation fog (ground fog) forms when the night time
sky is clear after a warm day: the ground radiates
heat, winds mix the cold air with surface air, and
this layer cools below the dewpoint (occurs often
in late summer/early fall)
Advection fog forms when warm moist air blows off
water and over much colder land – this is the fog
common in the Maritimes (e.g., Newfoundland &
Labrador) and in California (e.g., San Francisco)
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Precipitation is the falling of any form of water to
Earth’s surface; occurs when cloud droplets grow into
drops heavy enough to fall
In warm clouds condensation forms tiny droplets then
grow by bumping into and combining with other
droplets
In ice processes, supercooled water droplets
evaporate and are deposited on ice crystals, which
will fall when they become heavy enough
If the temperature in the lower part of the cloud is
warmer, the ice crystals melt and become rain (may
be sleet or freezing rain)
Rainfall is measured by a rain guage in mm –
measures the depth of water rain would leave if it
didn’t evaporate or soak into the ground
Snowfall is measured in cm or inches
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1.
Cloud Classification
Cloud Family
Average Height
Types
Features
Weather
High
Above
6000m
(20000 ft)
Cirrus
Cirrostratus
Cirrocumulus
Feathery, tufted. Good
Thin, smooth
Rain, snow
Granular
Good
patches
Middle
2000-6000m
(6500-20000 ft)
Altostratus
Altocumulus
High layers
Layers of puffs
“fish scales”
Rain/snow
Rain coming
Low
Below 2000m
(6500 ft)
Stratocumulus
Stratus
Nimbostratus
Low layers
Low sheets
Dark, grey
circles
Weak rain
Drizzle
Steady rain
Vertical
Development
500-18000m
(1600-60000 ft)
Cumulus
Cumulonimbus
Thick vertical
masses
Huge
“cauliflower” top
Showers
Thundershower
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2.
Types of Precipitation
Type
Size
H20
Description
Mist
0.005 to 0.05 mm
Liquid
From stratus clouds
Drizzle
<0.5 mm
Liquid
From stratus clouds
Rain
0.5 to 5 mm
Liquid
Nimbostratus or
cumulonimbus
Sleet
0.5 to 5mm
Solid
Frozen raindrops
Glaze
Layers 1mm to 2 cm
thick
Solid
Supercooled
raindrops freeze on
contact with objects
Rime
Variable
accumulations
Solid
Ice feathers form from
supercooled cloud/fog
Snow
1mm to 2cm
Solid
Crystals of frozen
water
Hail
5mm to 10cm+
Solid
Hard round pellets
Gaupel
2 to 5mm
Solid
“soft hail”