Chapter 4 (cont.)

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Chapter 4 (cont.)

• Precipitation

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How does precipitation form? Why do some clouds
generate precipitation and others do not? What
factors determine the various types of
precipitation?
To answer these questions, lets start by
examining the sizes of typical cloud and
raindrops.
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1. Growth by condensation
Fog and cloud drops form by the aggregation of
water vapor molecules by condensation on
hygroscopic cloud condensation nuclei (CCN), a
sub-set of the aerosol.
Initially, small droplets grow rapidly in an air
parcel that is cooling, but as they become
larger, their rate of growth decreases rapidly,
so that this process (condensational growth)
becomes too slow to produce raindrop size water
drops. (It would take days!) The excess water
vapor grows mainly on other activated CCN
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2. Growth by collision and coalescence.
In this process, large drops fall through smaller
drops, and collect many of those in their path,
thereby, growing even larger.
This mechanism works because large drops fall
more rapidly than small drops.
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What determines the fall speed of drops?
There are two forces that act on a falling
object, namely (1) the force of gravity which
tends to accelerate the object towards the
Earths surface, and (2) the frictional drag
caused by the air resistance.
When these two forces are exactly balanced, the
object falls at a steady, constant speed, the
terminal velocity.
Terminal velocities of larger drops are greater
than those of small drops.
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Collision and Coalescence Mechanism of Raindrop
Growth
Updrafts in clouds tend to hold falling drops
aloft. Drops with terminal velocities less than
the updraft velocity are swept up higher into the
cloud.
Drops only fall from the cloud when their
terminal velocity exceeds the updraft velocity.
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Collision and Coalescence Mechanism of Raindrop
Growth
Growth by collision and coalescence is enhanced
by
A wide spectrum of drop sizes, which therefore
implies a range of drop terminal velocities.
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Collision and Coalescence Mechanism of Raindrop
Growth
Growth by collision and coalescence is also
enhanced by
A high concentration of drops.
Strong cloud updrafts which hold the drops aloft
in the cloud and give them more time to grow.
The collision and coalescence process explains
the production of precipitation in warm clouds.

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Only relatively shallow clouds that do not extend
high into the troposphere contain nothing but
water drops. e.g. stratus cloud
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Deep clouds extend up into regions of the
troposphere where temperatures are well below
freezing. These cold glaciated clouds
consequently contain ice, as well as supercooled
water drops.
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Ice Formation in Cold Clouds
Small, pure water drops do NOT freeze at 0C.
As the temperature cools below 0C, larger water
drops tend to freeze first, before the smaller
drops.
As smaller water drops require colder
temperatures to freeze, the drop size of
supercooled water drops in a cold cloud tends to
decrease with height.
Spontaneous nucleation of ice (homogeneous
freezing) only occurs at very low temperatures.
Most ice formation in clouds results from the
action of Ice Nuclei.
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The saturation vapor pressure increases rapidly
with increasing temperature
How does the saturation vapor pressure vary with
temperature below 0?C?
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Ice Particles and the Formation of Precipitation
svp (ice) lt svp (liquid)
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vapor
3. The Bergeron Process
90 RH
When supercooled liquid water drops and ice
particles co-exist in the same air parcel, the
liquid drops near ice feel the drier shell
around ice, and they start to evaporate. This
water vapor deposits on the ice particles as fast
as it evaporates from the water drops. The net
result is that the ice crystals grow at the
expense of the water drops. This ice crystal
growth process, which promotes the rapid growth
of ice crystals, is known as the Bergeron
process. This process is the dominant
precipitation forming mechanism in most
places.
100 RH
90 RH
100 RH
90 RH
100 RH
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Ice Particles and the Formation of Precipitation
Ice particles sometimes collide and stick
together, forming a larger particle. This
process is called aggregation and leads to the
formation of ice crystal aggregates, commonly
known as snowflakes.
Some collisions result in splintering of the ice
crystals, thereby forming ice crystal fragments,
which in turn act as embryonic ice crystals.
Supercooled cloud droplets sometimes collide with
ice crystals and snowflakes. They freeze, and
stick to their surface. The resulting ice crystal
is said to be rimed.
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Rime Ice
Rime forms when supercooled water fog and cloud
drops impact a cold object. It not only leads to
the growth of graupel and hail but can form on
objects at the surface.
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Types of Precipitation Rain
Falling raindrops are usually wrongly depicted by
artists as being shaped like teardrops.
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Types of Precipitation Rain
Virga are falling streaks of drizzle and rain
that evaporate before reaching the ground.
Donna Charlevoix U. of Illinois
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Types of Precipitation Rain
Drizzle is composed of drops of diameter less
than 0.5 mm.
Raindrops have diameters between 0.5 mm and about
5 mm.
Showers fall from convective (cumiliform) clouds.

Continuous rain invariably falls from stratiform
clouds (e.g., nimbostratus).
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Types of Precipitation Snow and Ice
The type of crystal that forms reflects its
growth environment (primarily temperature). Plate
s grow at 0? to -4?C, -10? to -12?C, and -16? to
-22?C Dendrites grow at -12? to -16?C
Columns at -4? to -10?C and -22 to -50?C
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Sector Plates
Rasmussen Libbrecht, 2003
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Plates and Sector Plates
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Dendrites
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Dendrites
Rasmussen Libbrecht, 2003
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More dendrites
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More dendrites
Rasmussen Libbrecht, 2003
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Columns, Needles, and Rosettes
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Rosettes
Rasmussen Libbrecht, 2003
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Types of Precipitation Snow and Ice
Snow Grains are the ice equivalent of drizzle.
They are opaque, white ice particles of diameter
lt1 mm. They are usually associated with stratus
clouds.
5 km
Snow pellets (graupel) are opaque, white ice
particles formed by riming. Their diameter is 2
5mm. They fall from cumiliform clouds.
3 km
1 km
Hail and hailstones are transparent or partially
opaque ice particles of diameter 5 140 mm.
They form by riming of both graupel and frozen
drops. To grow larger than 5 mm, a hailstone
must make several up-and-down cycles in the cloud.
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Diameter 14 cm (5.5) Weight 0.7 kg (24 oz)
Cumulonimbus are the only clouds with
sufficiently strong updrafts to produce hail.
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Hail Damage
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Types of Precipitation Snow and Ice
When below-cloud temperatures are below freezing,
falling snow reaches the ground.
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Types of Precipitation Snow and Ice
When surface temperatures are significantly above
freezing, falling snow melts about 1000 ft (300
m) below the freezing level, reaching the surface
as rain. This reflects the time required to
acquire sufficient latent heat to effect melting.
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Types of Precipitation Snow and Ice
Snow Flurries are weak, intermittent light snow
showers that fall from developing cumulus clouds.
Snow Squalls are intermittent heavy snow showers
falling from cumulus clouds.
Steady snowfall is produced in winter by
nimbostratus clouds.
Blizzards are characterized by low temperatures,
high winds, large amounts of falling and blowing
snow, and poor visibility.
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Types of Precipitation Freezing Rain
Freezing rain occurs when rain or drizzle freezes
on contact with a frozen surface.
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Freezing rain mechanism

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Types of Precipitation Freezing Rain
Freezing rain is hazardous because it coats
surfaces with a very slippery layer of clear ice.
The ice can bring down power lines and cause
both personal and other accidents.
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Types of Precipitation Freezing Rain
Ice storms can break down trees, cause livestock
to slip and fall, and can even cause suffocation
due to ice clogging their nostrils. Birds have
been found frozen to trees, with their beaks
frozen shut, and unable to fly due to the ice
build-up on their wings.
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Types of Precipitation Freezing Rain
Freezing rain occurs in the Front Range area when
cold-air is dammed up against the mountains
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Types of Precipitation Sleet
Sleet consists of small ice pellets formed by the
re-freezing of small drops and/or partially
melted snowflakes. Re-freezing occurs when the
drops fall into a deep cold layer at the surface.