Transcript Chapter 5 - Weather Underground

Chapter 5: Cloud
Development and
Precipitation
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Atmospheric stability
Determining stability
Cloud development and stability
Precipitation processes
Precipitation types
Measuring precipitation
Atmospheric Stability
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We know that air rises, cools and
condenses into clouds
Atmospheric stability refers to condition of
equilibrium
We will refer to stable and unstable
environments
Atmospheric Stability
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A stable environment is one where the original
equilibrium is maintained (things return to where
they were
An unstable environment is one where things
move away from their original position
• Stability does not control
whether air will rise or sink.
Rather, it controls whether
rising air will continue to rise
or whether sinking air will
continue to sink.
Atmospheric Stability
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Remember, when air rises, it moves into an area
of lower pressure, expands, and cools. Sinking
air is the opposite
Adiabatic process – a process in which
exchanges no heat with its outside surroundings
(how can this happen)
Lapse rate – change of temperature with height
Dry adiabatic lapse rate - 10°C/1000m or
5.5°F/1000 feet
Atmospheric Stability
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But what happens when air cools (humidity?). If
we get condensation, then heat is released and
the process is no longer adiabatic.
Thus, we have the moist adiabatic lapse rate. Is
it less or more than the dry adiabatic lapse rate?
Moist adiabatic lapse rate - 6°C/1000 m or
3.3°F/1000 feet. Varies greatly with different
moisture content.
A Stable Atmosphere
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So how do we determine stability?
We raise a parcel and compare it to its
surroundings. If a parcel is colder than its
surroundings, it is more dense and will sink
(stable)
If a parcel is warmer, it is less dense and will rise
(unstable)
Environmental lapse rate – the rate at which air
changes in temperature with height
A Stable Atmosphere
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Absolute stability –
when a parcel is
colder than the
environment at all
levels (for both dry
and moist
adiabats)
If forced to rise,
clouds will have flat
tops and be thin
A Stable Atmosphere
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Atmosphere will be
stable with the
environmental lapse
rate is small
So when the air aloft
warms and surface
cools
This can happen during
radiational cooling
Influx of cold air
Air moving over cold
surface
A Stable Atmosphere
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Most stable
around
sunrise
Layer of fog
or haze can
be evidence
of a stable
atmosphere
Because stable atmospheres resist vertical movement, they trap
pollutants near the ground and can cause dangerous air quality
An Unstable Atmosphere
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Absolute instability –
occurs when air
parcels are warmer
than their
surroundings (parcel
will rise)
Warming of surface
air helps atmos to
becoming unstable
An Unstable Atmosphere
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Unstable atmospheres
occur when the
environmental lapse rate
steepens
Destabilizing processes:
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Solar heating of the surfance
Warm air brought by wind
Air moving over warm
surface
Superadiabatic lapse rate –
when environmental lapse
rate exceeds the dry
adiabatic lapse rate
Conditionally Unstable Air
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Conditional
instability – occurs if
we force a cool
parcel to a part of
the environment
where it condenses
and becomes
warmer
Level of free
convection – point
at which we don’t
need to force it up
anymore
Conditionally Unstable Air
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Look at the
environmental lapse
rate and make a
hypothesis of what
lapse rate you need
in order to have
conditionally
unstable air
You need an
environmental lapse
rate between the
moist and dry
adiabatic lapse rate
Average lapse rate in the troposphere is
6.5°C/1000 m. What is the average stability?
Convection and Clouds
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Primary ways clouds form:
Surface heating and convection
 Topographical uplift
 Convergence
 Lifting along a weather front
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Convection and Clouds
Convection and Clouds
Topography and Clouds
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Orographic uplift – forced lifting along a
topographic barrier
Rain shadow - Due to frequent westerly winds, the western
slope of the Rocky Mountains receives much more
precipitation than the eastern slope.
Convection and Clouds
Collision and Coalescence
Process
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How do raindrops
become large
enough to fall?
Condensation
alone is just not
enough
Collision and Coalescence
Process
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One process is
collision and
coalescence
Occurs in warm
clouds (tops
warmer than 0°C
• A typical cloud droplet
falls at a rate of 1
centimeter per second.
At this rate it would take
46 hours to fall one mile.
Collision and Coalescence
Process
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Clouds must have
varying droplet
sizes
Terminal velocity –
the point at which
gravity equals the
air resistance (falls
at constant speed
Collision and Coalescence
Process
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Larger drops
merge with smaller
drops in a
processing called
coalescence
Important factor is
how long the
droplet stays in the
cloud
Stepped Art
Fig. 5-9, p. 116
Ice Crystal Process
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Ice crystal or
Bergeron process
Occurs in cold
clouds – clouds
with temperatures
that drop to below
freezing
Bergeron process
states that liquid
and ice droplets
must co-exist in
clouds
Ice Crystal Process
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Supercooled water
droplets – droplets
that occur as liquid
below freezing
(middle portion of a
thunderstorm cloud)
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Occurs when there
are few ice nuclei
Ice Crystal Process
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Ice crystals form at the expense of water
droplets
Suppose we have one super cooled droplet and
one ice crystal in a saturated environment
Since saturated, number
molecules evaporating
and condense MUST be
equal!
More vapor molecules
over liquid because it is
easier to escape liquid to
vapor than ice to vapor
Ice Crystal Process
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Since more liquid molecules are going to vapor, it
takes more vapor molecules to condense to liquid
to keep the equilibrium (saturation)
Thus, the saturation
vapor pressure above
water is greater than that
above ice
This causes vapor
molecules to move
towards the ice
Removal of water vapor
cause water droplet to
shrink (not in equilibrium)
Ice Crystal Process
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Ice crystals may also grow larger by accretion
– when ice crystals collide with supercooled
dropets
Fig. 5-22, p. 124
Stepped Art
Fig. 5-22, p. 124
Cloud Seeding and
Precipitation
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Cloud seeding – pumping nuclei into
clouds to promote precipitation
Silver iodide – found to be a good ice
nuclei
• It is very difficult to determine whether a cloud seeding
attempt is successful. How would you know whether
the cloud would have resulted in precipitation if it hadn’t
been seeded?
Precipitation in Clouds
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Accretion
Ice crystal process
Rain
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Rain – falling droplet size greater than or
equal to 0.5 mm (0.02 in)
Drizzle – falling droplet size less than 0.5
mm (0.02 in)
Virga – rain that evaporates before it hits
the ground
Shower – brief downpours possibility as a
result of updrafts
Snow
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Snow – precipitation that falls to the
ground as ice crystals. Much precipitation
actually starts as snow
Fallstreaks – ice crystals and snowflakes
that fall from cirrus clouds
Dendrite – most common type of fernlike
snowflake
Blizzard – combination of strong winds,
low temperatures, and fine, dry snow
• Snowflake shape depends on both temperature and
relative humidity.
Sleet and Freezing Rain
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Sleet – precipitation that has thawed and frozen again
Freezing rain – supercooled droplets that reach the
ground and freeze on contact
Rime – accumulation of white ice that occurs when
supercooled droplets hit something
Snow Grains and Snow
Pellets
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Snow grains
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Solid
equivalent of
drizzle
Snow pellets
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Like grains,
but bounce
Hail
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Hail develops as droplets are uplifted in the
clouds above the freezing level
The droplets grows by accretion until it is large
enough for gravity to take it to the ground
Stepped Art
Fig. 5-35, p. 134
Instruments
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Standard rain gauge
Tipping bucket rain gauge
• It is difficult to capture rain in a bucket when the
wind is blowing strongly.
Doppler Radar and
Precipitation
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Radar
Doppler radar
Can detect
precipitation
intensity
 Can detect
movement
away or to
the radar
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Stepped Art
Fig. 5-39, p. 135