Transcript Stability and T

Static Stability and Cloud
Development
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Cloud Formation
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What is static stability?
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It describes what would happen to an air parcel if it
was forced upwards.
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How is air is cooled to the saturation point?
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Generally through vertical lifting
If atmosphere is stable:
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It could keep rising
It could return back to it’s original/starting height
Flat-layered type clouds form (stratus)
If atmosphere is unstable:
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Cumulus-type clouds form (puffy clouds)
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Stratus
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Cumulus
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Parcel rising and sinking in
the atmosphere
How does air get forced
upward?
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Topography
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Surface heating/convection
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Convergence of surface air
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Lifting associated with weather fronts
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Convection
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How to create an unstable
atmosphere?
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Get lots of warm air underneath cold air
Heating the surface considerably (convection)
 Wind/advection brings warm air in from another
region
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Add moisture near the surface
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moisture makes air more buoyant and able to rise
Bring in cold air to upper atmosphere
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troughs of low pressure do this
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How to create a stable
atmosphere
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Get cold air below warmer air.
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This is called a temeprature inversion, and
this prohibits convection
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Remove moisture near the surface
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A cool ocean nearby can accomplish this by
creating a marine layer
“drying out” the air makes air less buoyant and less
likely to rise
Bring in warm air to upper atmosphere
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ridges of high pressure do this
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Thunderstorms/
Lightning and
Tornadoes
Thunderstorms
Thunderstorms are generally classified into one
of two groups:
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Air Mass Thunderstorms
 These are not-so-intense storms that are
short-lived and localized.
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Mesoscale Convective Systems
 These systems lead to the generation of
numerous thunderstorms.
Air Mass Thunderstorms
These thunderstorms form within a single air mass
and are not tied to fronts or mid-latitude cyclones.
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Daily solar heating is primarily responsible for rising
air motion,
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producing rising cumulus clouds.
These storms may go through growing, mature and
dissipation stage
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In just an hour or two
Often associated with summer storms.
Air Mass Thunderstorms
•Air mass thunderstorms are “self extinguishing” –
their natural evolution forces their dissipation
An updraft is required to release the latent heat that
drives the thunderstorms. In the later stages, rainfall
will lead to air cooling and a downdraft. This largely
kills the updraft and thus the thunderstorm
Severe Thunderstorms
As the name suggests, these thunderstorms
are part of a mesoscale system. The
horizontal scale of up to a few hundred
kilometers.
The actual structure of an these storms can
vary considerably.
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Severe Thunderstorms
As the name suggests, these thunderstorms
are part of a mesoscale system. The
horizontal scale of up to a few hundred
kilometers.
The actual structure of an these storms can
vary considerably.
–mesoscale convective complexes
–squall line thunderstorms
–supercell thunderstorms
Severe Thunderstorms
Change in wind with
altitude important to
formation:
“Wind Shear”
Squall-line Thunderstorms
Severe Thunderstorms
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Characteristics
–Capable of producing large hail
–Strong gusty surface winds
–Flash floods
–Tornadoes
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Definition of Severe Thunderstorm:
–¾ inch hail or
–Surface wind gusts of 50 knots
Supercell Thunderstorms
Average # of days thunderstorms
observed
Average number of days hail observed
Lightning and Thunder
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Lightning is the discharge of electricity that
occurs within a thunderstorm.
The extreme heating associated with
lightning causes air to expand rapidly, and
produces sound waves we recognize as
thunder.
If you want to estimate the distance of an
approaching thunderstorm:
–Count the number of seconds between a lightning
strike and the sound of thunder
–For every five seconds, the storm is 1 mile away
Normal charge
separation in a mature
thunderstorm
 Charge separation not
fully understood, but
 rapid convection
(vertical motion) is
certainly important
Separation of Charge in
One theory suggests that
Clouds
the separation of charge
is due to collisions
between ice particles.
After exchanging charge,
the heavier ice crystals
settle toward the cloud
base.
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Development of lightning stroke
Cloud to Ground Lightning
Strike
Positive charge is drawn up to the
stepped leader. Usually through
the highest conducting object.
The return stroke can travel at 1 
108 ms-1 (roughly 1/3 the speed
of light.) This return stroke is
visible to the human eye.
There are commonly three or four
strokes along a common path,
which is why it is sometimes
appears like the lightning
flickers.
Why one shouldn’t
shelter under a tree
during a
thunderstorm…
Lightning Facts
It is estimated that globally 5000 people are killed by lightning
annually.
At any given moment there are ~ 1000 thunderstorms occurring
over the globe. Two thirds of all lightning strikes occur within
the tropics.
Most lightning strikes are NOT cloud to ground strikes (20%) rather cloud to cloud strikes are most common.
Aircraft are usually not damaged by lightning strikes.
Tornadoes …
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… are also called twisters or cyclones.
… are rapidly rotating winds that blow around a small
area of intense low pressure.
… come in many shapes, but mostly look like funnels
or tubes.
… often descend from large cumulonimbus clouds.
A funnel cloud is a tornado that doesn’t hit the
ground.
A waterspout is a tornado-like storm that occurs over
the ocean
Tornado Characteristics
Majority of tornadoes rotate counter-clockwise (cyclonic)
Most tornadoes only last a few minutes
Most tornadoes are ~ 100 – 600 m (300-2000 ft) in diameter
Power of the Wind
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The force, or power associated with the
wind is proportional to the cube of the wind
speed.
This means if the wind speed doubles, the
force increases by the factor of eight.
A wind speed of 100 mph is 8 times more
powerful than a 50 mph wind
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Enhanced Fujita Tornado
Damage Scale
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EF0...... Light damage. Peels surface off some roofs; some
damage to gutters or siding; branches broken off trees; shallowrooted trees pushed over. Winds of 65-85 mph
EFl...... Moderate damage. Roofs severely stripped; mobile
homes overturned or badly damaged; loss of exterior doors;
windows and other glass broken. Winds of 86-110 mph
EF2......Considerable damage. Roofs torn off well-constructed
houses; foundations of frame homes shifted; mobile homes
completely destroyed; large trees snapped or uprooted; lightobject missiles generated; cars lifted off ground. Winds of 111135 mph
Enhanced Fujita Tornado
Damage Scale
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EF3...... Severe damage. Entire stories of well-constructed houses
destroyed; severe damage to large buildings such as shopping malls; trains
overturned; trees debarked; heavy cars lifted off the ground and thrown;
structures with weak foundations blown away some distance. Winds of 136165 mph
EF4...... Devastating damage. Well-constructed houses and whole frame
houses completely leveled; cars thrown and small missiles generated.
Winds of 166-200 mph
EF5......Incredible damage. Strong frame houses leveled off foundations and
swept away; automobile-sized missiles fly through the air in excess of 100 m
(109 yd); steel reinforced concrete structure badly damaged; high-rise
buildings have significant structural deformation; incredible phenomena will
occur. Winds over 200 mph
So far only one EF5 tornado has been recorded since the Enhanced Fujita
Scale was introduced on February 1, 2007.
Tornado Formation
Formed in association with severe thunderstorms
Conditionally unstable atmosphere is important
Multiple tornadoes can come from a single storm (like a
supercell storm)
•Example: May 4-5, 2003, during a 24 hour period, there
were over 80 reported tornadoes.
Recipe for a tornado
Strong wind shear
- Can enhance rotation
- Caused mainly by jet stream
Warm moist air below dry colder
air
- Large instability
- Explosive growth due to
latent heat release
Supercell storms good candidate
for tornadoes (they already have
rotation).
Tornado occurrence?
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Tornadoes possible everywhere in the world, but
most are in the U.S. (tornado alley Texas –
Nebraska
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3/4 of the tornadoes occur from March to July,
with the maximum in _____.
May
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Jet stream is still a large influence
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late afternoon (4-6pm)
Most often occur in the _______________
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before sunrise
Least frequent ______________
Tornado incidence by state
25 year total
Why is Tornado Alley the most
likely place to get tornadoes?
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Perfect location for the mixing of air masses
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Warm, moist gulf air to the south
Cold, dry to the north/northeast
Rockies mountains to west/northwest
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Downslope flow is cool and dry
Right latitude for the polar jet stream
On Radar, the presence
of a hook echo
indicates a
mesocyclone; a region
in a thunderstorm very
likely to spawn a
tornado