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Static Stability and Cloud Static Stability and Cloud DevelopmentDevelopment

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Cloud FormationCloud Formation What is static stability?What is static stability?

It describes what would happen to an air parcel if It describes what would happen to an air parcel if it was forced upwards.it was forced upwards.

It could keep rising It could keep rising It could return back to it’s original/starting heightIt could return back to it’s original/starting height

How is air is cooled to the saturation point?How is air is cooled to the saturation point?

If atmosphere is stable:If atmosphere is stable:

If atmosphere is unstable:If atmosphere is unstable:

Generally through vertical liftingGenerally through vertical lifting

Flat-layered type clouds form (stratus)Flat-layered type clouds form (stratus)

Cumulus-type clouds form (puffy clouds)Cumulus-type clouds form (puffy clouds)

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StratusStratus

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CumulusCumulus

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Parcel rising and sinking in Parcel rising and sinking in the atmospherethe atmosphere

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How does air get How does air get forced upward?forced upward?

TopographyTopography

Surface heating/convectionSurface heating/convection

Convergence of surface airConvergence of surface air

Lifting associated with weather frontsLifting associated with weather fronts

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ConvectionConvection

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How to create an How to create an unstable atmosphere?unstable atmosphere?

Get lots of warm air underneath cold airGet lots of warm air underneath cold air Heating the surface considerably (convection)Heating the surface considerably (convection) Wind/advection brings warm air in from another Wind/advection brings warm air in from another

regionregion

Add moisture near the surfaceAdd moisture near the surface moisture makes air more buoyant and able to risemoisture makes air more buoyant and able to rise

Bring in cold air to upper atmosphereBring in cold air to upper atmosphere troughs of low pressure do thistroughs of low pressure do this

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How to create a stable How to create a stable atmosphereatmosphere

Get cold air below warmer air.Get cold air below warmer air. This is called a temeprature inversion, and This is called a temeprature inversion, and

this prohibits convectionthis prohibits convection A cool ocean nearby can accomplish this by A cool ocean nearby can accomplish this by

creating a marine layercreating a marine layer

Remove moisture near the surfaceRemove moisture near the surface ““drying out” the air makes air less buoyant and drying out” the air makes air less buoyant and

less likely to riseless likely to rise

Bring in warm air to upper atmosphereBring in warm air to upper atmosphere ridges of high pressure do thisridges of high pressure do this

Thunderstorms/Thunderstorms/

Lightning and Lightning and

TornadoesTornadoes

Thunderstorms are generally classified Thunderstorms are generally classified into one of two groups:into one of two groups:

Air Mass ThunderstormsAir Mass Thunderstorms

Mesoscale Convective SystemsMesoscale Convective Systems

ThunderstormsThunderstorms

These are not-so-intense storms that are These are not-so-intense storms that are

short-lived and localized. short-lived and localized.

These systems lead to the generation of These systems lead to the generation of

numerous thunderstorms.numerous thunderstorms.

These thunderstorms form within a single air These thunderstorms form within a single air mass and are not tied to fronts or mid-latitude mass and are not tied to fronts or mid-latitude cyclones.cyclones.

Daily solar heating is primarily responsible for Daily solar heating is primarily responsible for rising air motion, rising air motion, producing rising cumulus clouds. producing rising cumulus clouds.

These storms may go through growing, These storms may go through growing, mature and dissipation stagemature and dissipation stage In just an hour or twoIn just an hour or two

Often associated with summer storms.Often associated with summer storms.

Air Mass Air Mass ThunderstormsThunderstorms

AirAir MassMass

ThunderstormsThunderstorms•Air mass thunderstorms are “self extinguishing” – Air mass thunderstorms are “self extinguishing” –

their natural evolution forces their dissipationtheir natural evolution forces their dissipation

An updraft is required to release the latent heat that An updraft is required to release the latent heat that drives the thunderstorms. In the later stages, rainfall drives the thunderstorms. In the later stages, rainfall will lead to air cooling and a downdraft. This largelywill lead to air cooling and a downdraft. This largelykills the updraft and thus the thunderstormkills the updraft and thus the thunderstorm

As the name suggests, these As the name suggests, these thunderstorms are part of a mesoscale thunderstorms are part of a mesoscale system. The horizontal scale of up to a system. The horizontal scale of up to a few hundred kilometers. few hundred kilometers.

The actual structure of an these storms The actual structure of an these storms can vary considerably. can vary considerably.

SevereSevere ThunderstormsThunderstorms

As the name suggests, these As the name suggests, these thunderstorms are part of a mesoscale thunderstorms are part of a mesoscale system. The horizontal scale of up to a system. The horizontal scale of up to a few hundred few hundred kilometerskilometers. .

The actual structure of an these storms The actual structure of an these storms can vary considerably. can vary considerably.

SevereSevere ThunderstormsThunderstorms

–mesoscale convective complexes mesoscale convective complexes –squall line thunderstormssquall line thunderstorms–supercell thunderstormssupercell thunderstorms

SevereSevere ThunderstormsThunderstorms

Change in wind with Change in wind with altitude important to altitude important to formation:formation:““Wind Shear”Wind Shear”

Squall-line ThunderstormsSquall-line Thunderstorms

Characteristics Characteristics

Definition of Severe Thunderstorm:Definition of Severe Thunderstorm:

SevereSevere ThunderstormsThunderstorms

–Capable of producing large hailCapable of producing large hail–Strong gusty surface windsStrong gusty surface winds–Flash floodsFlash floods–TornadoesTornadoes

–¾ inch hail ¾ inch hail oror–Surface wind gusts of 50 knotsSurface wind gusts of 50 knots

Supercell ThunderstormsSupercell Thunderstorms

Average # of days thunderstorms Average # of days thunderstorms observedobserved

Average number of days hail observedAverage number of days hail observed

Lightning and ThunderLightning and Thunder Lightning is the discharge of electricity Lightning is the discharge of electricity

that occurs within a thunderstorm.that occurs within a thunderstorm. The extreme heating associated with The extreme heating associated with

lightning causes air to expand rapidly, lightning causes air to expand rapidly, and produces sound waves we and produces sound waves we recognize as thunder.recognize as thunder.

If you want to estimate the distance of If you want to estimate the distance of an approaching thunderstorm:an approaching thunderstorm:

–Count the number of seconds between a lightning Count the number of seconds between a lightning

strike and the sound of thunderstrike and the sound of thunder–For every five seconds, the storm is 1 mile awayFor every five seconds, the storm is 1 mile away

Normal charge Normal charge separation in a mature separation in a mature thunderstormthunderstorm

Charge separation not Charge separation not fully understood, but fully understood, but

rapid convection rapid convection (vertical motion) is (vertical motion) is certainly importantcertainly important

Separation of Charge Separation of Charge in Cloudsin CloudsOne theory suggests One theory suggests

that the separation of that the separation of

charge is due to charge is due to

collisions between ice collisions between ice

particles. After particles. After

exchanging charge, exchanging charge,

the heavier ice the heavier ice

crystals settle toward crystals settle toward

the cloud base.the cloud base.

++++

--

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Development of lightning Development of lightning strokestroke

Cloud to Ground Lightning Cloud to Ground Lightning StrikeStrike

Positive charge is drawn up to the Positive charge is drawn up to the

stepped leader. Usually through stepped leader. Usually through

the highest conducting object.the highest conducting object.The return stroke can travel at 1 The return stroke can travel at 1

10108 8 msms-1-1 (roughly 1/3 the speed of (roughly 1/3 the speed of light.) This return stroke is light.) This return stroke is visible to the human eye.visible to the human eye.

There are commonly three or four There are commonly three or four strokes along a common path, strokes along a common path, which is why it is sometimes which is why it is sometimes appears like the lightning appears like the lightning flickers.flickers.

Why one shouldn’t Why one shouldn’t shelter under a tree shelter under a tree during a during a thunderstorm…thunderstorm…

It is estimated that globally 5000 people are killed by It is estimated that globally 5000 people are killed by lightning annually.lightning annually.

At any given moment there are ~ 1000 thunderstorms At any given moment there are ~ 1000 thunderstorms occurring over the globe. Two thirds of all lightning occurring over the globe. Two thirds of all lightning strikes occur within the tropics.strikes occur within the tropics.

Most lightning strikes are NOT cloud to ground strikes Most lightning strikes are NOT cloud to ground strikes (20%) - rather cloud to cloud strikes are most (20%) - rather cloud to cloud strikes are most common.common.

Aircraft are usually not damaged by lightning strikes.Aircraft are usually not damaged by lightning strikes.

Lightning FactsLightning Facts

TornadoesTornadoes … …

… … are also called are also called twisterstwisters or or cyclonescyclones. . … … are rapidly rotating winds that blow around a are rapidly rotating winds that blow around a

small area of small area of intenseintense lowlow pressurepressure. . … … come in many shapes, but mostly look like funnels come in many shapes, but mostly look like funnels

or tubes. or tubes. … … often descend from large often descend from large cumulonimbuscumulonimbus cloudsclouds.. A A funnelfunnel cloudcloud is a tornado that doesn’t hit the is a tornado that doesn’t hit the

ground.ground. A A waterspoutwaterspout is a tornado-like storm that occurs is a tornado-like storm that occurs

over the oceanover the ocean

Tornado Tornado CharacteristicsCharacteristics

Majority of tornadoes rotate Majority of tornadoes rotate counter-clockwisecounter-clockwise (cyclonic) (cyclonic)

Most tornadoes only last Most tornadoes only last a few minutesa few minutes

Most tornadoes are ~ 100 – 600 m (300-2000 ft) in diameterMost tornadoes are ~ 100 – 600 m (300-2000 ft) in diameter

Power of the WindPower of the Wind

The force, or power associated with The force, or power associated with the wind is proportional to the cube the wind is proportional to the cube of the wind speed. of the wind speed.

This means if the wind speed This means if the wind speed doubles, the force increases by the doubles, the force increases by the factor of eight. factor of eight.

A wind speed of 100 mph is 8 times A wind speed of 100 mph is 8 times more powerful than a 50 mph windmore powerful than a 50 mph wind

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Enhanced Fujita Tornado Enhanced Fujita Tornado Damage ScaleDamage Scale

EF0EF0...... Light damage. Peels surface off some roofs; some ...... Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Winds of 65-85 mphshallow-rooted trees pushed over. Winds of 65-85 mph

EFlEFl...... Moderate damage. Roofs severely stripped; mobile ...... Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; homes overturned or badly damaged; loss of exterior doors; windows and other glass broken. Winds of 86-110 mphwindows and other glass broken. Winds of 86-110 mphEF2EF2......Considerable damage. Roofs torn off well-......Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off uprooted; light-object missiles generated; cars lifted off ground. Winds of 111-135 mphground. Winds of 111-135 mph

Enhanced Fujita Tornado Enhanced Fujita Tornado Damage ScaleDamage Scale

EF3EF3...... Severe damage. Entire stories of well-constructed houses destroyed; ...... Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance. Winds of 136-165 mphweak foundations blown away some distance. Winds of 136-165 mph

EF4EF4...... Devastating damage. Well-constructed houses and whole frame ...... Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated. Winds houses completely leveled; cars thrown and small missiles generated. Winds of 166-200 mphof 166-200 mph

EF5EF5......Incredible damage. Strong frame houses leveled off foundations and ......Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); steel reinforced concrete structure badly damaged; high-rise (109 yd); steel reinforced concrete structure badly damaged; high-rise buildings have significant structural deformation; incredible phenomena will buildings have significant structural deformation; incredible phenomena will occur. Winds over 200 mphoccur. Winds over 200 mph

So far only one EF5 tornado has been recorded since the Enhanced Fujita So far only one EF5 tornado has been recorded since the Enhanced Fujita Scale was introduced on February 1, 2007.Scale was introduced on February 1, 2007.

Tornado FormationTornado FormationFormed in association with Formed in association with severe thunderstormssevere thunderstorms

Conditionally unstable atmosphereConditionally unstable atmosphere is important is important

Multiple tornadoes can come from a single storm (like a Multiple tornadoes can come from a single storm (like a supercell storm) supercell storm)

•ExampleExample: May 4-5, 2003, during a 24 hour period, there : May 4-5, 2003, during a 24 hour period, there were over 80 reported tornadoes.were over 80 reported tornadoes.

Recipe for a Recipe for a tornadotornado

Strong wind shearStrong wind shear- Can enhance rotation- Can enhance rotation- Caused mainly by jet stream- Caused mainly by jet stream

Warm moist air below dry colder Warm moist air below dry colder airair - Large instability- Large instability

- Explosive growth due to - Explosive growth due to latent heat releaselatent heat release

Supercell storms good candidate Supercell storms good candidate for tornadoes (they already have for tornadoes (they already have rotation).rotation).

Tornado occurrence?Tornado occurrence? Tornadoes possible everywhere in the world, but Tornadoes possible everywhere in the world, but

most are in the U.S. (tornado alley Texas –Nebraskamost are in the U.S. (tornado alley Texas –Nebraska

3/4 of the tornadoes occur from March to July, with 3/4 of the tornadoes occur from March to July, with the maximum in _____.the maximum in _____.

Jet stream is still a large influenceJet stream is still a large influence

Most often occur in the _______________ Most often occur in the _______________

Least frequent ______________Least frequent ______________

MayMay

late afternoon (4-6pm)late afternoon (4-6pm)

before sunrisebefore sunrise

Tornado incidence by state Tornado incidence by state

25 year total25 year total

Why is Tornado Alley the Why is Tornado Alley the most likely place to get most likely place to get

tornadoes?tornadoes? Perfect location for the mixing of air Perfect location for the mixing of air massesmasses Warm, moist gulf air to the southWarm, moist gulf air to the south Cold, dry to the north/northeastCold, dry to the north/northeast Rockies mountains to west/northwestRockies mountains to west/northwest

Downslope flow is cool and dryDownslope flow is cool and dry Right latitude for the polar jet stream Right latitude for the polar jet stream

On Radar, the On Radar, the presence of a hook presence of a hook

echo indicates a echo indicates a mesocyclone; a mesocyclone; a

region in a region in a thunderstorm very thunderstorm very likely to spawn a likely to spawn a

tornadotornado