Thunderstorms and Lightning Jeff Gawrych Met 10

Thunderstorms and Lightning

Jeff Gawrych

Met 10

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

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,

These storms may go through growing, mature These storms may go through growing, mature and dissipation stageand dissipation stage

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

Air Mass Thunderstorms

Air Mass Thunderstorms•Air mass thunderstorms are “self extinguishing” –

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.

Severe Thunderstorms

Severe Thunderstorms

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

Squall-line Thunderstorms

Characteristics Characteristics

Definition of Severe Thunderstorm:Definition of Severe Thunderstorm:

Severe Thunderstorms

–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 Thunderstorms

Average # of days thunderstorms observed

Average number of days hail observed

Lightning and ThunderLightning is the discharge of electricity that Lightning is the discharge of electricity that occurs within a thunderstorm.occurs within a thunderstorm.

The extreme heating associated with lightning The extreme heating associated with lightning causes air to expand rapidly, and produces causes air to expand rapidly, and produces sound waves we recognize as thunder.sound waves we recognize as thunder.

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

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

Separation of Charge in Clouds

One 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.

++

-

-

Development of lightning stroke

Cloud to Ground Lightning Strike

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

10 108 8 msms-1-1 (roughly 1/3 the (roughly 1/3 the speed of light.) This return speed of light.) This return stroke is visible to the human stroke is visible to the human eye.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 Facts

Tornados …

… are also called ___________ or ____________. … are rapidly rotating winds that blow around a small area of __________________ … come in many shapes, but mostly look like funnels or tubes. … often descend from large ___________________. A __________ is a tornado that doesn’t hit the ground.

Tornado Characteristics

Majority of tornadoes rotate _________________ (cyclonic)

Most tornadoes only last ______________

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

Fujita Tornado Damage Scale

F0......Gale tornado...... winds of 40-70 mphSome damage to chimneys; branches broken off trees, pushes over shallow-rooted trees…

Fl...... Moderate tornado.....winds of 73-112 mphPeels surface off roofs; mobile homes destroyed.

F2......Significant tornado......winds of 113-157 mphConsiderable damage. Roofs torn off frame houses- mobile homes demolished; boxcars pushed over; larger trees snapped or uprooted-, light object projected like missiles.

Fujita Tornado Damage Scale

F3......Severe tornado......winds of 158-206 mphRoof and some wall torn off, well constructed houses, trains overturned; most trees in forest uprooted

F4......Devastating tornado......winds of 207-260 mphWell-constructed houses leveled, structures with weak foundations blown some distance; cars thrown…

F5......Incredible tornado......winds of 261-318 mphStrong frame houses lifted off foundations and carried considerable distances to disintegrate- automobile sized missiles fly in excess of 100 meters; trees debarked; steel reinforced concrete badly damaged.

Tornado Formation

Formed in association with _____________________Formed in association with _____________________

______________________ 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: April 3-4, 1974, during a 16 hour period, there : April 3-4, 1974, during a 16 hour period, there were 148 reported tornadoes.were 148 reported tornadoes.  

Recipe for a tornado

Strong wind shearStrong wind shear

Warm moist air below dry colder Warm moist air below dry colder airair

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

Tornado occurrence?Tornadoes possible everywhere in the world, but most are in the U.S. (tornado alley Texas –Nebraska

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

Most often occur in the _______________

Least frequent ______________

Tornado incidence by state

25 year total25 year total

Why is Tornado Alley the most likely place to get

tornadoes?

Perfect location for the mixing of air masses Warm, moist gulf air to the south Cold, dry to the north/northeast Rockies mountains to west/northwest

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