Thunderstorms and Lightning Jeff Gawrych Met 10

Thunderstorms are generally classified into one of two groups: Air Mass Thunderstorms Air Mass Thunderstorms  Mesoscale Convective Systems Mesoscale Convective Systems  Thunderstorms

These thunderstorms form within a single air mass and are not tied to fronts or mid-latitude cyclones. Daily solar heating is primarily responsible for rising air motion, These storms may go through growing, mature and dissipation stage Often associated with summer storms. Air Mass Thunderstorms

Air mass thunderstorms are “self extinguishing” –  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

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. Severe Thunderstorms

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

Squall-line Thunderstorms

Characteristics Definition of Severe Thunderstorm: Severe Thunderstorms –Capable of producing large hail –Strong gusty surface winds –Flash floods –Tornadoes –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 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:

Normal charge separation in a mature thunderstorm  Charge separation not fully understood, but 

Separation of Charge in Clouds One theory suggests that the separation of charge is due to collisions between ice particles. After exchanging charge, the heavier ice crystals settle toward the cloud base

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  10 8 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…

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. 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 ( ft) in diameter

Fujita Tornado Damage Scale F Gale tornado winds of mph Some damage to chimneys; branches broken off trees, pushes over shallow-rooted trees… Fl Moderate tornado.....winds of mph Peels surface off roofs; mobile homes destroyed. F Significant tornado......winds of mph Considerable 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 F Severe tornado......winds of mph Roof and some wall torn off, well constructed houses, trains overturned; most trees in forest uprooted F Devastating tornado......winds of mph Well-constructed houses leveled, structures with weak foundations blown some distance; cars thrown… F Incredible tornado......winds of mph Strong 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 _____________________  ______________________ is important  Multiple tornadoes can come from a single storm (like a supercell storm) Example: April 3-4, 1974, during a 16 hour period, there were 148 reported tornadoes.Example: April 3-4, 1974, during a 16 hour period, there were 148 reported tornadoes.

Recipe for a tornado  Strong wind shear  Warm moist air below dry colder air  Supercell storms good candidate for tornadoes (they already have 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 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