Chapter 10: Thunderstorms, Tornadoes, and Lightning

Thunderstorms

Storm containing lightning and thunder

Convective storms

Storms that form with rising air

Severe thunderstorms

Characterized by large hail, wind gusts greater than or equal to 50 knots, or a tornado

Ordinary Cell Thunderstorms

When the wind speed and wind direction do not drastically alter with height

Ordinary cell thunderstorm stages

Cumulus, mature, and dissipating

Multicell Thunderstorms

Contain a number of cells, each in a different stage of development

The Gust Front

Leading edge of the cold out flowing air

Microbursts

Downburst with winds extending four km or less

Squall-Line Thunderstorms

Line of multicell thunderstorms that form ahead of fronts

Derecho

Windstorm associated with a squall line

Mesoscale Convective Complex

A big circular convective weather system formed when several small individual multicell thunderstorms develop in magnitude

Supercell Thunderstorms

Strong vertical wind shear with a single violently rotating updraft

Pyrocumulus

Dense cumuliform cloud associated with fire

Outflow boundary

Occurs with a cold front with strong and gusty winds and where there are straight-line winds behind a gust front

Flash floods

Increase rapidly with little or no advance warning; often caused by stalled or slow thunderstorms

River flood

Occurs when a river overflows its banks or when its flow cannot be kept inside its channel

Distribution of Thunderstorms

More than 50,000 thunderstorms occur each day on Earth—over 18 million per year

Occurrence of thunderstorms

Over equatorial landmasses—about one every three days; prevalent over water along the intertropical convergence zone; less prevalent in polar and desert regions

Lightning

Discharge of electricity that occurs in mature cloud storms

Thunder

Booming sound wave caused by explosive expansion of air due to heat from lightning

Distance of Lightning

If you see lightning and hear the thunder 15 seconds later, the lightning stroke is about three miles away

Electrification of Clouds

Occurs when graupel and hailstones fall through supercooled liquid droplets and ice crystals, freezing and releasing latent heat

The Lightning Stroke

Involves a negative charge at the bottom of the cloud and a positive charge on the ground beneath it

Types of Lightning

Can take a variety of shapes and forms including forked, ribbon, bead, ball, sheet, and heat

Lightning Detection and Suppression

Lightning direction-finders detect radio waves produced by lightning; networks of magnetic devices are used to pinpoint locations

Lightning Distance Calculation

Count the seconds from the moment you see lightning until you hear thunder. Sounds take 5 seconds to travel a mile.

Tornado

Rapidly rotating column of air that blows around an intense area of low pressure.

Tornado Life Cycle

Stages include Dust-whirl, Mature, and Decay.

Tornado Occurrence and Distribution

The U.S. experiences most tornadoes.

Tornado Winds

Measured by Doppler radar.

Multi-vortex Tornadoes

Violent tornadoes that contain smaller whirls that rotate within them.

Seeking Shelter

Recommended locations include basement, storm shelter, or 'safe room.'

Enhanced Fujita Scale

Attempts to provide a wide range of criteria in estimating a tornado's winds by using a set of 28 damage indicators.

Tornado Outbreaks

Group or series of tornadoes spawned by the same thunderstorm, typically six or more.

Tornado Damage Classification Scale

Includes categories such as EF0, EF1 for weak tornadoes; EF2, EF3 for strong tornadoes; EF4, EF5 for violent tornadoes.

Destructive Potential of Wind

The pressure of the wind increases with the square of the wind speed, meaning if you double the wind speed, you get four times as much destructive potential.

Tornado Strength Variation

Tornadoes can strengthen or weaken dramatically, and shrink or enlarge, in just a few seconds.

Supercell Tornadoes

Form with supercell thunderstorms and are associated with hook echoes.

Nonsupercell Tornadoes

Tornadoes that do not occur in association with a pre-existing wall cloud of a supercell.

Waterspouts

A rotating column of air connected to a cumuliform cloud over a large body of water.

Tornadic Waterspout

A waterspout that formed over land and then traveled over water.

Doppler Radar Principle

The frequency of the returned radar pulse fluctuates as precipitation travels closer or away from the antenna.

Storm Chasing

Can be done informally or in organized tour groups.

VORTEX

Verification of the Origin of Rotational Tornadoes Experiment.

VORTEX2

A follow-up to the original VORTEX experiment conducted by the National Severe Storms Laboratory/NOAA.