Part 4. Disturbances

Chapter 11

Lightning, Thunder, and Tornadoes

Annual distribution of lightning strikes

There are about 40,000 thunderstorms daily on the Earth that produce lightning. A few produce tornadoes.

Development of lightning and thunder(a) Positive and negative

charges separate in the cloud.

(b) The step leader is a flow of negative charges (electrons) toward positive charges.

(c) A flow of positive charges moves toward the step leader.

(d) As the positive and negative charges combine, the lightning stroke is seen.

(e) A dart leader is a secondary stroke just after the first.Thunder arises from the rapid heating of the air by the lightning stroke, which

sends out a sound wave at 5 miles/sec.

Charge separation in a cloud Lightning only forms in clouds that extend into air that is below freezing

• Collisions between ice crystals and graupel may help transfer charge from solid ice to liquid films on some ice crystals

Positive charges tend to accumulate at the top of a cloud, negative charges in the lower part of a cloudRunaway discharge -- electrons accelerated to a very high speed, colliding with air molecules and creating more free electrons. High-speed moving electrons radiate light as lightning.

Strong electrical fields occur prior to lightning

Safest areas from lightning -- indoors or in an automobile. Outdoors, do not stand under tall objects. Do not touch telephones or electrical appliances. Lightning can strike in the same place twice!

A positive stroke can occur when thunderstorms become tilted

Types of lightning• Forked• Sheet (heat)• Ball • St. Elmo’s fire • Sprites • Blue jets

A blue jet

Types of ThunderstormsAir Mass

Frontal

Squall Line

Mesoscale Convective Complexes (MCC)

Air Mass thunderstorm lifecycle

Air Mass thunderstorms form in humid, unstable air. Each cell lasts no more than a few hours from development to dissipation.

Severe Thunderstorms• Winds exceed 93 km/hr (58 mph), have large

hailstones (1.9 cm; 0.75 in) or produce tornadoes

Mesoscale convective complex’s (MCCs)• Self-propagating thunderstorm systems

Mesoscale convective systems (MCSs) include MCCs and squall lines• Squall line -- linear band of thunderstorms

usually out ahead of a cold front (a type of MCS)

An MCC over South Dakota

A radar image of outflow boundaries

Outflow boundaries -- front edge of cold air flowing out away from a thunderstorm; a gust front occurs at an outflow boundary

Thunderstorm movement in an MCC

Movement and lifecycle of individual thunderstorm cells (A, B, C, D, E, F, and G) and how they change with time

Cells dissipating

Cells forming

Direction of movement of the line of thunderstorms

Direction of movement of the individual thunderstorm cells

A squall line (MCS)

This squall line is probably along or ahead of an advancing dry line

A radar image of a squall line

Wind shear and vertical motions in a squall line thunderstorm

Vertical wind profile in (a) here

Gust front-induced shelf and roll cloud

Internal structure of a supercell

A supercell thunderstorm is an extremely powerful thunderstorm cell. Supercell thunderstorms can spawn tornadoes.

Organization of a supercell and actual radar signature

The above are map views of a supercell thunderstorm.

Hook echo

Outflow boundary

Downbursts, Derechos, and MicroburstsDownbursts -- strong downdrafts in a mature thunderstorm

Derecho -- MCS-induced strong downdraft that can last for hours

Microburst -- small diameter downburst that usually lasts only a few minutes

Microbursts create aviation hazards

Tornadoes• Tornado characteristics and dimensions

– 100-yard average diameter – Movement = 50km/hr (30 mph) over 3-4

km (2-2.5 mi)– Winds = 65 km/hr (40 mph) to 450 km/hr

(280 mph)• Tornado formation

– Squall lines, MCCs, supercells, tropical cyclones

Tornado-producing supercell

Tornadoes typically drop out of the wall cloud on the southwest side of a supercell

A possible mechanism of tornadoformation

Tornado development along a convergence boundary

A non-supercell tornado development along outflow zone

Circular areas shows places where thunderstorm inflow and outflow circulation resulted in vorticity and tornado development

Global tornado frequency

Most of Earth’s tornadoes occur in the lower elevation areas of North America

“Tornado Alley”

Most tornadoes occur in the springtime, when the contrast between warm and cold air in the atmosphere is the greatest

-- Most injuries and deaths in tornadoes are in automobiles and mobile homes and are caused by flying debris-- US averages 91 tornado deaths each year-- Safest area in a tornado is in a basement or an interior room, away from windows

Multiple suction vortices greatly increase damage

Linear tornado damage path

• Tornado outbreaks– A single weather system producing a

large number of tornadoes

• Waterspouts– Similar to tornadoes– Develop over warm waters – Smaller and weaker than tornadoes

End of Chapter 11

Understanding Weather and Climate

4th Edition

Edward Aguado and James E. Burt