Chapter 10: Thunderstorms IIMet 10

A downburst is a downdraft that spreads out horizontally from the base of a thunderstorm.

A downburst with winds extending only 4 km or less is termed a microburst.

Microbursts are capable of blowing down trees.

Even with their small size, microbursts can have winds as strong as 146 knots (168 mph)!!

Microbursts are responsible for several airline crashes.

Damaging winds from thunderstorms

Flying into a microburst. At position (a), the pilot encounters a headwind; at position (b), a strong downdraft; and at position (c), a tailwind that reduces lift and causes the aircraft to lose altitude.

Dust clouds rising in response to a microburst near Denver, Colorado.

Tornadoes

A tornado is a rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground.

A tornado’s circulation is present on the ground either as a funnel-shaped could or as a swirling cloud of dust and debris.

Sometimes called twisters or cyclones, tornadoes can assume a variety of shapes and forms that range from twisting rope-like funnels, cylindrical-shaped funnels.

A funnel cloud, is a tornado that has not reached the ground.

Tornadoes

A majority of North American tornadoes rotate counterclockwise about their central core of low pressure.

The diameter of most tornadoes is between 100 and 600 m (~300-2000 ft.), although some are a few meters wide and others have diameters exceeding 1 mile!

Winds in tornadoes are very destructive. Most have winds that are 125 knots or less, but the most powerful have winds up to 220 knots!

Tornadoes

Table 10-2, p. 288

Tornado Winds

Total destruction caused by an F5 tornado, Oklahoma on May 3, 1999.

Fig. 10-32, p. 288

The total wind speed of a tornado is greater on one side than on the other.

When facing an on-rushing tornado, the strongest winds will be on your left side.

Fig. 10-31, p. 287

A powerful multi-vortex tornado with three suction vortices.

Fig. 10-29, p. 285

Average number of tornadoes during each month in the United States

Tornado Occurrence

Fig. 10-28, p. 285

Tornado incidence by state

number of tornadoes reported by each state during a 25-year period.

The lower figure is the average annual number of tornadoes per 10,000 square miles.

Table 10-1, p. 287

1. Spinning vortex tubes created by wind shear.

2. The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm

Features associated with a tornado-breeding supercell thunderstorm

Fig. 10-36, p. 290

A tornado-spawning supercell thunderstorm. A hook echo in its rainfall pattern on a Doppler radar screen. The colors red and orange represent the heaviest precipitation.

Fig. 10-37, p. 291

A classic tornadic supercell thunderstorm showing updrafts and downdrafts

Fig. 10-38, p. 291

Fig. 10-39, p. 292

Fig. 10-40, p. 293

(a)Along the boundary of converging winds, the air rises and condenses into a cumulus congestus cloud.

At the surface the converging winds along the boundary create a region of counterclockwise spin.

(b) As the cloud moves over the area of rotation, the updraft draws the spinning air up into the cloud, producing a nonsupercell tornado, or landspout.

Nonsupercell Tornadoes

Fig. 10-41, p. 294

Doppler radar display of winds associated with the supercell storm that moved through parts of Oklahoma City during the afternoon of May 3, 1999.

The close packing of the horizontal winds blowing toward the radar (green and blue shades), and those blowing away from the radar (yellow and red shades), indicate strong cyclonic rotation and the presence of a tornado.

Graduate students from the University of Oklahoma use a portable Doppler radar to probe a tornado near Hodges, Oklahoma.

Figure 3, p. 295

Lightning and Thunder

Lightning is simply a discharge of electricity, a giant spark, which usually occurs in mature thunderstorms.

Lightning may take place within a cloud, from one cloud to another, from a cloud to surrounding air, or from cloud to the ground.

Majority of lightning strikes occur within the cloud, while only 20% strike the ground.

A lightning stroke can heat the air to 30,000°C !!!

This extreme heating causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave– called thunder– that travels outward in all directions from the flash.

Lightning and Thunder

The lightning stroke can travel in a number of directions

Within a cloud, from one cloud to another cloud, from a cloud to the air, or from a cloud to the ground. Notice that the cloud-to-ground lightning can travel out away from the cloud, then turn downward, striking the ground many miles from the thunderstorm.

Clouds become electrified during the formation of precipitation when regions of separate charge exist within tiny cloud droplets and larger precipitation particles.

When falling precipitation collides with smaller particles, the larger precipitation particles become negatively charged and the smaller particles, become positively charged.

Updrafts within the cloud then sweep the smaller positively charged particles into the upper reaches of the cloud while larger negative charged particles settle toward the lower and middle parts of the cloud.

Electrification of Clouds

Fig. 10-21, p. 279

The generalized charge distribution in a mature thunderstorm.

The Lightning Stroke

Because unlike charges attract one another, the negative charge at the bottom of the cloud causes a region of the ground beneath it to become positively charged.

As the thunderstorm moves along, this region of positive charge follows the cloud like a shadow.

The positive charge is most dense on protruding objects, such as trees, poles, and buildings.

The difference in charges causes and electrical potential between the cloud and ground.

The development of a lightning stroke

(a) When the negative charge near the bottom of the cloud becomes large enough, a flow of electrons—the stepped leader—rushes toward the earth.

(b) As electrons approach the ground, a region of positive charge moves up into the air through any conducting object, such as trees, buildings, and even humans.

(c) When the downward flow of electrons meets the upward surge of positive charge, a strong electric current—a bright return stroke carries positive charge upward into the cloud.

A cloud-to-ground lightning flash hitting a 65-foot sycamore tree.

It should be apparent why one should not seek shelter under a tree during a thunderstorm.

Lightning Brush—Lightning protection

Cloud-to-ground lightning strikes in the vicinity of Chicago, Illinois.

Detected by the National Lightning Detection Network.

•The four marks on the road surface represent areas where lightning, after striking a car, entered the roadway through the tires.

•Lightning flattened three of the car’s tires and slightly damaged the radio antenna.

•The driver and a six-year-old passenger were taken to a nearby hospital, treated for shock, and released.

The lightning rod extends above the building, increasing the likelihood that lightning will strike the rod rather than some other part of the structure.

After lightning strikes the metal rod, it follows an insulated conducting wire harmlessly into the ground.

Summary of today’s lecture

Winds associated with thunderstorms- microbursts

Tornados: damage, winds, formation

Lightning: causes

Thunder: causes