BISL 04-Weather and Climate

125 The length of the path along the groundover which a tornado can move

miles

(200 km)

6 milesMaximum height that it can attain

(10 km)

FUJITA SCALEThe Fujita-Pearson scale wascreated by

Theodore Fujita

to classify tornadoesaccording to the damagecaused by the wind, from thelightest to the most severe.

F2

F4

F5

F3

Damage to

chimneys, treebranches broken

H

ouses uprooted fromtheir foundations and

dragged great distances

Solidly builtwalls blown

down

Roofs and walls

demolished, cars and

trains overturned

Mobile homesdestroyed, trees

felled

Mobile homes

ripped from their

foundations

F1

F0

WIND VELOCITY MILESPER HOUR (KM/H)

40-72(64-116)

73-112(117-180)

113-157(181-253)

158-206(254-332)

207-260(333-418)

261-320(420-512)

CATEGORYEFFECTS

TOP

The top of thetornado remainsinside the cloud.

PATHNormally the tornado pathis no more than 160 to330 feet (50-100 m) wide.

VORTEXColumn of air that formsthe lower part of atornado; a funnel thatgenerates violent windsand draws in air. Itusually acquires the darkcolor of the dust it sucksup from the ground, butit can be invisible.

MULTIPLEVORTICESSome tornadoeshave a numberof vortices.

SPIRALING WINDSFirst a cloud funnelappears that can thenextend to touch theground.

Some tornadoes areso powerful thatthey can rip theroofs off houses.

The tornadogenerally movesfrom thesouthwest to thenortheast.

2.ROTATIONThe circulation of the aircauses a decrease inpressure at the center ofthe storm, creating acentral column of air.

3.DESCENTThe central whirling columncontinues to descend withinthe cloud, perforating it inthe direction of the ground.

4.THE OUTCOMEThe tornado reaches theEarth and depending on itsintensity can send the roofsof buildings flying.

BEGINNING OF A TORNADOWhen the winds meet, theycause the air to rotate in aclockwise direction in theSouthern

H

emisphere and in

the reverse direction in theNorthern

Hemisphere.

1.

How They Form

Where and When

Most tornadoes occur in agricultural areas.

The

humidity and heat of the spring and summer are

required to feed the storms that produce them. In order togrow, crops require both the humidity and temperaturevariations associated with the seasons.

TornadoesAgricultural areas

Convection

Spinningfunnel of air

Strong wind

MildWind

Warm andhumid wind Cold anddry wind

Humidwind

Storm

Cumulonimbus

300 Maximum velocity the tornadowinds can attain

miles

per hour(480 km/h)

1,000tornadoes are generatedon average annually inthe United States.

3:00 The period of the day withthe highest probability oftornado formation

P.M.

-9:00

P.M.

0.6 mile (1 km)

Maximumdiameter

Tornadoes begin to form when a current of warmair ascends inside a cumulonimbus cloud and

begins to rotate under the influence of winds in theupper part of the cloud. From the base of the column, airis sucked toward the inside of the turning spiral.

The air

rotates faster as it approaches the center of the column.This increases the force of the ascending current, andthe column continues to grow until it stretches from highin the clouds to the ground. Because of their shortduration, they are difficult to study and predict.

52

ME

T

EORO

LOGICA

L

P

H

ENOMENA

WEATHER AND CLIMATE

53

Lethal ForceT

ornadoes are the most violent storms of nature.

T

hey are

generated by electrical storms (or sometimes as the result of ahurricane), and they take the form of powerful funnel-shaped

whirlwinds that extend from the sky to the ground. In these storms,moving air is mixed with soil and other matter rotating at velocities ashigh as 300 miles per hour (480 km/h).

T

hey can uproot trees, destroy

buildings, and turn harmless objects into deadly airborne projectiles. Atornado can devastate a whole neighborhood within seconds.