Essentials of Ecology

S50 SUPPLEMENT 8

Tropical cyclones are spawned by the formation

of low-pressure cells of air over warm tropical

seas. Figure 9 shows the formation and structure

of a tropical cyclone. Hurricanes are tropical

cyclones that form in the Atlantic Ocean; those

forming in the Pacifi c Ocean usually are called

typhoons. Tropical cyclones take a long time to

form and gain strength. As a result, meteorolo-

gists can track their paths and wind speeds and

warn people in areas likely to be hit by these

violent storms.

For a tropical cyclone to form, the tem-

perature of ocean water has to be at least

27 °C (80 °F) to a depth of 46 meters (150 feet).

A tropical cyclone forms when areas of low

pressure over the warm ocean draw in air from

surrounding higher-pressure areas. The earth’s

rotation makes these winds spiral counter-

clockwise in the northern hemisphere and

clockwise in the southern hemisphere (Fig-

ure 7-3, p. 142). Moist air warmed by the heat

of the ocean rises in a vortex through the center

of the storm until it becomes a tropical cyclone

(Figure 9).

The intensities of tropical cyclones are rated

in different categories based on their sustained

wind speeds: Category 1: 119–153 kilometers

per hour (74–95 miles per hour); Category 2:

154–177 kilometers per hour (96–110 miles per

hour); Category 3: 178–209 kilometers per hour

(111–130 miles per hour); Category 4: 210–

249 kilometers per hour (131–155 miles per

hour); and Category 5: greater than 249 kilome-

ters per hour (155 miles per hour). The longer

a tropical cyclone stays over warm waters, the

stronger it gets. Signifi cant hurricane-force

winds can extend 64–161 kilometers (40–100

miles) from the center, or eye, of a tropical

cyclone.

Figure 10 shows the change in the average

surface temperature of the global ocean between

1871 and 2000. Note the rise in this tempera-

ture since 1980. These higher temperatures,

especially in tropical waters, may explain why

Tornadoes and Tropical Cyclones

Are Violent Weather Extremes

Sometimes we experience weather extremes.

Two examples are violent storms called torna-

does (which form over land) and tropical cyclones

(which form over warm ocean waters and some-

times pass over coastal land).

Tornadoes or twisters are swirling funnel-

shaped clouds that form over land. They can

destroy houses, cause other serious dam-

age, and kill people in areas where they touch

down on the earth’s surface. The United States

is the world’s most tornado-prone country, fol-

lowed by Australia.

Tornadoes in the plains of the midwestern

United States usually occur when a large, dry,

cold-air front moving southward from Canada

runs into a large mass of humid air moving

northward from the Gulf of Mexico. Most

tornadoes occur in the spring and summer

when fronts of cold air from the north penetrate

deeply into the midwestern plains.

As the large warm-air mass moves rapidly

over the more dense cold-air mass, it rises

swiftly and forms strong vertical convection

currents that suck air upward, as shown in

Figure 7. Scientists hypothesize that the rising

vortex of air starts spinning because the air near

the ground in the funnel is moving more slowly

than the air above. This difference causes the air

ahead of the advancing front to roll or spin in a

vertically rising air mass or vortex.

Figure 8 shows the areas of greatest risk from

tornadoes in the continental United States.

Severe

thunderstorm

Descending

cool air

Rising

warm air

Rising

updraft

of air

Severe thunderstorms
can trigger a number
of smaller tornadoes
Tornado forms when
cool downdraft and
warm updraft of air
meet and interact
Warm
moist
airdra
wnin

Highest risk

Lowest risk

0

5

10

15

20

25

30

35

40

Figure 7 Formation of a tornado or twister. Although twisters can form at any time of the year, the

most active tornado season in the United States is usually March through August. Meteorologists cannot

tell us with great accuracy when and where most tornadoes will form.

Figure 8 States with very high and high tornado risk in the continental United States. (Data from NOAA)

Data and Map Analysis

1. How many states have areas with a risk factor of 25 or higher? How many have areas with a risk factor
   of 20 or higher?


2. What is the level of risk where you live? If you live in a far western state, does this mean you are guaran-
   teed never to see a tornado in your area?