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from the Gulf of Mexico at the surface. Above this warm, moist
layer is a layer of dry, unstable air that has just crossed the Rocky
Mountains, moved along rapidly by the jet stream. The stage is
now set for some event, such as a cold air mass moving in from
the north, to shove the warm, moist air upward, and the result
will be violent thunderstorms with tornadoes.

CONCEPTS Applied

Tornado Damage
Tornadoes are rated on wind speed and damage. Here is
the scale with approximate wind speeds:
0: Light damage, winds under 120 km/h (75 mi/h).
Damage to chimneys, tree limbs broken, small trees
pushed over, signs damaged.
1: Moderate damage, winds 121 to 180 km/h (76 to
112 mi/h). Roofing materials removed, mobile homes
and moving autos pushed around or overturned.
2: Considerable damage, winds 181 to 250 km/h
(113 to 155 mi/h). Roofs torn off homes, mobile
homes demolished, boxcars overturned, large trees
snapped or uprooted, light objects become missiles.
3: Severe damage, winds 251 to 330 km/h (156 to
205 mi/h). Roofs and some walls torn off homes,
whole trains overturned, most trees uprooted, cars
lifted off the ground and thrown.
4: Devastating damage, winds 331 to 418 km/h
(206 to 260 mi/h). Homes leveled, cars thrown,
large missiles generated.
5: Incredible damage, winds 419 to 512 km/h
(261 to 318 mi/h). Homes demolished and swept
away, automobile-sized missiles fly though the air
more than 100 yards, trees debarked.

FIGURE 23.18 A tornado might be small, but it is the most
violent storm that occurs on Earth. This tornado, moving across
an open road, eventually struck Dallas, Texas. Hurricanes
What is the difference between a tropical depression, a tropi-
cal storm, and a hurricane? In general, they are all storms with
(about 4 to 5 mi) on the ground, sometimes skipping into the strong upward atmospheric motion and a cyclonic surface wind
air, then back down again. The bottom of the column moves circulation (Figure 23.19). They are born over tropical or sub-
across the ground at speeds that average about 50 km/h (about tropical waters and are not associated with a weather front. The
30 mi/h). The speed of the whirling air in the column has been varieties of storm intensities are classified according to the speed
estimated to be up to about 480 km/h (about 300 mi/h), but of the maximum sustained surface winds.
most tornadoes have winds of less than 180 km/h (112 mi/h). A tropical depression is an area of low pressure around which
The destruction is produced by the powerful winds, the sud- the winds are generally moving at 55 km/h (about 35 mi/h) or
den drop in atmospheric pressure that occurs at the center of less. The tropical depression might dissolve into nothing, or it
the funnel, and the debris that is flung through the air like pro- might develop into a more intense disturbance. A tropical storm
jectiles. A passing tornado sounds like very loud, continuous is a more intense low-pressure area with winds between 56 and
rumbling thunder with cracking and hissing noises that are 120 km/h (about 35 and 75 mi/h). A hurricane is a very intense
punctuated by the crashing of debris projectiles. low-pressure area with winds greater than 120 km/h (about
On average, several hundred tornadoes are reported in the 75 mi/h). A strong storm of this type is called a hurricane if it
United States every year. These occur mostly during spring and occurs over the Atlantic Ocean or the Pacific Ocean east of the
early summer afternoons over the Great Plains states. Texas, international date line. It is called a typhoon if it occurs over the
Oklahoma, Kansas, and Iowa have such a high occurrence of North Pacific Ocean west of the international date line.
tornadoes that the region is called tornado alley. During the A tropical cyclone is similar to the wave cyclone of the
spring and early summer, this region has maritime tropical air mid-latitudes because both have low-pressure centers with a

23-13 CHAPTER 23 Weather and Climate 577