Weather and storms in the tropics are discussed in this chapter. Several unusually strong and destructive hurricanes have affected the United States, Mexico, and countries of the
Caribbean in recent years, so this subject should be especially relevant and interesting to students.
The chapter begins by comparing atmospheric conditions and storms in the tropics to those at middle latitudes. Horizontal temperature and pressure gradients are generally fairly weak at low latitudes, but moisture is abundant and thunderstorm formation is common. Zones of convergence, such as might be associated with an easterly wave or might be found near the
ITCZ, will sometimes cause thunderstorm development in lines or clusters. On a few occasions each year special conditions will cause a mass of thunderstorms to become better organized and to develop into a tropical storm or hurricane. Hurricanes form above (and derive much of their energy from) a warm surface layer of ocean water. Hurricanes generally form between 5 and 20 degrees latitude, where steering winds will cause an overall east to west movement. Schematic diagrams illustrate the air motions in a mature hurricane and point out the characteristic eye, eye wall, and spiral rain band features.
Several infamous hurricanes are described, along with the unique hazards associated with powerful hurricanes. In addition to powerful winds and flooding caused by heavy rainfall, extensive damage can be caused by the hurricane storm surge. Several recent examples of destructive and costly hurricanes are given, and we see that providing timely and accurate warning of a hurricane's approach and landfall is a particularly demanding task. A list of names proposed for future Northern Atlantic and Eastern Pacific hurricanes is included.
1. Video footage of recent hurricanes is available for purchase from a variety of sources.
See a recent issue of Weatherwise magazine for advertisements.
A one-hour program about hurricanes has also been produced by the NOVA television series. This program includes dramatic aerial footage from inside the eye of Hurricane Gilbert.
2. During the winter, examine a surface weather map in class and compare temperatures to the north and the south of the polar front. There is often a very sharp temperature gradient. Locate the polar jet stream on an upper-level chart and determine the region where the maximum winds are found.
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1. Depending on the time of the year students may be interested in plotting and following hurricane or tropical storm trajectories on a tracking chart. Students can investigate how large, synoptic scale weather features affect the storm's movement.
2. Have students prepare a report on a strong recent hurricane or a hurricane of historical interest. A suitable choice could be made from the list in the text. Some additional possibilities can be found in the list of the ten deadliest hurricanes to have struck the United States in the
20th century (ref: J. Williams, The Weather Book , 2nd ed., p. 143, Vintage Books, New York,
1997).
3. Using the Virtual Hurricane section of the ThomsonNow web site, make separate graphs of wind speed vs. distance from the eye as you approach the eye from the north, south, east and west. Does the relationship of wind speed vs. distance change with your direction of approach? Try to explain your results.
1. A weak trough of low pressure in the tropics. They generally travel from east to west. On the eastern side, where the southeasterly winds converge, rising air generates showers and thunderstorms.
2. Streamlines are useful because they show where surface air converges and diverges.
3. Typhoon.
4. Horizontal structure: As one approaches the hurricane, the sky becomes overcast with cirrostratus clouds; barometric pressure drops slowly at first, then more rapidly as we move closer to the center. Winds blow from the north and northwest with ever-increasing speed as we near the eye. The high winds, which generate huge waves over 10 m (33 ft) high, are accompanied by heavy rainshowers. As we move into the eye, the winds slacken, rainfall ceases, and the sky brightens, as middle and high clouds appear overhead. Vertical structure: Near the surface, moist tropical air flows in toward the hurricane’s center. Adjacent to the eye, this air rises and condenses into huge
5.
6. cumulonimbus clouds that produce heavy rainfall, as much as 25 cm (10 in.) per hour. Near the top of the clouds, the relatively dry air, having lost much of its moisture, begins to flow outward away from the center. This diverging air aloft actually produces an anticyclonic (clockwise in the
Northern Hemisphere) flow of air several hundred kilometers from the eye.
Air is sinking in the eye, causing adiabatic warming and preventing cloud development.
Hurricanes form over tropical waters where the surface winds are light, the humidity is high in a deep layer extending up through the troposphere, and the surface water temperature is warm, typically 26.5°C (80°F) or greater, over a vast area. Aloft, the trade wind inversion must be weak and upper-level winds must not be strong.
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7. Convergence of winds along the ITCZ; formation of a low pressure area along a wave in the ITCZ; convergence along the eastern size of an easterly wave; convergence along a midlatitude front that has moved into the tropics.
8. a. Water vapor evaporating from the ocean surface. b. The maximum strength a hurricane can achieve is proportional to the difference in air temperature between the tropopause and the surface, and to the potential for evaporation from the sea surface.
9. No, because the energy supply (evaporation of large amounts of surface water) is unavailable.
10. Northern side, because on this side the easterly winds will be enhanced by the westward movement of the storm; eastern side, because after the storm turns northward the southerly winds will be enhanced by the northward movement of the storm.
11. Wind shear, passage over cooler or shallower water, increased ocean wave activity, passage over land.
12. Initially, the mass of thunderstorms with only a slight wind circulation is known as a tropical disturbance, or tropical wave. The tropical disturbance becomes a tropical depression when the winds increase to between 20 and 34 knots and several closed isobars appear about its center on a surface weather map. When the isobars are packed together and the winds are between 35 and 64 knots, the tropical depression becomes a tropical storm. The tropical storm is classified as a hurricane only when its winds exceed 64 knots (74 mi/hr).
13. Similarities: both have central cores of low pressure and winds that spiral counterclockwise about their respective centers (Northern Hemisphere). Differences: A hurricane derives its energy from the warm water and the latent heat of condensation, whereas the midlatitude storm derives its energy from horizontal temperature contrasts. The vertical structure of a hurricane is such that its central column of air is warm from the surface upward; consequently, hurricanes are called warmcore lows. A hurricane weakens with height, and the area of low pressure at the surface may actually become an area of high pressure above 12 km (40,000 ft). Mid-latitude cyclones, on the other hand, are cold-core lows that usually intensify with increasing height, with a cold upper-level low or trough often existing above, or to the west of the surface low. A hurricane usually contains an eye where the air is sinking, while mid-latitude cyclones are characterized by centers of rising air. Hurricane winds are strongest near the surface, whereas the strongest winds of the mid-latitude storm are found aloft in the jet stream.
14. Steering winds in the tropics are usually easterly.
15. The huge waves and flooding associated with the storm surge usually cause the most destruction.
16. Flooding.
17. The combined effect of high water (which is usually well above the high-tide level), high winds, and the net Ekman transport toward the coast, produces the storm surge—an abnormal rise of several meters in the ocean level—which inundates low-lying areas and turns beachfront homes
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into piles of splinters. The storm surge is particularly damaging when it coincides with normal high tides.
18. The name is assigned when the storm reaches tropical storm strength.
19. Spin-up vortices (mini-swirls).
20. Over the Gulf of Mexico, Katrina gradually turned northward toward Mississippi and
Louisiana. As the powerful Category 5 hurricane moved slowly toward the coast, its rainbands near the center of the storm began to converge toward the storm’s eye.
This process cut off moisture to the eyewall. As the old eyewall dissipated, a new one formed further away in a phenomenon meteorologists call eyewall replacement.
21. To forecast the intensity and movement of a hurricane, meteorologists use numerical weather prediction models. Information from satellites, buoys, and reconnaissance aircraft (that deploy dropsondes into the eye of the storm) are fed into the models. The models then forecast the intensity and movement of the storm. There are a variety of forecast models, each one treats some aspect of the atmosphere (such as evaporation of water from the ocean’s surface) in a slightly different manner. Often, the models do not agree on where the storm will move and on how strong it will be.
22. When a hurricane poses a direct threat to an area, a hurricane watch is issued. When it appears that the storm will strike an area within 24 hours, a hurricane warning is issued.
23. In attempts to reduce their winds.
24. Sea surface temperatures offshore of New Jersey are warmer than those offshore of
Oregon. Also, a typical path of Atlantic Hurricanes passes westward across the Atlantic toward the
Caribbean Sea, then turns northward along the eastern shore of North America, passing close to
New Jersey. Typical paths of Pacific hurricanes pass nowhere near Oregon.
1. (a) The winds should change from E to NE to N to NW. (b) Probably the strongest winds would occur with the east or northeast wind coming from the ocean as surface friction would tend to slow down the wind blowing from other directions. (c) The lowest sea level pressure would most likely occur with the north wind. At this point the storm would be directly offshore, its point of closest approach.
2. (a) Elena: Category 3 (b) Ivan: Category 3 (c) Hugo: Category 4 (d) Olaf: Category 4
3. Possible reasons include: the hurricane can pass over an area of warm surface ocean water, it can move away from the equator where the Coriolis force is stronger, it can experience decreasing levels of vertical wind shear, the trade wind inversion can weaken, upper-level divergence can increase.
4. In May the surface ocean water in the subtropics is still relatively cool from the winter. In
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October the water is still warm from the summer.
6. The rapid rate of evaporation cools the water.
7. It would not necessarily begin with the letter C because several tropical storms may have formed but not developed into full-blown hurricanes before this.
8. To the east. Winds circulate clockwise around tropical cyclones in the Southern
Hemisphere, so the onshore winds (and the storm surge) will arrive to the east of Darwin.
9. Yes. The tropical storm should be given a new name because it is now an eastern Pacific storm, which is named according to a different list than north Atlantic storms.
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