Presentation on theme: "Tropical Storms and Hurricanes"— Presentation transcript:
1Tropical Storms and Hurricanes Chapter 12Tropical Storms and Hurricanes
2Extremely strong tropical storms go by a number of different names, depending onwhere they occur. Over the Atlantic and theeastern Pacific they are known as hurricanes.Those over the extreme western Pacificare called typhoons; those over theIndian Ocean and Australia are cyclones.
3Middle- and upper-level air along the eastern side of anticyclones sinks as it approaches the west coastsof adjacent continents. Because the air does notdescend all the way to the surface, a subsidence inversioncalled the trade wind inversion forms above the surface.The air below the inversion, called the marine layer,is cool and relatively moist.
4Hurricanes have sustained wind speeds of 120 km/hr (74 mph) and are typically about 600 kilometers (350 mi) wide.Sea level pressure near the center of a typical hurricane isaround 950 mb, but pressures as low as 870 mb have beenobserved for extremely powerful hurricanes.
5Hurricanes obtain most of their energy from the latent heat released by condensation and are mostcommon where a deep layer of warm water fuels them.August and September are the prime hurricane monthsin the Northern Hemisphere, while January to Marchis the main season in the Southern Hemisphere.
7The hurricane eye is a region of relatively clear skies, slowly descending air, and light winds.Along the margin of the eye lies the eye wall,the zone of most intense storm activity withthe strongest winds, thickest cloud cover, andmost intense precipitation of the entire hurricane.
9Tropical disturbances are disorganized groups of thunderstorms having weak pressure gradients andlittle or no rotation. Most tropical disturbances thatenter the western Atlantic and become hurricanesoriginate in easterly waves, large undulations orripples in the normal trade wind pattern.
10Easterly waves have surface convergence and cloud cover east of the axis and divergence to the west.
11When a tropical disturbance develops to the point where there is at least one closed isobar on a weather map, thedisturbance is classified as a tropical depression.If the depression intensifies further and maintainswind speeds above 60 km/hr, it becomes a tropical storm.A further increase in sustained wind speeds to120 km/hr creates a true hurricane.
12A hypothetical hurricane moves northward at 50 km/hr. Along the right-hand side, the 200 km/hr winds are in the same direction as themovement of the storm, so there is a net-wind speed of 250 km/hr.On the left side, the net winds are southward at 150 km/hr.
13Hurricanes form only where the ocean has a deep surface layer with temperatures above 27 °C (81 °F).The need for warm water precludes hurricane formationpoleward of about 20 degrees becausesea surface temperatures are usually too low there.
14Hurricane formation also depends on the Coriolis force, which must be strong enough to prevent filling ofthe central low pressure. The absence of aCoriolis effect at the equator prohibitshurricane formation between 0° and 5° latitude.
15Hurricanes and tropical storms have a tendency to move north or northeast out of the tropics along the southeast coast of North Americaand often move in wildly erratic ways. After making landfall,a tropical storm may die out completely within a few days.
16Storm surge is a rise in water level induced by the hurricane. Strong winds blowing toward a coast force surface waterslandward and thereby elevate sea level. Low atmosphericpressure in a hurricane also contributes to the storm surge.For every millibar of pressure decrease,the water level rises 1 cm.
17The 1900 Galveston hurricane was the deadliest in U.S. history.
18When forecasters predict that an approaching hurricane will reach land in more than 24 hours, they issue ahurricane watch. If it is expected to make landfallover the United States within 24 hours,they issue a hurricane warning.
19The Saffir-Simpson scale classifies hurricanes into five categories, with increasing numberscorresponding to lower central pressures,greater wind speeds, and larger storm surges.
20The next chapter examines weather forecasting and analysis.