Chapter 12 Tropical Storms and Hurricanes Extremely strong tropical storms go by a number of different names, depending on where they occur. Over the Atlantic and the eastern Pacific they are known as hurricanes. Those over the extreme western Pacific are called typhoons; those over the Indian Ocean and Australia are cyclones. Middle- and upper-level air along the eastern side of anticyclones sinks as it approaches the west coasts of adjacent continents. Because the air does not descend all the way to the surface, a subsidence inversion called the trade wind inversion forms above the surface. The air below the inversion, called the marine layer, is cool and relatively moist. Hurricanes 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 is around 950 mb, but pressures as low as 870 mb have been observed for extremely powerful hurricanes. Hurricanes obtain most of their energy from the latent heat released by condensation and are most common where a deep layer of warm water fuels them. August and September are the prime hurricane months in the Northern Hemisphere, while January to March is the main season in the Southern Hemisphere. Hurricanes around the globe. The 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 with the strongest winds, thickest cloud cover, and most intense precipitation of the entire hurricane. A cross section of a typical hurricane. Tropical disturbances are disorganized groups of thunderstorms having weak pressure gradients and little or no rotation. Most tropical disturbances that enter the western Atlantic and become hurricanes originate in easterly waves, large undulations or ripples in the normal trade wind pattern. Easterly waves have surface convergence and cloud cover east of the axis and divergence to the west. When a tropical disturbance develops to the point where there is at least one closed isobar on a weather map, the disturbance is classified as a tropical depression. If the depression intensifies further and maintains wind speeds above 60 km/hr, it becomes a tropical storm. A further increase in sustained wind speeds to 120 km/hr creates a true hurricane. A hypothetical hurricane moves northward at 50 km/hr. Along the right-hand side, the 200 km/hr winds are in the same direction as the movement 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. Hurricanes form only where the ocean has a deep surface layer with temperatures above 27 °C (81 °F). The need for warm water precludes hurricane formation poleward of about 20 degrees because sea surface temperatures are usually too low there. Hurricane formation also depends on the Coriolis force, which must be strong enough to prevent filling of the central low pressure. The absence of a Coriolis effect at the equator prohibits hurricane formation between 0° and 5° latitude. Hurricanes and tropical storms have a tendency to move north or northeast out of the tropics along the southeast coast of North America and often move in wildly erratic ways. After making landfall, a tropical storm may die out completely within a few days. Storm surge is a rise in water level induced by the hurricane. Strong winds blowing toward a coast force surface waters landward and thereby elevate sea level. Low atmospheric pressure in a hurricane also contributes to the storm surge. For every millibar of pressure decrease, the water level rises 1 cm. The 1900 Galveston hurricane was the deadliest in U.S. history. When forecasters predict that an approaching hurricane will reach land in more than 24 hours, they issue a hurricane watch. If it is expected to make landfall over the United States within 24 hours, they issue a hurricane warning. The Saffir-Simpson scale classifies hurricanes into five categories, with increasing numbers corresponding to lower central pressures, greater wind speeds, and larger storm surges. The next chapter examines weather forecasting and analysis.