Chapter 12Chapter 12

Tropical Storms and HurricanesTropical Storms and Hurricanes

Extremely strong tropical storms go by a number of different names, depending on

where they occur. Over the Atlantic and theeastern Pacific they are known as hurricanes.

Those over the extreme western Pacificare 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 Marchis 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, andmost intense precipitation of the entire hurricane.

A cross section of a typical hurricane.

Tropical 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 or

ripples in the normal trade wind pattern.

Easterly waves have surface convergence and cloud covereast of the axis and divergence to the west.

When a tropical disturbance develops to the point wherethere is at least one closed isobar on a weather map, the

disturbance 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 to 120 km/hr creates a true hurricane.

A hypothetical hurricane moves northward at 50 km/hr. Along theright-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.

Hurricanes form only where the ocean has a deepsurface layer with temperatures above 27 °C (81 °F).

The need for warm water precludes hurricane formationpoleward 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 aCoriolis effect at the equator prohibits

hurricane formation between 0° and 5° latitude.

Hurricanes and tropical storms have a tendency to move north ornortheast 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 waterslandward 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 hurricanewill reach land in more than 24 hours, they issue ahurricane 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 hurricanesinto five categories, with increasing numbers

corresponding to lower central pressures,greater wind speeds, and larger storm surges.

The next chapter examines The next chapter examines weather forecasting and analysis.weather forecasting and analysis.