Chapter 11: THE HURRICANE

Hurricane Climatology: Engine Specs

Figure 11.2 defines the tropical basins in which hurricanes usually form, and shows some of the tracks commonly followed by hurricanes that form in these basins. The water in these birth basins is warm, the most critical factor for the ignition of a hurricane's heat engine. Ocean currents play an important role in fashioning these breeding grounds. Currents flowing from higher latitudes can put a chill on low-latitude waters, as the California and Peru currents do along much of the west coast of the Americas (refer to Figure 3.12 to see these currents). Or poleward-flowing currents can warm mid-latitude waters, as the Gulf Stream does over the western North Atlantic Ocean. In essence, cold ocean currents shrink the latitudinal range in which hurricanes can be born and thrive, while warm ocean currents expand the range.

Gearing Up for Hurricane Season: Setting the Timing and Rolling to the Starting Line

The Atlantic Basin, which includes the North Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico, is the source region for virtually all hurricanes that directly impact the contiguous United States. Since 1945, an average of nine tropical storms have formed in this basin each year, with five to six becoming hurricanes, on average.

Tropical storms that develop in June are early birds, typically forming over the southwestern Caribbean or the southern Gulf of Mexico, where shallower water depths (compared to the open Atlantic Ocean) allow the water to warm faster. Hurricane Allison was such a storm, coming ashore near Apalachee Bay, FL, on June 5, 1995, the earliest that a hurricane has hit the contiguous United States in over 100 years of records. Of the 59 tropical storms that have formed in June in the Atlantic Basin in the period 1886-1995, only two formed outside of the Gulf of Mexico or the Caribbean Sea. As waters gradually warm over the tropical North Atlantic, storm breeding grounds spread eastward, with many late-season storms developing off the coast of western Africa. These storms are sometimes called Cape Verde storms, named for the tropical islands that lie about 700 km (435 mi) off the west coast of Africa.

The eastern Pacific Basin is typically more active than the Atlantic Basin. As evidence, consider that the waters off the west coast of Central America boast the world's highest number of births of tropical storms per square kilometer. The high birth rate can be attributed, in part, to high water temperatures that rise above 84oF (29oC) in August and September. On average, 16 tropical storms form in this basin each year, and an average of eight or nine become hurricanes. The tracks of hurricanes spawned in this hotbed of storms are limited to the north by cool waters: hurricanes never strike southern California, given the vast fortification of chilly waters brought south by the California current. However, the remnants of eastern Pacific hurricanes can bring unusual summer rains and occasionally gusty winds to southern California. In September 1939, the remnants of an eastern Pacific hurricane managed to produce sustained winds of 52 km/hr (32 mph) in Los Angeles.

When it comes to producing tropical storms and hurricanes, the western North Pacific never sleeps. Typhoon season runs throughout the year, given the enduring warmth of ocean waters in this basin. It is not unusual for thirty or more tropical storms to form in this basin in one year.

Hurricane Season: The Tropics in Overdrive

When a powerful hurricane struck the Florida Keys on Labor Day 1935, killing 408 people, Ernest Hemingway remarked that there was no autumn, only "a more dangerous summer" surrounding his tropical home at the tip of Florida. Over the years, residents of the tropics and subtropics of the Northern Hemisphere have learned from experience that the time for a flurry of tropical storms and hurricanes lags the summer solstice by two months or so. Figure 11.3 shows the daily frequency of tropical storms and hurricanes for the Atlantic Basin from May 1 to December 31. In this basin, the official hurricane season extends from June 1 until November 30, but it could be argued that there is a hurricane season within the hurricane season þ the peak activity of hurricanes spans from August through October (the period when tropical waters are warmest). About 90% of all days when hurricanes are in existence in the Atlantic Basin fall within this period. The peak around September 10 coincides with the climatological peak in ocean temperatures.

Baseball season begins in April with anxious speculation about the pennant races. Hurricane season begins with similar but more sobering speculation about how active the tropics will be in the coming months. Since 1950, there have been as many as twelve hurricanes in a year (in 1969) and as few as two (in 1982) in the Atlantic Basin. The average is around 5.7 (the greatest number of named storms in any year since 1950 is 19, in 1995). Dr. William Gray, a well-known hurricane researcher at Colorado State University, has been issuing forecasts of the number of tropical storms and hurricanes that will form in the Atlantic Basin during a given hurricane season, drawing upon climatological statistics and current weather patterns (such as the strength of stratospheric winds and whether an El Nino is in progress). Dr. Gray's hurricane forecasts for each hurricane season since 1984, issued in early June of that year, and the observed number of hurricanes are given in Table 11.1.

Table 11.1: Forecasts of the number of hurricanes in the Atlantic Basin, issued in early June of each year by Dr. William Gray, and the actual number of hurricanes that were observed, for each year since 1984.


                         Number of      Number of
               Year      Hurricanes     Hurricanes
                         Forecasted     Observed

               1984           7         5
               1985           8         7
               1986           4         4
               1987           5         3
               1988           7         5
               1989           4         7
               1990           7         8
               1991           4         4
               1992           4         4
               1993           7         4
               1994           5         3
               1995           8         11
               1996           6         9

As shown in Table 11.1, Dr. Gray's forecasts have been perfect three times, off by one hurricane twice, by two hurricanes four times, and by three hurricanes four times; on average, the forecasts are off by 1.7 hurricanes. Interestingly, a forecaster who stays close to the average usually won't go too far astray, given that only twice during the period 1984-96 did the number of hurricanes fall outside the range of three to eight. In fact, simply alternating forecasts of five and six hurricanes during the period 1984-96 would have produced an average error of 1.9 hurricanes. Note from the table that during nine of the thirteen years, Dr. Gray correctly predicted whether the observed number of hurricanes would be above or below the average of 5.7. When tropical storms are included in the total, Dr. Gray's predictions have been on the correct side of the mean eleven out of the thirteen years.

With an overview of where and when hurricanes form, we now turn to the details of how the greatest heat engine on earth gets up and running on all cylinders.

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