A World of Weather, Edition 3: Chapter 10 Introduction

CHAPTER 10

TROPICAL WEATHER, PART II: HURRICANES Hurricane Camille (Figure 10.1) was one of the most powerful storms of the 20th century, making landfall along the Mississippi Coast on August 17, 1969. At its peak intensity over the very warm offshore waters of the Gulf of Mexico, Camille's central pressure fell into the barometric cellar at 905 mb, while its sustained winds revved up to an awesome 300 km/hr (185 mph). These readings easily qualified for the highest rating on the Saffir-Simpson Damage Potential Scale (Table 10.1), giving Camille the dubious distinction of being one of only two Category-5 hurricanes ever to make landfall in the United States in recorded weather history (the Labor-Day hurricane that lashed the Florida Keys in 1935 was the other).

Table 10.1:  Saffir-Simpson Hurricane Damage Potential Scale

   Category      Pressure (mb; inches mercury)   Wind (knots; mph)   Storm Surge (m; ft)

1: Minimal         980        28.94             64-82     74-95         1.0-1.7   4-5
2: Moderate        965-979    28.50 - 28.91     83-95     96-110        1.8-2.6   6-8
3: Extensive       945-964    27.91 - 28.47     96-113    111-130       2.7-3.8   9-12
4: Extreme         920-944    27.17 - 27.88     114-135   131-155       3.9-5.6   13-18
5: Catastrophic    < 920       < 27.17           >135      >155          >5.6     >18

At the coastal town of Pass Christian, MS, some residents at the Richelieu Apartments (Figure 10.2a) did not evacuate as Camille bore down on the central Gulf Coast. That decision cost many lives (legend suggests that residents had thrown a "hurricane party," but that story now appears to be a myth). Fierce onshore winds on Camille's right-forward flank (as viewed looking down over the eye, the relatively calm center of the storm) pushed a huge volume of water onto land - the storm surge - raising coastal sea levels by nearly 7.6 m (about 25 ft), with battering waves breaking atop the monstrous tide. Camille's storm surge leveled the apartment complex (Figure 10.2b).

Had Camille been born over the western north Pacific Ocean, it would have been called a typhoon. Had the storm formed over the Indian Ocean or near Australia, it would have been called a tropical cyclone. Regardless of place of origin or title bestowed by local cultures, hurricanes are all the same type of powerful weather system. Formally, a hurricane is a low pressure system of tropical origin that sustains a fierce surface wind of at least 119 km/hr (74 mph).

On the way to becoming a hurricane, an intensifying tropical low passes through three distinct phases of development. In the stage prior to hurricane status, when sustained surface winds reach 62 km/hr (39 mph) or greater, a developing system is called a tropical storm and christened with a male or female name (see Table 10.2). We will discuss the first two developmental stages shortly.

On average, about 80 tropical storms form worldwide each year. Though seemingly small in number, these storms attract a great deal of attention from meteorologists. A swirling, galactic-arm appearance on satellite images (see Figure 10.3a and Figure 10.3b), together with an eye staring menacingly into space, absolutely invites investigation by the scientifically curious. Much more importantly, hurricanes destroy property and take human life - tolls that can reach staggering proportions when these powerful storms pass over highly populated, low-lying coastal areas.