Most people are aware of the basic dangers associated with a hurricane: high winds, tidal surges, tornadoes, torrential rain and inland flooding.
There are a few facts about these dangers, however, that may not be so obvious, and they could impact how we prepare or evacuate from these threats.
Increases in wind speed can affect the amount of force impacting structures along the storm’s path. For example, doubling the wind velocity in a storm generally quadruples the potential wind force on a stationary object. Therefore, a 100 mph wind exerts around four times the amount of pressure on a structure as a 50 mph wind. Even a modest 16 percent increase from 60 mph to 70 mph can increase wind pressure by more than a third. For this reason, it can be misleading to simply look at the Saffir-Simpson category for an approaching storm. For example, Category 1 hurricanes can vary from 74 mph sustained winds up to 96 mph sustained winds (nearly doubling the amount of potential destructive wind pressure from the lower end of the category to the higher).
The power of moving water
People often underestimate the power of moving water, either from a surge or inland flooding. Water weighs about 1,700 pounds per cubic yard. Six inches to a foot of flowing water can sweep an adult off his feet. Two feet of moving water can wash away an automobile. As little as 4 feet of moving water can destroy a wall.
The larger a storm’s wind field radius the more potential there is for destructive storm surges and wind damage. For example, relatively small Category 3 Hurricane Alicia produced a much smaller storm surge than Category 2 Hurricane Ike, despite Alicia’s stronger sustained wind speeds and more favorable (for surge potential) landfall location.
Large, powerful storms can take hurricane force or higher winds far inland, making some evacuation sites less desirable. Large Category 4 Hurricane Carla produced sustained winds of 100 mph or more 150 miles inland, which is why you need to avoid being in the path of such storms when choosing an evacuation locale.
Speed of forward motion
How fast a hurricane is traveling can make a big difference in the level of damage experienced. A fast-moving system will bring a smaller time period of destructive winds than a slow-moving system, whose additional hours of peak winds leads to increased exposure to strong winds.
Also, slow-moving systems tend to produce greater amounts of flooding rains than faster-moving systems, where the duration of rainfall is briefer. On the other hand, a rapidly advancing storm can mean less time for preparation and evacuation.
Your location relative to a hurricane’s path can make a big difference in its impact in your home area or evacuation site. Generally speaking, being in the northeast quadrant of a hurricane will result in stronger winds, heavier rains and higher tornado potential than being to the southwest of a land-falling system.
Galveston weather history has many examples of storms making landfall 75 miles to 100 miles southwest of Galveston resulting in much greater damage than equivalent systems coming ashore 75 miles to 100 miles to the east.
The bottom line: Never underestimate the power of hurricanes or tropical storms and their potential to inflict damage on people and property in their paths.