Hurricane Categories and Their Meaning
If you live near the coast, you probably gnawed your nails as Irma went from tropical storm to hurricane and the category numbers went up. Irma is now a Category 5 hurricane, but what exactly do these categories mean?
Hurricane categories are based on wind speed. Specifically, the highest wind speed that persists for one minute at 10 meters or 33 feet above the ground. The higher the wind speed, the more wind damage we can expect – although this does not apply to other types of damage such as flooding. Strong winds can knock over power lines (category 1), uproot trees (category 2) and flatten houses (category 5).
Here’s a breakdown according to the National Oceanic and Atmospheric Administration :
- Tropical Depression : Wind speeds of 38 miles per hour or less .
- Tropical storm : wind speeds of 39 to 73 miles per hour.
- Category 1 : wind speeds between 74 and 95 mph. “Some damage,” including collapsed power lines, broken tree branches, and damage to roofs and gutters of well-built homes.
- Category 2 : wind speeds between 96 and 110 mph. “Significant damage,” including uprooted trees, “serious roof and siding damage” to homes, and power outages that last for days or weeks.
- Category 3 : wind speeds between 111 and 129 mph. “Destructive damage”, including roof decks and gable ends torn away from houses.
- Category 4 : wind speeds between 130 and 156 mph. “Catastrophic damage”, including the dismantling of roofs and walls of houses; most of the area will be “uninhabitable for weeks to months.”
- Category 5 : wind speeds of 157 mph and above. A “large percentage” of homes will be completely destroyed.
However, wind speed does not fully describe what to expect from a hurricane. Size matters too. The stronger the storm, the more moisture it is going to shed in the form of rain on the affected areas. Hurricanes and their weaker cousins like tropical storms can also cause flooding due to storm surges . A storm surge occurs when water from the ocean hits land as an unusually high tide.
Katrina, for example, had slowed to Category 3 by the time she made landfall near New Orleans in 2005, but she caused a massive storm surge, heavy rain, and several side effects in the form of tornadoes. All this together has made Katrina one of the most deadly Atlantic hurricane ever on record .
Meteorologists can predict these other effects, such as precipitation and storm surges, but predictions are only correct when the storm is close to shore. Harvey’s impending flood only became apparent two days before the hurricane , when it may have been too late to try to evacuate the entire city. NOAA has a surge forecasting tool that emergency responders and forecasters can use to plan for the future.
To find out how much damage a storm can cause, visit the National Hurricane Center for the latest forecasts.