Hurricane classification – measuring the strength & intensity of the powerful storms

With Harvey fresh on our minds and eyes on Hurricane Irma, this week’s weather question covers classifications of hurricanes.

As meteorologists, we use the Saffir-Simpson scale to rate hurricanes.

The 1 to 5 scale is based on two factors: the highest wind speed reached for one minute within the storm (maximum sustained winds) and potential property damage.

A hurricane is considered a Category 1 when max sustained winds are 74 to 95 miles per hour.

It is a Category 2 when winds are 96 to 110 miles per hour.

Once wind speeds reach 111 to 129 miles per hour, it is a Category 3 and considered a ‘major hurricane’ because of the potential for significant loss of life and damage. For reference, Hurricane Katrina made landfall in Louisiana as a Category 3.

A Category 4 hurricane has wind speeds of 130 to 156 miles per hour. “Catastrophic damage will occur,” according to NOAA.

Harvey made landfall to Texas as a Category 4.

A Category 5 hurricane has wind speeds of 157 and above. This is the highest and most dangerous classification of a hurricane.

The strongest hurricane with the longest duration is now Hurricane Irma.

Hurricane Allen (1980) is the strongest hurricane on record in the Atlantic Basic (if including Gulf of Mexico and Caribbean Sea) with max sustained wind speed of 190 mph.

The strongest hurricane on record in the world is Hurricane Patricia (2015) with 1-minute max sustained wind at 213 mph.

For more details on magnitude of damage to specific structures, power outages and effects on water supply click here.

Along with wind, there are other important factors to measure the intensity and impacts of hurricanes like pressure and storm surge – which is more destructive than wind speed alone.

Storm surge is usually the greatest threat to life and property for those along the coast where a hurricane hits. A storm surge is the water pushed onshore from the storms’ wind. Combine this with tides moving in, and water levels can rise up to 30 feet bringing danger and destruction to any structures, vehicles or people in the path.

The National Weather Service uses a model called SLOSH (Sea, Lake and Overland Surges from Hurricanes) to help estimate potential storm surges.

One way to measure the intensity of a hurricane is through atmospheric pressure. In the most basic sense, the lower the atmospheric pressure, the more intense the hurricane. There are a few ways to measure atmospheric pressure. For hurricanes, it is most common to use millibars (mb).

Standard pressure in millibars is 1013.25. Strong high pressure (typically fair weather) is 1030 mb.

Typical low pressure (storm system) is 1000 mb.

The lowest pressure in a hurricane ever recorded is 870 mb from Typhoon Tip in October 1979 – which impacted the Philippines.

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