Surface Pressure a Better Indicator of Hurricane Damage Potential, New Study Says

Surface pressure is a better benchmark than sustained wind speeds for forecasting hurricane damage potential, according to a new study from Colorado State University.

Wind speeds have been used for decades in categorizing hurricanes, but the new study, led by noted hurricane season forecaster Dr. Philip Klotzbach, says that the amount of damage that a hurricane causes is better related to a storm's minimum sea-level pressure.

Surface pressure – the same metric that we all use to guess if the weather is getting better or worse by seeing if the pressure is rising or falling – is already a common test of strength used in hurricanes and storm systems around the globe.

Generally, the lower the central pressure, the stronger the storm. The lowest pressure in a hurricane is always found at its center, or in its eye.

This change from wind speeds to pressure is important for a few reasons – the first being better communication of threats.

In the current scale, the Saffir-Simpson Hurricane Wind Scale (SSHWS), not all hurricanes within a certain category are created equal. The scale only takes into account the highest wind speeds within the given storm. In fact, in some cases, a Category 2 hurricane could be worse than a Category 4. More on this later.

The scale says nothing about storm surge, inland flooding, tornadoes, storm size or storm duration over a community.

Nearly nine of every 10 hurricane- and tropical-storm-related direct deaths come from water-related hazards – not from wind, which is what the scale is built around.

Sea-level pressure, however, is more directly related to all threats in a hurricane's arsenal.

A hurricane's pressure is generally lower as wind speeds increase, but pressure is also lower when a storm is larger in size.

A storm that is larger in size usually has a bigger storm surge, brings more rainfall and may have more embedded tornadoes. A larger storm is also more likely to bring damaging hazards to larger population centers.

Two examples of failures by the current SSHWS are Hurricane Ike in 2008 and Superstorm Sandy in 2012.

Hurricane Ike was a Category 2 on the SSHWS but had a pressure of 950 millibars when it arrived on the Texas coast. This was the third-lowest pressure for a landfalling Category 2 since 1900.

Ike caused about $30 billion in damage, according to the National Hurricane Center. In 2009, Ike ranked as the second-costliest hurricane to make landfall in the United States. Since then, more damaging storms have made landfall, but Ike remains as the sixth-most-damaging hurricane.

The best example of a SSHWS failure (along with its giant communication failures) was Hurricane Sandy, better known as Superstorm Sandy. It had a minimum pressure of 942 millibars at landfall, 8 millibars lower than Ike's landfall.

While it became post-tropical (or more winterlike), Sandy had the lowest pressure for any Category 1-equivalent storm since 1900. Based on the proposed pressure scale, Sandy would have been a Category 4 hurricane.

Sandy's enormous size – more than 850 miles in diameter – drove a catastrophic storm surge into New York and New Jersey.

Superstorm Sandy caused $65 billion in damage in the mid-Atlantic and in the Northeast, currently the fourth-most-damaging tropical cyclone in the Atlantic.

A few other reasons this change is important:

• For accuracy: Wind speeds are often estimated by hurricane hunter reconnaissance planes and in between surface observations if there are any in the area. Pressure is easier to measure with more accuracy.
• For better data: Barometers (the instrument that measures pressure) are generally much more sturdy than anemometers (the instrument that measures wind speeds). Barometers last longer when hurricane conditions are occurring.
• For a better description of a storm as a whole, meteorologically: In order for pressure to drop in a cyclone's center, every part of the hurricane must work together as a cohesive system. This means that when pressure is lower, the whole system is healthier. This is contrary to the wind speed point of view, which only looks at one small part of a hurricane while other parts of the system may be ill-formed or ragged.
• For historical comparison: Pressure data is available as far back as 1851 for landfalling hurricanes and to 1979 for all named tropical systems in the Atlantic.

The authors of the study state, "While no scale will ever perfectly account for the totality of storm risk to life and property (e.g., inland flooding), any improvements to better explain and warn the potential hurricane impacts to an increasingly vulnerable coastal and inland population is, in our view, a worthwhile endeavor."

The History of the Saffir-Simpson Hurricane Wind Scale

The Saffir-Simpson Hurricane Scale was first developed in the late 1960s and early 1970s by Herbert Saffir, a consulting engineer who lived in Florida, and Dr. Robert Simpson, who was then the director of the National Hurricane Center, to classify hurricanes.

Saffir began working on the scale in 1969 after his work on a United Nations commission, which had been looking at preventing damage to low-cost housing in Hurricane Alley, turned up no options for quantifying hurricane damage.

The first version of the scale showed up in 1972.

Saffir turned his construction-based scale, known at the time as the "Disaster Potential Scale for Atlantic Hurricanes," over to the National Hurricane Center, where Simpson added storm surge and flooding to the simple 1-to-5 scale, which was designed to communicate damage akin to the Richter Scale for earthquakes.

A similar version to the scale, shown below, was run in the guidebook for hurricanes – NOAA's National Hurricane Operations Plan – from 1972 to 1996.

This study isn't the first to bring sea-level pressure to the Saffir-Simpson scale.

Previous versions listed central pressures typically associated with each category due to a relationship that exists between pressure and wind, but the details can vary depending on the nature of each particular hurricane. Also, storm surge was quantified by category, but that was removed from the operations plans in 1997 and from the official scale in 2009.

The scale is an unofficial extension of the Beaufort Scale for wind, which was generally used in the days of shipping and boating. The SSHWS starts at Beaufort Force 12, or at a wind speed of 74 mph.

Note that hurricanes were also loosely rated on the Fujita Scale for a time.

Today, the SSHWS is strictly a wind scale, a 1-to-5 categorization based on the hurricane's intensity at the indicated time. The scale provides examples of the type of damage and impacts in the U.S. associated with winds of the indicated intensity. In general, damage rises by about a factor of four with every category increase.

However, hurricanes with large wind fields can produce storm-surge heights that are much higher than is average for a given category, as was the case with Category 2 Hurricane Ike in 2008.

Conversely, very compact hurricanes – even if extremely strong windwise and with very low central pressures, as was the case with Hurricane Charley in 2004 – can produce surges substantially lower than what was included in the original scale.

This scale does not account for how much rainfall or how many tornadoes a hurricane will produce, either.

Tropical storms can create more damage than Category 5 hurricanes in some cases – Tropical Storm Allison in 2001, for example.

Smaller changes to the scale regarding translations to knots were made in 2012.

The SSHWS has been tweaked since its inception, and conceivably will be changed in the future, perhaps with pressure in mind once again.

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