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# How strong can a hurricane get?

8 September 2017

For two and a half days, Irma converted heat into kinetic energy so efficiently that it maintained an intensity at or even exceeding the theoretical limit.

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Intense hurricanes, such as this year’s Hurricane Irma and 2015’s Hurricane Patricia, often make headlines for their extreme maximum wind speeds. Irma peaked at 185 mph/298 kph for much of 6–7 September, while Patricia reached an almost unbelievable 215 mph/346 kph. Such extreme wind speeds provoke questions about how fast the winds in a hurricane can possibly get. So what is the theoretical upper limit?

To answer this question, we can use a theoretical hurricane model developed by MIT atmospheric scientist Kerry Emanuel (see these papers from 1988 and 1995). According to the model, a hurricane can be regarded as a Carnot heat engine. The storm extracts heat from the warm tropical ocean (the water acts as fuel) and converts some of that heat into kinetic energy (that is, wind). The kinetic energy of the hurricane’s winds is ultimately dissipated due to surface friction.

When there is no intervention from external factors, a hurricane will intensify until kinetic energy production balances kinetic energy dissipation to create a steady state. Once that balance is reached, we can equate two mathematical expressions: the work produced by the hurricane and the dissipation of kinetic energy. From the resulting equation, we can obtain a formula that explicitly evaluates a hurricane’s maximum wind speed Vmax, a value known as maximum potential intensity:

$Vmax = (Tocean−TaloftTaloft)E$

The variables in the equation are the temperature of the ocean surface (Tocean); the temperature at the cloud-top level, usually 12–18 km (Taloft); and E, a factor that determines how fast heat moves from the ocean to the atmosphere. E depends on the temperat­ure, humidity, and pressure of the air just above the ocean surface. The formula shows clearly that a hurricane’s maximum potential intensity goes up when the storm encounters warm waters, or when the temperature at the cloud-top level is low. Hurricanes are especially efficient when there is a large temperature difference between the surface and the level at which the air is exhausted of moisture.

All the variables in the equation can be obtained from instruments such as buoys, satellites, and weather balloons. Assuming typical conditions in the tropical western Pacific Ocean, the place with the most cond­­ucive conditions for hurricane formation on Earth, we get a Vmax value of 200 mph. Assuming that maximum potential intensity theory is appropriate, we can say that the highest wind speed any hurricane can produce is about 200 mph.

When we plug values into the Vmax equation that are typical of the tropical Atlantic Ocean around this time of year, the maximum potential intensity is ~180 mph. This value is remarkably close to Hurricane Irma’s actual peak intensity of 185 mph. In fact, Irma and Patricia were a bit stronger than their theoretical Vmax. That is not totally uncommon. Current research is trying to understand why this is possible and how the theory can be improved.

Despite the few outlier cases, maximum potential intensity theory is generally useful for predicting the upper limit of hurricane intensity. Hurricanes rarely meet their Vmax because the environmental conditions are hardly ever 100% ideal and other external factors may disrupt intensification. And when hurricanes do reach their maximum potential intensity, they typically do so only for short periods.

Hurricane Irma is unique in that it sustained an intensity close to Vmax for a remarkably long time. Irma maintained category-5 (157+ mph) status for more than 60 consecutive hours, the longest such period for any Atlantic hurricane on record. Irma is a storm for the record books, and it intrigues scientists who are trying to better understand the mechanics of these violent atmospheric phenomena.

Falko Judt is an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. He researches tropical meteorology—hurricanes in particular—and atmospheric predictability.

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