Hurricanes are massive engines which convert heat to kinetic energy. They draw evaporative heat from the ocean which then becomes the wind and rain of the storm. It’s not possible to directly measure the energy involved in a storm system, but there are two main methods of developing an estimate.
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The first is by sampling the rain rate across the storm, and estimating total rain flux. Rain flux is converted to total volume of rain, either per day or across the hurricane’s life span. The latent heat of condensation for the total volume of rain is calculated. Per NOAA, an average hurricane releases 6×10^14 watts per day. (That’s scientific notation for six times 10 to the 14th power, or one hundred million millions.)
The second method is to sample wind speeds across the storm, and estimate the dissipation of the wind by friction. This gives a result, for an average hurricane, of 1.5×10^12 watts per day (a million millions). The two methods differ significantly, by two orders of magnitude. Worldwide about 3×10^12 watts of electricity are generated every day. By the wind calculation method, a hurricane generates half that much. By the rain calculation method, a hurricane generates 200 times as much. The difference between the rain and wind calculations–a factor of 400–shows that the huge majority of the storm’s energy–about 99.6%–is spent in the rain.
Another question which is difficult to answer exactly is how big a role hurricanes play in the transport of heat from the tropics to the poles. As a hurricane moves across the ocean, the wind stirs the surface waters to a depth of 100 m or more. A model based on the 1996 hurricane season gave the result that about 1.4×10^15 (a billion millions) watts of heat were brought north in the waters of the North Atlantic alone due to hurricane mixing. So hurricanes might be responsible for most of the heat carried north in the Atlantic. They are an essential part of the global weather and climate systems.
Tomorrow: ocean surface currents.
Be well!
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This post was previously published on Dailykos.com and is republished on Medium.
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