When direct measurement of wind speed is not possible, one common method for estimating the intensity of tropical cyclones is the Dvorak technique. Developed three decades earlier by American meteorologist Vernon Dvorak, the technique estimates maximum wind speed by analysing subtle differences between satellite images taken in visible and infrared light. The Dvorak technique does not, however, measure storm speed directly. Some meteorologists believe that under certain conditions it estimates maximum wind speed above its actual value. [source]
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JPL
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The data in the accompanying image come from NASA's Jet Propulsion Laboratory (JPL) and the Indian Space Research Organisation. [source] [source]
Because meteorological agencies, considering the possible dangers, do not send reconnaissance aircraft to observe tropical cyclones in the Pacific, and because instruments installed at ground stations rarely survive these storms, storm-warning centres and other research groups must rely on estimates derived from the Dvorak technique. American meteorologist Eric Holthaus said that Super Typhoon Haiyan, which had struck the Philippines the previous week, even exceeded the highest value defined on the Dvorak scale (8.0). [source]
Another approach uses a scatterometer, a type of microwave radar, to measure the strength of tropical storms. For example, this instrument aboard the Indian Space Research Organisation's Oceansat-2 satellite could be used to measure storm strength over the ocean surface. On November 6, 2013, Oceansat-2 measured Haiyan over the ocean at 9:30 a.m. local time, as shown in the image. The arrows indicate wind direction and the colours indicate wind speed, with purple representing stronger winds. As described in the source, the strongest part of the storm was northeast of the storm centre, or eye. [source]
According to Oceansat-2 data processed by researchers at NASA JPL using empirical methods, the storm's measured intensity peaked between 190 and 300 kilometres per hour—enough, as the original article puts it, to completely destroy virtually anything at the surface. [source]
Bryan Stiles of JPL cautioned that Haiyan's maximum strength was probably greater than Oceansat-2 measured. Under the algorithm developed by his group, Oceansat-2 data were averaged over an area greater than 24 square kilometres, producing a value somewhat below the storm's absolute peak intensity. On that basis, Stiles estimated a minimum wind speed of about 240 kilometres per hour, roughly 20 percent higher than the Oceansat-2-derived figure discussed in the article. His team had not yet had sufficient time for more detailed calculations. [source]
Brian McNoldy of the University of Miami argued that because a scatterometer's resolution is normally about 25–50 kilometres, the instrument cannot estimate the most extreme winds. His group was therefore working on another method based on the disturbance produced at the ocean surface by the storm.
In that method, what is actually measured is not wind speed but the difference in the way radiated energy is scattered from the ocean surface. Once this quantity is determined, a complex model is used to calculate the wind speed responsible for the surface disturbance.