40th Anniversary of the Dvorak technique

Illustration of cloud classification chart from Dvorak's 1975 paper.

Illustration of cloud classification chart from Dvorak’s 1975 paper.

In May of 1975, Vernon Dvorak published his technique for estimating tropical cyclone strength using satellite pictures.  Prior to the use of his technique, hurricane specialists would resort to a subjective evaluation of such images to guess at storm intensity.  At the time, Dvorak worked at NOAA’s Applications Group of the National Environmental Satellite Service and had spent the last three years distilling insights gained from these subjective judgements into an objective method of evaluating a satellite photograph to reach an estimate. He compared various cloud features such as the eye, central dense overcast, and rain bands with aircraft flights and data on storm winds from other platforms to reach a T (for tropical) number.  This T number could then be referenced on a chart to yield an estimated maximum wind speed and lowest central pressure.  He also included a means of estimating trends in the change of cloud appearances over 24 hours in order to forecast the storm’s intensity change.

The Dvorak Technique proved a big improvement over previous methods and soon spread to other tropical forecast centers around the world.  And Dvorak continued to refine his methodology, fine-tuning his estimates for different ocean basins and adapting the use of enhanced infrared satellite for night-time estimates.  There are recognizable problems with the Technique, with different estimates possible from different experts, but it is still the primary tool for estimating tropical cyclone intensity over the open sea in those ocean basins where no aircraft reconnaissance is available.

References

Vernon F. Dvorak, 1975: Tropical Cyclone Intensity Analysis and Forecasting from Satellite Imagery. Mon. Wea. Rev., 103, 420–430.

doi: http://dx.doi.org/10.1175/1520-0493(1975)1032.0.CO;2