The Intersection

Alberto: It’s Gonna Be Close

The line of demarcation between a tropical storm and a hurricane is, obviously, somewhat arbitrary in nature. Why should the official cutoff point be sustained winds of 74 miles per hour, rather than 73 mph or 75 mph? Defined in terms of their true meteorological species, all three storms are tropical cyclones–period.

From a psychological point of view, however, the question of whether Florida gets hit by a “hurricane” this early in the season, as opposed to a mere “tropical storm,” obviously makes a lot of difference. So that’s what everyone is going to be watching for. The latest advisory from the guys at the National Hurricane Center–written in their inimitable all caps elliptical style–makes it clear that this one is going to be close: “A FORECAST OF ALBERTO POSSIBLY REACHING HURRICANE STRENGTH BEFORE LANDFALL APPEARS TO STILL BE IN ORDER.”

Stay tuned….


  1. #1 Clyde Soles
    June 13, 2006

    Since you’re writing the book, get back to us on the historical reason for the 74 mph cut-off. Why did Beaufort stop at that particular number (actually 73 in some sources)? It isn’t a neat figure in any conversion so where does it come from? As you say, it’s psychological but that’s enough to trigger a mandatory evacuation.

  2. #2 Chris Mooney
    June 13, 2006

    Don’t know that I’ve come across an answer to this question….

  3. #3 did
    June 13, 2006

    I believe it’s a conversion from 65 knots. 1 knot = roughly 1.15 mile/hour.


  4. #4 llewelly
    June 13, 2006

    Why did Beaufort stop at that particular number (actually 73 in some sources)?

    The earliest version of the scale was entirely qualitative; it had only general descriptions of the effects of wind associated with each force number. Beaufort ended his scale at the wind force that could typically be expected to destroy the canvas sails of his time. Later, wind speed values in knots (not mph) were determined experimentally. So why it is 64 knots and not 63 or 65 is presumably in part due to the strength of sails at the time the scale was designed, and in part due to artifacts of experiment design. As to 73 vs 74, I suspect that is due to the fact that many people truncate instead of rounding. 64 kt is approx 73.65 miles per hour. In practice, that means little, since (nearly?) all meteorological agencies estimate (usually from multiple measurements and/or satellite images) wind speeds in increments of 5 knots. As far as the NHC is concerned, a TC is either 60 kt, and therefor not a hurricane, or 65 kt, and a hurricane.

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