What you see is not what you think you're getting

Tools

By Kevin White

Story Created: Oct 3, 2006 at 2:56 PM CDT

Story Updated: Oct 10, 2006 at 1:20 PM CDT

By KSN Chief Meteorologist Dave Freeman

No, I'm not talking about the latest consumer fraud! But I am talking about something that fools just about everyone -- from the nervous ninny who is afraid of every dark cloud in the sky to the seasoned, trail-bitten storm chaser. It is the wind speed! We see a constant stream of calls and emails here in the WeatherLab from folks who just can't understand why there is no warning on their thunderstorm -- it just has to be severe because the trees are bending and swaying and the windows are rattling! But in many of those cases, folks are surprised to find out that the wind speed is nowhere near the severe thunderstorm criteria of 58 mph. Even veteran, trained storm spotters and researchers have been shown to be poor estimators of wind speed!

Fortunately, we have a very helpful tool that has been around for centuries! In 1805, Sir Francis Beaufort of England developed a scale that was designed to help sailors match what they were seeing with their eyes to actual wind speed. It was then modified to provide useful guides for us landlubbers. By making careful observations, we can use the Beaufort Scale (see below) to come up with a more reliable estimate of wind speed. For example, let's take the situation I mentioned above: Whole trees are bending and swaying and the windows are rattling. Is that a severe thunderstorm? Let's ask Sir Francis! By following the Beaufort Scale we see this category:

Near Gale

32 to 38 mph

Whole trees in motion. Inconvenience felt when walking against the wind.

So, that horrible storm that is making the trees sway and the windows rattle is only producing 32-38mph winds! That is way below the severe weather criteria of 58mph, which falls into this category on the Beaufort Scale:

Whole Gale

55 to 63 mph

Considerable structural damage occurs, especially on roofs. Small trees may be blown over and uprooted.

(By the way, the reason the severe weather criteria is 58 mph is because that is 50 knots, the unit of speed used to measure wind in aviation.)

Below I have listed the entire Beaufort Scale so that you can bookmark this page or even print it out. Keep the Scale handy and when you see strong winds, use the scale to get an idea of what type of wind you are actually experiencing.

Remember, if you see wind that reaches 58 mph or greater, please share that information with local law enforcement, the National Weather Service, and of course the KSN WeatherLab. We'll use that information to help keep you and your neighbors safe!

Estimating Wind Speeds with Visual Clues
Beaufort number	Description	Speed	Visual Clues and Damage Effects
0	Calm	Calm	Calm wind. Smoke rises vertically with little if any drift.
1	Light Air	1 to 3 mph	Direction of wind shown by smoke drift, not by wind vanes. Little if any movement with flags. Wind barely moves tree leaves.
2	Light Breeze	4 to 7 mph	Wind felt on face. Leaves rustle and small twigs move. Ordinary wind vanes move.
3	Gentle Breeze	8 to 12 mph	Leaves and small twigs in constant motion. Wind blows up dry leaves from the ground. Flags are extended out.
4	Moderate Breeze	13 to 18 mph	Wind moves small branches. Wind raises dust and loose paper from the ground and drives them along.
5	Fresh Breeze	19 to 24 mph	Large branches and small trees in leaf begin to sway. Crested wavelets form on inland lakes and large rivers.
6	Strong Breeze	25 to 31 mph	Large branches in continuous motion. Whistling sounds heard in overhead or nearby power and telephone lines. Umbrellas used with difficulty.
7	Near Gale	32 to 38 mph	Whole trees in motion. Inconvenience felt when walking against the wind.
8	Gale	39 to 46 mph	Wind breaks twigs and small branches. Wind generally impedes walking.
9	Strong Gale	47 to 54 mph	Structural damage occurs, such as chimney covers, roofing tiles blown off, and television antennas damaged. Ground is littered with many small twigs and broken branches.
10	Whole Gale	55 to 63 mph	Considerable structural damage occurs, especially on roofs. Small trees may be blown over and uprooted.
11	Storm Force	64 to 75 mph	Widespread damage occurs. Larger trees blown over and uprooted.
12	Hurricane Force	over 75 mph	Severe and extensive damage. Roofs can be peeled off. Windows broken. Trees uprooted. RVs and small mobile homes overturned. Moving automobiles can be pushed off the roadways.

Severe Thunderstorm Warning is issued for winds greater than 58 mph.

Dave Freeman
KSN Chief Meteorologist
weatherlab@ksn.com

KSN WeatherLab

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