How to Estimate Wind Speed

October 30, 2012

Hurricane Sandy is lashing the Northeast as I write this. Heavy winds and powerful gusts are hitting the region from Boston to North Carolina, with maximum gusts in many areas expected to reach well above 50 miles per hour. Here’s a quick refresher on how to estimate wind speed at your location.

The following information has been adapted from the modern version of the Beaufort Scale, which ranks wind speed from 0 (calm) to 12 (hurricane force) based on its observable effects on water and land features.

If You Are Inland
Pay attention to the trees. Note how they move in the wind and the type of damage they are sustaining. If you’re out in the wind, notice how the wind is (or isn’t) affecting your ability to move around.

  • Are only leaves and small twigs in motion?
    •  It’s a gentle breeze (Class 3), with wind speeds of 8 to 12 mph.
  • Are small branches moving as well? 
    • It’s a moderate breeze (Class 4), with wind speeds of 13 to 18 mph. 
  • Are entire small trees swaying in the wind? 
    • It’s a fresh breeze (Class 5), with wind speeds of 19 to 24 mph.
  • Are large branches on large trees moving in the wind? Is it difficult or near-impossible to use an umbrella? 
    • It’s a strong breeze (Class 6) with wind speeds of 25 to 31 mph.
  • Are entire large trees being pushed around? Is the wind making it difficult to walk? 
    • It’s a moderate gale (Class 7), with wind speeds of 32 to 38 mph.
  • Are twigs and small branches being snapped off trees? Is it extremely challenging to walk at all? 
    • It’s a gale (Class 8), with wind speeds of 39 to 46 mph.
  • Are larger branches snapping off and some smaller trees being blown over? All but impossible to walk? 
    • It’s a strong gale (Class 9), with wind speeds of 47 to 54 mph. 
  • Are entire large trees snapping at the trunk or being blown over from the roots? 
    • It’s a whole gale (Class 10), with wind speeds of 55 to 63 mph.
  • Is there extensive and widespread damage, including numerous tree blow downs and structural damage to roofs and buildings?
    • It’s a violent storm (Class 11), with wind speeds of 64 to 73 mph.
  • Is it atmospheric Armageddon, with debris and unsecured objects flying around and widespread devastation?
    • It’s a hurricane (Class 12), with wind speeds in excess of 74 mph. 

As you can see from this quick overview, there’s a crucial (and dangerous) tipping point somewhere between 45 to 55 mph, when the wind force begins to cause significant damage and power outages as fallen branches and trees hit power lines. Keep in mind also that other factors play into this, including whether or not trees are still in leaf (and thus are more affected by damaging winds) and whether the ground is saturated from heavy rainfall, which increases the likelihood that a tree will be uprooted at lower wind speeds than might be indicated above.

If You Are by the Ocean
In addition to the cues outlined above, wave characteristics and the behavior of the water surface can be telling indications of wind speed.

  • Is the wind generating large wavelets? Are their crests starting to break with scattered whitecaps?
    •  It’s a gentle breeze (Class 3), with wind speeds of 8 to 12 mph.
  • Are whitecaps frequent on waves that reach up to six feet in height?
    • It’s a moderate breeze (Class 4), with wind speeds of 13 to 18 mph. 
  • Are small amounts of spray now being blown around amidst extensive whitecaps?
    • It’s a fresh breeze (Class 5), with wind speeds of 19 to 24 mph.
  • Are white foam crests present on many waves? Is spray becoming airborne at times?
    • It’s a strong breeze (Class 6) with wind speeds of 25 to 31 mph.
  • Is some foam from wave crests being blasted into streaks in line with the wind direction? Are there moderate amounts of airborne spray?
    • It’s a moderate gale (Class 7), with wind speeds of 32 to 38 mph.
  • Are the watery tops of large breaking waves being blasted into spindrift? Are there extensive linear foam streaks and airborne spray?
    • It’s a gale (Class 8), with wind speeds of 39 to 46 mph.
  • Is airborne spray becoming so intense that it’s beginning to affect visibility?
    • It’s a strong gale (Class 9), with wind speeds of 47 to 54 mph. 
  • Has the sea surface assumed a white appearance due to large patches of foam? Is airborne spray noticeably reducing visibility?
    • It’s a whole gale (Class 10), with wind speeds of 55 to 63 mph.
  • Do large patches of foam now cover most of the sea surface? Do large amounts of airborne spray significantly reduce visibility?
    • It’s a violent storm (Class 11), with wind speeds of 64 to 73 mph.
  • Is the sea completely white with foam and spray? Is the air filled with driving spray, all but eliminating visibility? Are you scared out of your mind?
    • It’s a hurricane (Class 12), with wind speeds in excess of 74 mph.

It’s easy to overestimate wind speed. These simple guidelines can help you gauge more precise estimates. It might not make the most thrilling story later (“The wind must have been over 100! I swear!”) but it will certainly be more accurate.

Be safe out there.

“Equipped” is an AMC Outdoors blog, written by Matt Heid.

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Matt Heid

Equipped blogger Matt Heid is AMC's gear guru: He loves gear and he loves using it in the field. While researching several guidebooks, including AMC's Best Backpacking in New England, he has hiked thousands of miles across New England, California, and Alaska, among other wilderness destinations. He also cycles, climbs, and surfs.