Weather for Woodsmen, Beaufort’s Scale and Estimating Wind Speed by Observation ©

 

 

So just how fast was the wind blowing?  Do you know?  Photograph by the Author, taken  on August 27, 2020.

 

The third in a series of articles on weather for woodsmen, for the previous article, go HERE – Author’s Note

Can you estimate how fast the wind is blowing from the things you see around you?  If so, how fast is the wind blowing in the picture above, do you know?  And is it important to know how strong the wind is blowing anyways?

 

Mankind has been studying the weather and observing the winds and their effects on the land and the sea for the last 200,000 years, but it wasn’t until Francis Beaufort designed the scale bears his name that the study and description of winds was organized and classified¹. 

 

The original Beaufort scale, from “Beaufort scale of wind force as adapted for use on forested areas of the northern Rocky Mountains”, by George M. Jemison, page 77.

And according to Francis Beaufort, and the other meteorologists who have revised and updated his scale over the following two centuries, you can estimate the speed of the wind by observing its effects on the land and sea that it blows upon.  In 1960, Beaufort’s scale of wind force reached its modern form, when probable wave heights and probable maximum wave heights were added.  Probable wave height refers to the height of waves experienced far from land in the open sea, remember waves build as they approach shore, because drag from the bottom slows the wave and when that happens the waves get closer together and taller.  And the probable maximum wave height is the height of the highest wave that can be expected in a 10-minute period².

 

A modern example of Beaufort’s Scale, from Sailing: A Beginner's Guide, by David Seidman.

So, you can estimate the speed of the wind by observing its effects on the land and sea that it blows upon.  But is it important to know the wind speed?  Yes, there are several situations where knowing the wind speed is helpful, such as when you are travelling by boat over large bodies of water or when you are travelling where dust or snow could be blown around reducing visibility, however there is one instance where knowing the wind speed is crucial and that is when there is a possibility of wind chill and hypothermia!

 

“Blow, winds, and crack your cheeks! rage! blow!”

William Shakespeare, King Lear, Act III, Scene II

 

An excerpt from "Historia de gentibus septentrionalibus", 1555, by Olaus Magnus, from Wikimedia, HERE.

Knowing the wind speed allows you calculate the wind chill, which is the combined effect of the air temperature and wind speed.  The wind chill is always less than the actual air temperature and knowing what it is allows you to determine how long you can safely be out in the weather without risking frostbite or hypothermia.  Alan E. Nourse, in The Best About Backpacking, wrote “a two-mile-an-hour breeze can drag down body temperature as effectively as a twenty-mile gale if the victim’s clothes are wet”.  And remember hypothermia isn’t just something that happens in the winter, more people die of hypothermia during the spring, summer or fall, than during the winter.

 

NOAA Wind Chill chart.

One way to estimate the wind speed is to watch a flag and use math.  When looking at a flag, estimate the angle of the flag compared to the flagpole and take the angle of the flag and divide it by the number four.  The answer will give you the wind speed in miles per hour³.

 

ANGLE OF FLAG / 4 = WIND SPEED in MPH

 

·       If the flag is not moving and down on the pole, or if it only occasionally stirs, then the wind is calm.  On the chart below the wind would be class 0 if the flag is down and class 1 if only occasionally stirs.

 

·       If the flag is extended at 45^o, then wind is about 11 mph (about 18 kph), and on the chart below the wind would be class 3.

 

·       If the flag flaps lightly and is more than 45^o but less than a fully extended 90^o, then wind speed is between 12 to 18 mph (between 19-29 kph), and on the chart below the wind would be class 4.

 

·       If the flag is fully extended to 90^o and waving over its entire length, the wind speed is between 19 to 24 mph (between 30-39 kph), and on the chart below the wind would be class 5.

 

·       If the flag is stretched and flapping fast, the wind speed is between 25 to 31 mph (40-50 kph), and on the chart below the wind would be class 6.

 

·       If the flag is stretched and flapping hard, the wind speed is between 32 to 37 mph (51-59 kph), and on the chart below the wind would be class 7.

 

However, the easiest way to estimate the wind speed in the field is to look at a flag or, you can use the movements of the trees, bushes, and grass, and compare what you see to the chart below.

 

Excerpts from George Jemison’s Tables 2 and 3, and from Sailing: A Beginner's Guide, by David Seidman, combined into one chart.  Graphic by the Author.

 

So, know you know how to estimate wind speeds.  By looking again at the front picture and the chart above, can you estimate how fast the wind was blowing?  To find out the answer go to note⁴.

 

The break wall on the Buffalo Outer Harbor measures 14 feet (4.3 meters) above the water and the waves on August 27, 2020, which were crashing over it!  Photograph by the Author.

Don’t forget to come back next week and read “Hypothermia!  It can Happen at Any Time, or Anywhere ©”, where we will talk about hypothermia and how stay warm and safe.

 

I hope that you continue to enjoy The Woodsman’s Journal Online and look for me on YouTube at BandanaMan Productions for other related videos, HERE.  Don’t forget to follow me on both The Woodsman’s Journal Online, HERE, and subscribe to BandanaMan Productions on YouTube.  If you have questions, as always, feel free to leave a comment on either site.  I announce new articles on Facebook at Eric Reynolds, on Instagram at bandanamanaproductions, and on VK at Eric Reynolds, so watch for me.

 

That is all for now, and as always, until next time, Happy Trails!

 

 

Notes

 

¹ Captain Beaufort wasn’t the first to try to classify wind speed, others, such as Daniel Defoe who had created a 12-point scale in 1704 that was very similar to Beaufort’s later scale, had beaten him to it.  However, Beaufort’s scale was the first to gain acceptance and come into general use.

 

From “Beaufort: National Meteorological Library and Archive Fact sheet 6 – The Beaufort Scale”, by the Met Office, page 3.

 

² According to the authors of “Beaufort: National Meteorological Library and Archive Fact sheet 6 – The Beaufort Scale”, “Strictly, it applies only when the sea is fully developed; that is, when waves have reached their maximum height for a particular wind speed.  Care must be exercised when the fetch and duration of the wind are limited (the fetch is the distance over which the wind has blown, and the duration the time it has been blowing).  It is also worth remembering that the appearance of the sea’s surface is influenced not only by wind but also by swell (waves from far away), precipitation, tidal streams and other currents.”

 

Ibid., page 8.

 

³ From Paul Wetzl, “How to use a flag to find the wind speed”.

 

⁴ The wind on August 27, 2020, averaged 17.9 mph (28.8 kph) and at noon when that picture was taken the wind was blowing at an average of 18.1 mph (29.1 kph).  That means it was a class 4, moderate wind.

 

From “World Weather, Weather in Buffalo, August 27”, HERE.

 

 

Sources

 

Jemison, George M.; “Beaufort scale of wind force as adapted for use on forested areas of the northern Rocky Mountains”, Journal of Agricultural Research, Vol. 49, No. 1, July 1, 1934, [Forest Service, United States Department of Agriculture, Washington, D.C.], page 77 to 82, https://www.fs.fed.us/rm/pubs_exp_for/priest_river/exp_for_priest_river_1934_jemison01.pdf, accessed June 5, 2022

 

Met Office; “Beaufort: National Meteorological Library and Archive Fact sheet 6 – The Beaufort Scale”, [Met Office, © 2010], https://www.metoffice.gov.uk/binaries/content/assets/metofficegovuk/pdf/research/library-and-archive/library/publications/factsheets/factsheet_6-the-beaufort-scale.pdf, accessed June 5, 2022

 

Seidman, David; Sailing: A Beginner's Guide, [A & C Black (Publishers) Ltd, 1995]

 

Van Lear, Denise; The Best About Backpacking, [Sierra Club, San Francisco, 1974], page 355

 

Wetzl, Paul; “How to use a flag to find the wind speed”, Youngstown Weather, May 11, 2021, Updated June 4, 2021, [© 2022 Nexstar Media Inc.], https://www.wkbn.com/weather/weather-for-kids-how-to-use-a-flag-to-find-the-wind-speed/, accessed June 6, 2022