Win the ‘Wind Speed’ with Beaufort’s Wind Scale

Why ‘Wind Speed’ plays an important role about how 'Pedestrian Perceive Wind Comfort'?

Yellow wind sock on a wooden post (Photo by Dlanor S on Unsplash)

For cities, it is vital that pedestrians are thermally comfortable and safe when they are walking. The mechanical wind effect on pedestrians are assessed, typically with Beaufort Wind Scale. It is an international wind scale (see Table 1 below) which is measured at 10m above ground are reduced to about 75% at head height (Bennett 2007) (Stathopoulos 2009).
 
Table 1: Beaufort Wind Scale used on land (ACSE 2003, reported by Stathopoulos, 2009)  

Beaufort's Number	Description	Wind Speed at 1.5 m height (m/s)	Effects observed on land	Effects on Pedestrians
B0	Calm	0.55	Calm, Smoke rises vertically.	No feeling. Oppressed
B1	Light Air	0.55 -1.39	Smoke Drift indicates wind direction but not wind vanes.	Hard to feel
B2	Light Breeze	1.67 - 3.05	Wind felt on exposed skin.	Wind felt on face
B3	Gentle Breeze	3.33 - 5.28	Light flags extended.	Hair blows in the wind
B4	Moderate Breeze	5.55 - 8.05	Dust and loose paper raised.	Dust raised and hair blown
B5	Fresh Breeze	8.33 - 10.83	Small trees in leaf begin to sway.	Wind is strong enough. Limit of land wind
B6	Strong Breeze	11.11 - 13.89	Large branches in motion, Whistling heard in telegraph wires, Umbrellas used with difficulty.
B7	Near Gale	14.17 - 16.95	Whole trees in motion, Inconvenience felt in walking.
B8	Gale	16.67 - 20.55	Breaks twigs of trees, Generally impedes progress.
B9	Strong Gale	20.83 - 24.17	Slight structural damage occurs e.g. to roofing shingles, TV antenna etc.
B10	Storm	24.44 - 28.33	Seldom experienced inland, Trees uprooted, Considerable structural damage occurs
B11	Violent Storm	28.61 - 32.22	Very rarely experienced, Accompanied by widespread damage.
B12	Hurricane	>32.22

A simple rule of thumb has been provided by Wise (1970) and Pendwarden (1973) and reported by Stathopolous is as follows (Bennett 2007):

V = 5 m/s or 18 km/h onset of discomfort
V = 10 m/s or 36 km/h definitely unpleasant
V = 20 m/s or 72 km/h dangerous

Here, V is the mean wind speed.

Conditions for pedestrians are considered acceptable, if V > 5 m/s and happens less than 20% of the time (Penwarden and Wise 1975). In addition to the Beaufort Numbers of Table 1 above, the following table outlines relationship between activities, areas and pedestrian perception of wind environment (see Table 2 below).

Table 2. Comfort and Safety Criteria against different Beaufort Scale [read in conjunction with the Table 1 above and reported by Jessica Bennet (Bennett 2007)]  

Activity		Areas Applicable	Perceptible	Tolerable	Unpleasant	Dangerous
Walking		Pavements	B5	B6	B7	B8
Strolling		Parks Entrances	B4	B5	B6	B8
Standing, Seating	Short Time	Parks Plazas	B3	B4	B5	B8
	Long Time	Street Cafes Theaters	B2	B3	B4	B8
Acceptable if speed occurs less than				Once a Week	Once a Month	Once a year

The usually recognised effects of winds are along these lines (Bennett 2007):

10 m/s	The limit of comfort when sitting or standing for long periods in an open space.
15 m/s	The limit of acceptability for comfort while walking.
18 m/s	The threshold of danger level.
23 m/s	Completely unacceptable for walking.

Now that you know which speed has what effect and little bit of physics behind wind speed, ‘You can Win it for Comfort’. Remember to walk within 11 m/sec of wind speed. Happy Walking! 

References: 
1. Bennett, Jessica. 2007. “Wind Design Guide.” New Zealand. http://www.victoria.ac.nz/architecture/centres/cbpr/publications/architectural-aerodynamics/pdfs/BBSC_433_Jessica-Bennett_Wind-Design-Guide.pdf.
2. Penwarden, A. D., and Alan Frederick Edward Wise. 1975. Wind Environment around Buildings. UK: Dept. of the Environment, Building Research Establishment : H. M. Stationery Off.
3. Stathopoulos, Ted. 2009. “Wind and Comfort.” In . Florance, Italy. http://www.iawe.org/Proceedings/5EACWE/K04.pdf.