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

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

Why does wind speed play a critical role in shaping pedestrians’ perception of wind comfort?

Figure 01: Yellow wind sock on a wooden post (Photo by Dlanor S. on Unsplash)

For cities, it is vital that pedestrians feel both thermally comfortable and safe while walking. The mechanical effects of wind on pedestrians are typically assessed using the Beaufort Wind Scale, an internationally recognized standard (see Table 1 below). Wind speeds measured at 10 m above ground level are approximately reduced to 75% at pedestrian head height (1.5 m), as noted by Bennett (2007) and Stathopoulos (2009).

Table 1. Beaufort Wind Scale adjusted for Land Conditions (ACSE, 2003; reported in Stathopoulos, 2009)

Beaufort 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 sensation; air feels oppressive
B1	Light Air	0.55 -1.39	Smoke drift indicates wind direction, but not 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 blown slightly
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 feels strong; limit of comfortable walking
B6	Strong Breeze	11.11 - 13.89	Large branches in motion; whistling in telegraph wires; umbrellas difficult to use	Walking becomes difficult
B7	Near Gale	14.17 - 16.95	Whole trees in motion,	Walking very inconvenient
B8	Gale	16.67 - 20.55	Twigs broken from trees; progress generally impeded	Walking extremely difficult
B9	Strong Gale	20.83 - 24.17	Slight structural damage (e.g., roofs shingles, antennas)	Hazardous for pedestrians
B10	Storm	24.44 - 28.33	Rare inland; trees uprooted; considerable structural damage	Unsafe outdoors
B11	Violent Storm	28.61 - 32.22	Very rarely experienced; widespread damage	Dangerous; not walkable
B12	Hurricane	>32.22	Severe destruction	Impossible for pedestrians

 

A simple rule of thumb was proposed by Wise (1970) and Penwarden (1973), and later reported by Stathopoulos (Bennett, 2007), which can be summarized as follows:

• V = 5 m/s (18 km/h): Onset of discomfort
• V = 10 m/s (36 km/h): Definitely unpleasant
• V = 20 m/s (72 km/h): Dangerous

Here, V represents the mean wind speed. Pedestrian wind conditions are considered acceptable if wind speeds exceed 5 m/s less than 20% of the time (Penwarden & Wise, 1975). In addition to the Beaufort numbers presented in Table 1, Table 2 below outlines the relationship between pedestrian activities, types of areas, and perceptions of the wind environment.

Table 2. Comfort and safety criteria in relation to the Beaufort Scale (to be read in conjunction with Table 1; adapted from Jessica Bennett, 2007)

Activity	Applicable Areas	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
Standing / Seating (Long Time)	Street Cafés, Theaters	B2	B3	B4	B8
Acceptable if wind speed occurs less than			once a week	once a month	once a year

Commonly recognized wind speed thresholds for pedestrian comfort are as follows (Bennett, 2007):
10 m/s: Limit of comfort for sitting or standing for extended periods in open spaces.
15 m/s: Upper limit of acceptable comfort while walking.
18 m/s: Threshold of danger.
23 m/s: Completely unacceptable for walking.

With an understanding of how wind speeds affect comfort and the basic physics behind wind behavior, you can optimize your walking experience. Aim to walk in conditions where the wind speed is below 11 m/s. 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.