The 50% porosity figure is a design convention from wind-tunnel and field studies, not a natural constant. It controls where the drift lands, and it stops being the right number when the site has two wind directions.

Where the 50% porosity figure comes from

window SNOW SEASONwritten 2026-08-21responsible Windrow Field Manual (a named masthead, not a person)

50% is a convention from a single-wind-direction study

[['Solid', 'Immediately downwind, tall', 'Never, for road protection'], ['About 50%', 'Long shallow drift in the design zone', 'Single prevailing wind, setback available'], ['Above 70%', 'Very little deposition', 'Not enough capacity to protect a road']]

p	r	o	b	e		c	a	r	d	:		p	o	o	s	i	t	y	a	g	a	i	n	s
P	o	r	o	s	i	t	y
D	r	i	f	t		p	o	s	i	t	i	o	n
W	h	e	n		i	t		i	s		t	h	e	r	i	g	h	t	c	h	o	i	c	e

What porosity actually controls

Porosity sets how much air passes through the fence and therefore how much the wind speed drops on the lee side, and over what distance. That in turn sets where the snow can no longer be carried. Everything about the 50% figure is downstream of that single mechanism.

Study setting	Finding	Limit of the finding
Wind tunnel, single direction	About 50% porosity gives the longest, shallowest drift	Uniform snow, no vegetation, no second wind
Field trial, open plain	Confirms the drift shape at similar porosity	One wind direction for the season
Site with two wind directions	No single porosity optimises both	The convention is applied outside its evidence

Where the number stops working

The manual's position is that 50% is a good default and a poor rule. It was derived where one wind direction dominates and the fence can be set far enough back. On a site where the wind swings, a fence at 50% will place the drift correctly for one direction and wrongly for the other, and the second case is the one that gets complained about.

This is contestable. Plenty of practitioners treat 50% as a standard to be met, and on the sites they work it is the right answer. The manual is arguing about the sites where it is not, and asking for the wind rose to be checked before the number is used.

What to check before using the figure

The wind rose for the site, not the nearest town.
Whether the setback land is available for the drift the fence will produce.
Whether the road's own embankment is a second snow source, which no fence porosity can fix.

Where the numbers come from

S1How Do Snow Fences Work?, Road Weather Management Technical Brief, Federal Highway Administrationsupports the porosity behaviour that the 50% convention comes from
S2How Do the New Snow Fences Work?, Road Weather Management Technical Brief, Federal Highway Administrationsupports the drift-length behaviour of the newer fence designs
S3Cost Benefit, Living Snow Fences, Minnesota Department of Transportationsupports the cost-benefit framing used when a fence is justified
S4GeoPAK Snowdrift Analysis Tool, Minnesota Department of Transportationsupports the snowdrift analysis a porosity choice has to satisfy

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Put this page down Put this page down when you have looked up your site's wind directions, not just the porosity number.