Wind Gradients

[Cold_Bore_88]

Lately I have begun to focus my training and research on the great equalizer. Wind. I have been learning how to measure your target in wind speed, average wind speeds at various distances, etc. The one thing I don't see a lot of info on is wind gradients. Are there any hard and fast rules/tricks to wind gradients? For example;

I am shooting at 1200 yards with a .308 175 gr bullet running about 2650 (I am basically hitting a fly ball to center field). According to AB, I will need 15.6 MRAD in elevation to get there. Wind is 10 mph, full value, at me and the target (trying to keep it simple here). Understanding that my bullet will travel over 50 ft above my head (I think I calculated that right), should I assume a standard wind speed increase at higher elevations of 3-5 mph? How do y'all account for wind gradients/speeds at higher elevations?

 

[THEIS]

Hi,

Short answer is Yes there is a formula to account for wind speed at certain elevation based off wind speed at ground.

LLLJP Wind Shear Formula (Power law)
The wind speed at a certain height above ground level is:
u=(uref)*((z/zref)α)
where u and uref are the mean wind speeds at the heights z and zref, respectively. The assumption of a normal wind profile or the power law relation is a common approach used in the wind energy industry to estimate the wind speed u at a higher elevation (z) using surface (usually at 10 m) or tower measurements of wind speeds Uref at reference height zref. The shear exponent (α) is typically assumed to be equal to 0.2.

EDITED TO ADD: Nice article for formula reference.
https://www.inti.gob.ar/cirsoc/pdf/accion_viento/2005-10-dregger.pdf

"Real" World answer is that there are way way too many things that will cause "interference" with the above formula since the wind can be deflected, tunneled, etc by the terrain, terrain composition, and terrain obstructions.

You would be better off (If you are not already doing so) to think of wind as water. Imagine your shooting area in regards to how water would flow, deflect, eddie, rise and drain. That is what the wind is doing, you just cannot see it as you would if it were actually water.

Sincerely,
Theis

 

[LastShot300]

In my experience, trying to account for the wind gradient is a waste of time. it's akin to chasing ghosts. You cannot actually measure it and any formula will always be an approximation though. Measure it at the muzzle like anybody else and you'll probably be good to start with.

 

[Deleted member 113831]

The height of the wind gradient that you shoot through on flat ground is determined by terrain and wind speed. Example: Shooting across an open field where vegetation isn't more than a couple of feet high with a wind that is 1- 2mph. It is very possible that the wind speed 20ft off the ground is double what you are feeling on your face.

Now up that wind to 6mph on your face, and the difference between what you feel and what it is at 20ft might be 2mph, so it would be 8mph. By the time you get to 8mph on your face, you might add 1mph additional to your call. As the wind speeds up, it flattens the layer where friction with the ground slows it down.

Now consider terrain such as a line or shelter belt of trees. Depending on how thick the trees are, the turbulence area on the leeward side could be as much as 20 times the height in say, a 15mph wind. But again, this depends on speed. Wind is like water in that the faster you push it, the longer it takes to turn corners.

A slow 4mph wind might flow over a hill smoothly, while a 10 mph wind might cut across the top of it creating a compression updraft on the windward side, and a wind shadow and a rotary zone on the leeward side.

 

[TripleBull]

I understand the use of water as a sort of visualization guide, as long as you keep in mind the differences in density, compressibility, surface tension and all those other little details.

 

[TiroFijo]

Of course wind velocity varies above ground. But not only is tricky to try to estimate the wind velocity above the LOS, but also you have to rememberr that the closer to the muzzle, the most effect wind has on the bullet deflection...

 

[flyright]

Wind is "the ghost in the machine" but the more you shoot (practice) the more you get a "feel" for its effect. When sticking with one caliber - you get the "feel" quicker. Your wind holds become second nature and get to be easier. Now, with gusty 25+ mph winds it can be frustrating, especially at 800 yds plus and many shooters bow out. It is the nature of ELR shooting. All part of the challenge.

 

[Deleted member 113831]

I was dealing with some tricky wind just yesterday. Shooting at 1416 yds from a ridge, down into a valley. We were in a wind shadow at the firing position for about 200 yards out. The wind at the target was showing 3-6mph, but the laminar wind out in the middle was 6-8mph.

The wind was being compressed through a narrow valley, and then breaking out into a wide open bowl. So we were getting 90 degree right to left wind near our position, just beyond the wind shadow we were in. Then, we were getting wind from about 120 degrees out in the middle.

Extremely tough wind to call.

 

[Howland]

Hi,

Short answer is Yes there is a formula to account for wind speed at certain elevation based off wind speed at ground.

LLLJP Wind Shear Formula (Power law)
The wind speed at a certain height above ground level is:
u=(uref)*((z/zref)α)
where u and uref are the mean wind speeds at the heights z and zref, respectively. The assumption of a normal wind profile or the power law relation is a common approach used in the wind energy industry to estimate the wind speed u at a higher elevation (z) using surface (usually at 10 m) or tower measurements of wind speeds Uref at reference height zref. The shear exponent (α) is typically assumed to be equal to 0.2.

EDITED TO ADD: Nice article for formula reference.
https://www.inti.gob.ar/cirsoc/pdf/accion_viento/2005-10-dregger.pdf

"Real" World answer is that there are way way too many things that will cause "interference" with the above formula since the wind can be deflected, tunneled, etc by the terrain, terrain composition, and terrain obstructions.

You would be better off (If you are not already doing so) to think of wind as water. Imagine your shooting area in regards to how water would flow, deflect, eddie, rise and drain. That is what the wind is doing, you just cannot see it as you would if it were actually water.

Sincerely,
Theis

Which succinctly explains why on a certain golf course with two par 3 holes the same length adjacent to each other both hitting across a steep draw, it can be a one club wind on both tees and greens but I'll use a wedge on one and a 7 iron on the other.

I think I'll copy that and tape it to my grips.