Shooting across valleys and windage?

[EXPRESS]

I know, as a helicopter pilot that wind in gullies can be up to 20% stronger when channeled up a valley.
The THLR range cards have a little diagram showing the shape and depth of a series of valleys, and what additional clicks are needed when shooting across such valleys or canyons. The diagram seems flawed in that it only give a number of clicks and there no explanation of the relevant distances involved.

However this is the only reference I can find to the phenomenon in shooting.

So I recently set up new turret label for a new load as part of my shooting routine I decided to shoot one cold bore shot every couple of days at a steel target 640 yards from my house.

Each day I've had the same kind of wind, and given the same windage; around .4 MIL for a 4-4.5mph 10 o'clock wind. The shot crosses a fairly "average" valley.

Three shots so far have all hit within 1" of each other, but all .3MIL downwind.

I will double check zero, but I can't help but wonder if it is something is amiss?

The windage comes from the drop chart ballistics, which has been trued and verified out to 1356 yards so I can confidently say that we can it has to be very close to perfect.

Amyone else ever have this issue?

 

[EXPRESS]

Sorry, please ignore the email at the top of my post. I wrote it out on my phones' notepad then pasted it here without realizing that was there. Now there is no "edit" button.

Back to the subject, the rifle was checked and I shot with it last weekend so I have no reason to believe that it lost zero.

The wind strengthens in valleys, but this applies only to the middle section, so lets say that over a 640 yard valley shot, wind gains stregnth for the middle 250 yards of the bullets' path.
In this case I was reading a 1 - 2 m/sec wind (2 m/sec is 4.5mph) so it would have to double for the middle 250 in order to give the actual result I got.

I suppose that, in light winds, it's actually pretty easy to double. From 4.5 to 9mph isn't that much.

I will have to wait for better conditions as it's stormy and blustery right now, to verify my wind on level ground.

 

[EricL]

This seams to be a problem most of us in the mountains or hills face, and as near as I can tell it just takes practice to figure out. It does sound like to me that you already have a lot better ideal of what is going on than most folks, for sure with your occupation its something you deal with all the time. By the way 1'' at 640 is an awesome group!! If you figure out a good equation to get rid of 80% of the guess work let me know!

 

[EXPRESS]

I should clarify a bit, that when I said "up to 20%" increase in wind up the middle of a valley, I should have said as an average. With a light wind, it can easily double for instance. To go from a 3 to a 6 mph wind is nothing, while a 30 is not so easily going to become a 60mph wind, onviously.

I shot again yesterday in a 20 mile wind, same direction, and put the shot into the same group. Yesterday I gave it 1.1mil of windage. It's somewhat frustrating that while I continue to dial what wind I think I need, to be off by the same 3 clicks each time.
I'll try to find the time to check zero at 100.

Also the group is not inside 1" - the shots are about 1" from each other, making for a 4 shot group inside 3" so far. The group size is still great to me, because these are cold bore, different day setup shots in different conditions.

 

[FEENIX]

I should clarify a bit, that when I said "up to 20%" increase in wind up the middle of a valley, I should have said as an average. With a light wind, it can easily double for instance. To go from a 3 to a 6 mph wind is nothing, while a 30 is not so easily going to become a 60mph wind, onviously.

I shot again yesterday in a 20 mile wind, same direction, and put the shot into the same group. Yesterday I gave it 1.1mil of windage. It's somewhat frustrating that while I continue to dial what wind I think I need, to be off by the same 3 clicks each time.
I'll try to find the time to check zero at 100.

Also the group is not inside 1" - the shots are about 1" from each other, making for a 4 shot group inside 3" so far. The group size is still great to me, because these are cold bore, different day setup shots in different conditions.

Montanan's knows a thing or two about winds and it's always a challenge from open prairies to cross canyons ...

Below is a video of a gentle zyphyr while antelope hunting ...

[ame]https://youtu.be/0g8VvpqdtkQ?t=1[/ame]

gun)

 

[Zen Archery]

That video...