Elevation and Humidity

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Wildernesshunter

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I started shooting at age three....killed my first deer at age 5. I have been around guns and shooting and reloading my whole life. I feel a little deprived that no one ever educated me on the effects of humidity and elevation on ballistics....and that my scientifically trained mind (Bachelors of Science), never initiated the research until recently.....yeah, I'm 48 years old.

About two weeks ago, I started playing with some of the ballistic calculators that are available on the web, notably the one by Hornady. Initially, I was trying to decide if I wanted to buy a bolt action 7.62x39 and then I started researching the ballistics of hand loads.

At some point, I clicked on the Advanced feature and too my surprise, it allowed be to input elevation and humidity....along with some other inputs. So I started playing. Living in Colorado, this is actually a big deal, as most ballistics data is defaulted at about 1,000 elevation and 60-80% humidity.

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So here are a couple of comparisons for the 7.62x39 with a 150gr Speer SP Hand Load......

Sea Level with 80% humidity and 60 degrees (maybe a winter hunt in Louisiana)

FPS/ENERGY
Muzzle = 2,400/1,918
400 yds = 1,659/916 (with 200yd zero, drop is -33.5")


8,000 feet elevation with 20% humidity and 30 degrees (Colorado hunt)

FPS/ENERGY
Muzzle = 2,400/1,918
400 yds = 1,800/1,079 (with 200yd zero, drop is -30.7")

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Here are a couple of comparisons for the 30'06 with a 165gr Speer SP Hand Load......

Sea Level with 80% humidity and 60 degrees (maybe a winter hunt in Louisiana)

FPS/ENERGY
Muzzle = 2,920/3,124
400 yds = 2,120/1,647 (with 200yd zero, drop is -21")


8,000 feet elevation with 20% humidity and 30 degrees (Colorado hunt)

FPS/ENERGY
Muzzle = 2,920/3,124
400 yds = 2,276/1,899 (with 200yd zero, drop is -19.4")

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Drop was the first thing I looked at and expect the most change, but at 400 yards it is only 2-3" for both calibers. That is a loss of .5 to .75 MOA....just because of environmental conditions. (Maybe this is why the NRA loves to do competitions at the Whittington Center outside of Raton, NM. That range is probably about 6,000-7,000 feet elevation.)

Energy is really were the difference lies. The 7.62x39 increased in energy from Louisiana to Colorado by 163 ftlbs. The 30'06 increased 252 ftlbs. Holding to most state legalities of hunting loads (1,000 ft lbs of energy or more)....I would be ok at 300 yards in LA, but 400 yards in CO.

I suppose some will see this as insignificant, petty differences.....but it was an education for me.

Just wanted to share my learning and get shooters thoughts and comments.
 
Thanks for the informative post, Big John.

The data makes sense, as both lower humidity and thinner atmosphere will decrease drag, thus preserving velocity and energy downrange.

Looks like it would behoove us to check zero at or near our intended hunting environment, e.g., a person from the Florida coast making a hunting trip to Colorado (or Wyoming, or my home state, Nevada) would do well to check zero at a local range before heading-out to the mountains...
 
Like all smart hunters, my dad would always shoot three rounds upon arriving wherever he was hunting. It probably had more to do with a confirmation that the scope was still on, but maybe he knew something he never told me......??
 
Contrary to what you might expect, high humidity reduces air density, thus reducing drag. If you doubt that (as I did at first), then google the phrase "effect of humidity on air density." The explanation is that water vapor is a gas which is less dense than oxygen or nitrogen. Dry air at high elevation would partly counter balance the density reduction that you get with high elevation, compared to wet air at low elevation.
 
Just ran the numbers on the ballistics calculator and the difference on 0% humidity and 100% humidity at any elevation is incredibly minimal. Only 4-8 points on velocity and energy.....


But good point that humidity has little effect. That in fact, the air density caused by elevation is really the issue.
 
Just ran the numbers on the ballistics calculator and the difference on 0% humidity and 100% humidity at any elevation is incredibly minimal. Only 4-8 points on velocity and energy.....
Click to expand...

I just ran the numbers for my go-to 308 load. By changing the elevation from sea level to 7000' and humidity from 78% to 25% ( typical differences between GA and CO) I gained 152 fps, 207 ft lbs energy, and 2" less drop at 400 yards in Colorado compared to GA.

That load would be borderline for elk at 400 yards here in GA, but the extra 200 ft lbs of energy at 7000' would allow a little more margin for error. Just about 1750 ft lbs vs 1535 ft lbs.
 
I began rifle hunting in the Midwest and West in the mid-70's. The highest elevation I've hunted was just below the Continental Divide in the Bob Marshall Wilderness. My 5 x 5 elk was so close and walking toward me it was almost point shooting. My small black bear was 400+ yards and dropped instantly when I shot with the cross-hairs slightly behind the shoulder. Elevation never crossed my mind, and I've have never had a guide or outfitter say anything about adjusting a scope for elevation. On a very few occasions I've had the opportunity to shoot at paper within 25 miles or so of the hunting location, but I have never adjusted the scope. On one trip I missed a Mule Deer and discovered a screw was missing from the variable power adjustment ring. The guide handed me his rifle, I asked how it was zeroed, he said something like "on the target." I held the cross hairs on the chest and the deer dropped immediately. I was very lucky he had that rifle. :)
 
Had a good example of this during this hunting season.

My kid has one of those fancy Range Finding Burris scopes. You have to sit it up for expected drop do to altitude (among other things).

He did and had a successful elk hunt at 9500 ft in the southern part of the state (WY).

Then day before yesterday, he took his same rifle, took a good broadside shot at a deer at 560 yards. The scope called the range right, which matched my range finders, but the hold dot was off, elevation wise. Shot about 11 inches low, kicking up dirt under the buck.

Did the numbers on my phone's ballistic program, yup, there was a 10 inch difference.

I don't worry about it for my rifles, I have the BR2 G7 range finder which makes the corrections for you.

Yesterday, he was using my rifle, I called the corrections based on my BR2, at 440 yards, perfect neck shot.

As I said, long range shooting is about the weaponization of math.
 
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