Humidity the multiplier


Well-Known Member
Nov 3, 2008
Central Pennsylvania
So this humidity thing was bugging me. I decided to run the numbers through JBM to see if my theory held weight and whether or not that I have in fact read an article discussing this very issue. So all things being equal; Baro, temp, altitude, I just changed the humidity to see what effect it changed on velocity and flight. Here is what I came up with:

Rifle is an assumed load because I was crunching numbers for another thread:
338 Norma 2750fps 300grnSMK

First the numbers
75 degrees temp, 100%Humidity, 1000' elevation, Baro 28.82 (close enough)
39.55 MOA 1506fps 1510 ft. lbs 1340yrds

Same as above except the Humidity is changed to 30%
39.04MOA 1505fps 1508 ft. lbs. 1330yrds.

So as you can see, all things being equal which can happen in PA except the humidity, a more humid environment allows for better ballistics. Can somebody explain this? Now granted it is not very much and I'm sure only really noticeable at extreme range, but it is still a factor. Hopefully Bryan Litz is paying attention.

The higher the humidity, the less dense the air, the more efficient the bullet will move though the air.

Contrary to what our senses tell us (high humidity make the air feel heavier or thicker) high humidity air is less dense air than dry air (low relative humidity air). Bullets will cut through less dense air more efficiently.

As you confirmed, humidity plays a relatively minor role compared to temperature and atmospheric pressure. It can just about be ignored, but if you know the humidity and you're shooting very long range, and your ballistics software can correct for it, then I do enter it as part of environmental conditions for predicted drops.
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