Air temperature changes bullet trajectory by increasing or decreasing atmospheric density. This seems counter intuitive to some folks, but cold air makes a given load strike lower. Hot air makes the same load strike higher. Shooters can count on this:
- Expect to shoot under your point-of-aim in extreme cold, over in extreme heat.
- The longer the range, the more significant these temperature-induced differences in impact points.
- Ballistic calculators can predict how much temperature changes bullet trajectory, giving you data so you can compensate.
Now let’s investigate how this happens.
Cold air is more dense than hot air, so it slows bullets. Hot air expands, reducing molecular density and drag. The result is predictable in pattern, if not degree. Bullets strike higher.
Veteran shooters might accuse me of belaboring the obvious here, but I can promise you there are shooters reading this who do not know how temperature changes bullet trajectory. You’ll read an embarrassing example of this near the end of this article.
All of us were once ignorant about how air temperature changes bullet trajectory, so let’s cut everyone some slack. When I first got interested in ballistics, it took me a bit of cogitating before I could believe that cold air, which always feels brisk and light to me, could be heavier (more dense) than hot air, which always feels heavy and oppressive.
I resolved this intuitive roadblock by thinking of heating water on a stove. Heat it in a covered pot and it expands so much it can literally blow the lid off. Heat it long enough uncovered and the pot is soon empty. All that dense, heavy water has expanded and gotten so light that it floated away! Hot air expands the same way. With molecules farther apart, there are fewer of them to drag on your bullets and slow them down.
If the water analogy doesn’t work for you, consider gunpowder. With the addition of a bit of heat, it expands from a relatively dense, compact solid into a gas with so much volume that it drives heavy bullets down tight, high-friction barrels to go flying at thousands of feet per second. That’s a lot of expansion.
Here’s an easy way to remember how air temperature changes bullet trajectory: Heat rises and so does bullet impact. Cold sinks and so does bullet impact.
Another thing about air that’s a bit hard to visualize is how much drag it exerts on bullets. It seems that a slim, pointy bullet could zip through the atmosphere slicker than a razor through hot butter. How can a little air significantly slow down a bullet? Well, think about pedaling a bicycle into a strong headwind. That’s just air, yet pushing against it demands real work, real energy. Bullets are subject to this same force. They have to power their way through nitrogen (about 78% of the atmosphere,) oxygen (21%,) argon (just under 1%,) carbon dioxide (0.039%,) trace gases (0.003%,) and water (about 2%.) All these molecules create friction. The colder they are, the more densely they’re packed together. Your bullet uses more of its kinetic energy plowing through them. Thus air temperature changes bullet trajectory.
The reason I’m addressing this topic at this time is because of a news release I saw recently from ballisticapp.com, manufacturer of one of the best ballistic phone apps. Some folks at Ballistic got their wires crossed because the new release included this misinformation:
“Or consider the effect temperature has on your ammunition. Vaughn notes that temperature differences don’t affect the path of the bullet per se. Rather, the air temp impacts how fast your round’s gun powder burns and how much pressure it develops. For example, when Vaughn’s 243 Ackley Improved handload is fired at 20 degrees Fahrenheit, it has a muzzle velocity of 3,175 fps and stays super-sonic out to 1,350 yards. But take that same round and fire it off at 80 degrees? Now, the muzzle velocity is 3,286 fps, and the bullet stays super-sonic out to 1,450 yards.”
The effect Mr. Vaughn is addressing is powder temperature. Perhaps he was misquoted about air temperature. Nonetheless, this shows how easy it is for misinformation to slip out and spread. Air temperature changes bullet trajectory. Count on it.
Ron Spomer has hunted — and missed — in temperatures as hot as 100-degrees F. and as cold as -20-degrees F. He blames air temperature.