How much do temperature, humidity, and altitude affect the velocity of a bullet, and therefore its range? I used the online ballistics program at JBMballistics.com to find an answer to the question. For the “test” bullet, I chose a common caliber, .308, and a particular bullet that has a G7 BC (more accurate) given by Bryan Litz, based on his own investigation (even more accurate), shot at 2860 fps.
When we go from zero to 100% humidity, the effect on the velocity of the bullet is limited: only 5.9 fps for a temperature of 50 degrees F (at 1000 yards). At 75 degrees, the air is capable of holding more water vapor, so the effect is more pronounced: 14.3 fps. At 100 degrees, the difference goes up to 31.2 fps. So the effect of humidity on bullet ballistics is greater at higher temperatures. But it is still fairly limited.
The effect of temperature is more pronounced. At 0% humidity, the temperature increase from 50 F to 100 F gives you 91.8 fps more at 1000 yards, and 117.1 fps more at 100% humidity. That additional 100 fps or so implies that the effective range is significantly increased. If we consider that the greatest distance with at least 1300 fps and at least 500 ft lbs is the point of maximum effective range for a round, then going from 50 F to 100 F extends the range, for the above ballistic example, from 890 yards to 955 yards, a 65 yard advantage.
The bullet drop is also affected. The drop at 50 F and 0% humidity is 346.3 inches (200 yd zero, at 1000 yds), and at 100 F it’s 324.9 inches, for a difference of 21.4 inches, solely due to temperature.
But altitude, which is basically just a difference in air pressure, makes even more of a difference.
There is a 230.4 fps increase from sea level to 5000 ft.
Denver CO is just over 5000 ft altitude, so it is not as if no one lives at such a high location. Let’s compare 2 locations, Denver versus a sea level site, and let’s add in a temperature and humidity difference:
1) Denver, 100 F, 100% humidity: 1516.7 fps at 1000 yards, 1297.5 at 1200 yards.
2) Sea level, 0 F, 0% humidity: 1497.9 at 700 yards, 1299.6 at 825 yards, 1050.0 fps at 1000 yards
The Denver location gives 1516.7 fps at 1000 yards, compared to 1050.0 fps for a sea level location with large temperature and humidity differences. The 1200 yard shot in Denver has about the same velocity as the 825 yard shot at sea level. So altitude, humidity, and temperature together can made a huge difference in the range of a cartridge.