Although it’s the least influential of the three environmental variables, humidity does have an influence on bullet trajectory.
- Contrary to common thought, higher humidity results in thinner air, this affects bullet trajectory
- Therefore, a bullet travels easier through humid air.
- However, humidity is the least influential environmental factor.
- Without a ballistic calculator, the way to track the net effect of all environmentals is density altitude.
- It's a figure that shows the cumulative effect of all three environmental variables.
- This includes pressure, temperature and humidity.
In the last two columns we explored the first two elements of the three external/environmental variables: air density and temperature. In this column, we’re going to discuss the third element, humidity, and introduce a way to account for all three elements at once as it relates to bullet trajectory.
Does Humidity Even Matter?
If you’ve followed along so far, this one might be a curveball.
As air density increases, the bullet experiences more resistance as it flies through the air and therefore slows down more than it would in less dense air.
Higher density = slower bullet = lower impact on a target.
Sounds simple enough, right?
Well, contrary to common thought, higher humidity results in thinner air. Yes, you read that right. Despite how it feels to walk out into high humidity (the air feels “thicker”), the air is actually less dense. Therefore, it has the opposite ballistic effect that you might expect — a bullet travels easier through humid air.
Yes, I know what you’re thinking — water is denser than air. That’s true with water in its liquid state. As a gas, it displaces the air molecules and actually results in less matter for the bullet to pass through.
The good news is that humidity is the least influential of the environmental effects and can be ignored in most cases.
Putting It All Together: Density Altitude
How are you supposed to track the net effect of changes in air pressure, temperature and humidity? Well, without a ballistic calculator, it can be maddening. An increase in one variable makes the air thinner while an increase in another makes the air thicker.
There are two ways to keep track of the net effect of environmentals:
- Always use a ballistic calculator and have it do the work for you
- Focus only on the “density altitude”
Density altitude is a normalized figure that represents the cumulative effect of all three environmental variables. It’s a figure that’s calculated off of a set of “standard” conditions (pressure, temperature and humidity) at sea level and then representing all three as the altitude you’d have to be at in order to experience your current conditions. Essentially, if the net effect of the variables results in thinner air, then your density altitude value will be higher because with these “standard” variables assumed, you’d have to be at a higher altitude to experience the thinner air.
By using density altitude, you can understand bullet trajectory and record your elevation data required to hit certain targets at your current density altitude. Then, when you change locations — or the environmental variables change — you can look to see what the new density altitude is.
Of course, you’ll likely need to start with a ballistic calculator to gather/confirm your elevation data. However, you can record your new data for that new density altitude. Then, whenever you experience that same/similar density altitude again (whether it’s due to your actual altitude change or the net effect of changing environmental variables), you can reference the data you recorded for that density altitude and start shooting.
Yes, you still need to worry about the environment. However, by using density altitude, you can reduce all of the variables to one single value to track.
What Does It All Mean?
Our march through these ballistic topics in the past few columns can be summarized simply like this:
The amount your bullet drops on the way to the target – bullet trajectory – isn’t really about how far away the target is; it’s about how long it takes the bullet to get to the target. If one bullet leaves the gun faster than another similar/same bullet, or it starts at the same speed but travels through the air more efficiently, then it won’t drop as much.
Also, a bullet’s time of flight at a certain distance isn’t a constant. Environmental variables can allow a bullet to better retain its speed or slow down more, thereby resulting in a different time of flight. The same can be said when considering how much of an effect wind will have.
Therefore, the only things that change a bullet’s path are gravity and wind. And, anything that changes the time it takes for a bullet to reach a particular target will change how much of an effect that gravity and wind can have.
Your job is to learn what your bullet does in certain conditions, record its performance, and track how it changes as certain variables change. Then, you can use your information to predict how your bullet will behave in similar conditions in the future.
Remember, know what your bullet does at certain density altitudes and you’ll be able to predict what it will do wherever and whenever you find yourself shooting next.
Editor's Note: This article original appeared in the November 2017 issue of Gun Digest the Magazine.