How Does Altitude Affect Bullet Trajectory?
Altitude plays a crucial role in determining bullet drop.
So, when shooting over long distances, you must account for it. Otherwise, you're most likely going to miss your shot.
This post analyzes the effects of altitude on a bullet during a flight. The idea is to pass knowledge and, hopefully, increase your long-range hunting success.
So, what's the effect of altitude on your bullet? Most importantly, how can you counter the effects?
First, we can't discuss how altitude affects a bullet path without talking about barometric pressure. Why? …because altitude variations cause changes in air pressure. As the altitude rises, air pressure drops.
So, when firing at a higher altitude, you're shooting through thinner air. As a result, your bullet travels faster in a flatter trajectory. The opposite is true.
The bullet impact will be significant if you don't account for this trajectory change. The solution? You must recalculate the trajectory based on the atmospheric pressure at the shooting time. You'll also need to update the changes as your shooting session continues.
Let's say you're shooting over a distance of 800 meters.
If you were to change your firing point to another location sitting at 1,000 meters (3,300 ft) at a higher altitude, your bullet impact would be about 20 inches above your target point.
Throw in a 5 degrees centigrade temperature change, and the miss increases to about 28 inches.
A 28 inches miss is equivalent to a complete miss at a human-sized target if you're aiming at the center mass as seen in the image below.
The question then becomes:
How Does Altitude and Air Pressure Affect a Bullet?
As stated, altitude and barometric pressure go hand in hand when talking about a bullet trajectory. Let's differentiate the two:
Altitude: This refers to a shooter's height in relation to the sea level.
Barometric pressure: Also known as atmospheric pressure, this refers to the weight of the air.
The standard sea pressure is 1013.25 hPa (hectopascal). Still, the pressure can change, irrespective of the altitude. In extreme weather conditions, for instance, the sea level pressure can drop below 900pHa, the same average pressure reading at a 1,000-meter altitude.
That said, here's how altitude affects the outcome of a long-range shot.
You can expect lower pressure at high altitudes because of less air density, and thinner air. As a result, there's less air resistance or drag on your bullet.
The ripple effect is a relatively more efficient flight, enabling the bullet to travel further. If you don't make adjustments, the bullet's impact will be high.
A lower altitude means you're closer to sea level. So, you should expect higher air pressure, meaning thicker, dense air.
As a result, as your bullet encounters more air pressure, it slows down relatively quickly, reaching transonic speed sooner. The bullet's impact will be low if you don't make any adjustments.
Side Note: Transonic speed refers to the speed at which bullets cross the sound barrier, transitioning from supersonic to subsonic speed. At supersonic speed, the bullet loses stability, and you cannot predict its trajectory. The bullet reaches its effective range.
Altitude Effect on Trajectory in Numbers
The altitude’s effect on the trajectory is negligible at short distances. At 100 meters, for instance, the change is almost unnoticeable. However, the effect increases as you move past 300 meters, as shown in the table below.
According to the table above, your bullet's trajectory flattens in line with altitude change and air density.
You can also notice that your gun’s effective range gets better with altitude.
A bullet fired from the same gun can travel an extra 250 meters at an altitude of 2,000 meters compared to shooting at sea level.
Besides, at 300 meters, the bullet drop difference is only a few centimeters. This explains why experienced shooters may not notice this altitude effect on their bullet trajectories because they don’t account for it during shooting. However, for long-range shooting, the effect is crucial.
Side Note: It is important to remember that absolute pressure decreases at 33.85hPaf for every 300 meters/ 1,000 ft you climb. Or an equivalent of 11pHa for every 100 meters / 300 ft you climb.
How Can You Compensate for Altitude and Pressure?
You can use a ballistics calculator to adjust for the trajectory changes caused by altitude variations. All you need to do is to enter the actual pressure, also known as absolute pressure, into the calculator, and the program will show you the new trajectory.
Side Note: Many ballistics programs support corrected pressure or station pressure. Things can be confusing if you don’t understand this pressure — which takes us to our next point.
Understanding Corrected and Standard Pressure
Below is a breakdown of the two pressures to help you accurately correct your bullet’s trajectory.
Corrected pressure refers to the pressure adjusted to an equivalent of sea level. It is referred to as sea level barometric pressure.
Your local weather station, in most cases, reports corrected pressure. You can also find this reading online and on weather apps. The pressure is adjusted to sea level pressure to give you a standard reference for pressure changes.
Put differently, the correct pressure reading doesn’t show the actual pressure during shooting. So, it'd be best to avoid it for long-range shooting.
If you must use corrected pressure, the calculation below will help you generate a good approximate reading that you can enter into your ballistics calculator.
- METRIC CALCULATION = Barometric Pressure (hPa) minus (Altitude in Meters / 9) = Station Pressure (hPa)
- IMPERIAL CALCULATION = Barometric Pressure (inHg) minus (Altitude in Feet / 1000) = Station Pressure (inHg)
Station pressure refers to pressure you feel or record on location when taking a reading. You can read station pressure using a weather meter.
Side Note: Many smartwatches can record air pressure accurately. Still, most displays corrected pressure, but you can leverage 3rd party apps like Ambient Pressure to get a station pressure reading.
…before you go, let’s look at one last crucial topic
Standard Pressures at Different Altitudes
You can use the chart below to determine the atmospheric conditions at the range if your station or barometric pressure isn’t available.
Remember, these figures may change with elevation climb because many environmental conditions affect the day’s barometric pressure. These include temperature, humidity, weather, and diurnal pressure variation.
The Bottom Line
Knowing altitude affects bullet drop goes a long way in helping you make accurate long-distance shots. And, with the right ballistics program, you should have a problem predicting trajectories accurately.
Also, don’t forget to tell us how the information shared in this article helped you improve your long-range shooting accuracy in the comment section below.
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