October 19th, 2014

How Altitude and Air Pressure Influence Ballistics

Suunto AltimeterOne of our readers asked “What effect does altitude have on the flight of a bullet?” The simplistic answer is that, at higher altitudes, the air is thinner (lower density), so there is less drag on the bullet. This means that the amount of bullet drop is less at any given flight distance from the muzzle. Since the force of gravity is essentially constant on the earth’s surface (for practical purposes), the bullet’s downward acceleration doesn’t change, but a bullet launched at a higher altitude is able to fly slightly farther (in the thinner air) for every increment of downward movement. Effectively, the bullet behaves as if it has a higher ballistic coefficient.

Forum member Milanuk explains that the key factor is not altitude, but rather air pressure. Milanuk writes:

“In basic terms, as your altitude increases, the density of the air the bullet must travel through decreases, thereby reducing the drag on the bullet. Generally, the higher the altitude, the less the bullet will drop. For example, I shoot at a couple ranges here in the Pacific Northwest. Both are at 1000′ ASL or less. I’ll need about 29-30 MOA to get from 100 yard to 1000 yards with a Berger 155gr VLD @ 2960fps. By contrast, in Raton, NM, located at 6600′ ASL, I’ll only need about 24-25 MOA to do the same. That’s a significant difference.

Note that it is the barometric pressure that really matters, not simply the nominal altitude. The barometric pressure will indicate the reduced pressure from a higher altitude, but it will also show you the pressure changes as a front moves in, etc. which can play havoc w/ your calculated come-ups. Most altimeters are simply barometers that read in feet instead of inches of mercury.”

As Milanuk states, it is NOT altitude per se, but the LOCAL barometric pressure (sometimes called “station pressure”) that is key. The two atmospheric conditions that most effect bullet flight are air temperature, and barometric pressure. Normally, humidity has a negligible effect.

It’s important to remember that the barometric pressure reported on the radio (or internet) may be stated as a sea level equivalency. So in Denver (at 6,000 feet amsl), if the local pressure is 24″, the radio will report the barometric pressure to be 30″. If you do high altitude shooting at long range, bring along a Kestral, or remember to mentally correct the radio station’s pressure, by 1″ per 1,000 feet.”

You can do your own experimental calculations using JBM Online Ballistics (free to use). Here is an extreme example, with two printouts (generated with Point Blank software), one showing bullet trajectory at sea level (0′ altitude) and one at 20,000 feet. For demonstration sake, we assigned a low 0.2 BC to the bullet, with a velocity of 3000 fps.

Trajectory of Bullet fired at Sea Level

Trajectory of Bullet fired at 20,000 feet

if you want to learn more about all aspects of External Ballistics, ExteriorBallistics.com provides a variety of useful resources. In particular, on that site, Section 3.1 of the Sierra Manual is reprinted, covering Effects of Altitude and Atmospheric Pressure on bullet flight.

Permalink Tech Tip 5 Comments »
June 21st, 2008

Timex Altimeter, Barometer, Thermometer Watch

Temperature, Atmospheric Pressure, and Altitude — all these things can and do affect bullet ballistics. Moreover, temperature changes can alter powder performance and chamber pressures. The smart reloader takes the predicted ambient temperature into account. And when calculating your come-ups and windage click values, you need to factor in Temperature, Air Pressure, AND Altitude.

Kestrel 4000 Weather TrackerHow do you keep track of these important variables when you’re shooting? The Kestrel 4000 Pocket Weather Tracker®, is compact and provides a wealth of information: atmospheric pressure, altitude, density altitude, temperature, humidity, wind speed, wind chill, dew point, and heat index. We like the Kestrels and recommend them. But at $279.00, a Kestrel 4000 is too expensive for many shooters.

Timex now offers a solution. For about $56.00-$80.00 street price, the Timex Expedition® Adventure Tech™ watch (item T41501EA) displays Altitude, Barometric Pressure, and Temperature (ABT). The thermometer function records temps from 14° F to 140° F. The Altimeter reads from -2,296 feet to +29,520 feet with 10-foot resolution. The Barometer tracks current and sea level pressure, and provides a graph of pressure changes over time.

Timex Altimeter Watch

Encased in a stainless housing with a resin band, the Timex watch is water resistant to 50 meters, and offers many other nice features, including dual time zones, countdown timer, alarm, and backlit nightlight (very handy).

There are other quality watches, such as the Suunto Core, that provide Altitude, Atmospheric Pressure, and Temperature read-outs. But, at under $70 street price, the Timex costs one-fourth the price of the Suunto. Amazon.com is currently offering the Timex Temp/Altitude/Barometer watch for $55.97 – $79.95 (depending on source). It is offered with a silver band as well as the black shown above.

Useful Tool or Just Another Gadget?
Is this watch really useful or just another piece of male jewelry? Well just last week a friend was developing loads for his PPC. While chronographing his loads he wanted to input the ambient temperature, and current air pressure in his log. We just had to guess on those numbers. It felt like 90°+, but it may have been in the mid-eighties. With the Timex we’d have had reliable data (Note: it’s best to take the watch off your arm when gauging air temp). For this editor, the Timex ABT watch is a useful tool. I bought one and will be using it during load development and when shooting competitively.

Here’s an owner’s review: “I have been through a Nike ABT, a Swiss Army ABT, and a High Gear ABT watch this year alone. None of them, which cost quite a bit more, have been as accurate and durable as this Timex. It has a better fit, was easy to use and read and just plain ‘on the money’ as far as forecasting weather and altitude. Also, I have not had to take it off my arm for 10-15 mins to get a decent temperature rating. It has always been within 1-3 degrees of actual temp. And I live in the Carribean in a tropical climate… so, for it to be this accurate is a statement in itself.”

And here’s an interesting comment from one purchaser: “I bought this watch to set the jetting on my racing go kart. I needed to know field elevation, and temperature. Plus changes in barometric pressure as the day went on. Most teams use a weather station and computer that cost over $700 for this purpose. My watch and accompanying chart (for the motor) has given me the same results.” If this watch works for racers, it can work for our “internal combustion” precision bullet-launching machines as well.

Permalink Gear Review, Hot Deals 1 Comment »