Altitude vs. Barometric pressure

[Mikecr]

As mentioned earlier in the thread, DA is calculated from measurement of station Pressure, Temp, and Rh. These things(all 3) have to be measured, so it makes sense to simply use them directly.
A side problem with DA, at least in discussion, is that folks don't understand that DA has nothing to do with altitude. There are also devices out there that dismiss RH, providing pressure altitude in place of actual density altitude, because even the makers of these devices do not know the difference..

This weakness(dismissing Rh) also presents in ballistic software, and it matters because MACH# is affected by Rh. Again, the makers miss it.
This whole thread demonstrates that shortcuts lead to misunderstandings.

 

[RTK]

Humidity has the smallest affect on DA, temperature and pressure altitude the most.

 

[User4302021]

As mentioned earlier in the thread, DA is calculated from measurement of station Pressure, Temp, and Rh. These things(all 3) have to be measured, so it makes sense to simply use them directly.
A side problem with DA, at least in discussion, is that folks don't understand that DA has nothing to do with altitude. There are also devices out there that dismiss RH, providing pressure altitude in place of actual density altitude, because even the makers of these devices do not know the difference..

This weakness(dismissing Rh) also presents in ballistic software, and it matters because MACH# is affected by Rh. Again, the makers miss it.
This whole thread demonstrates that shortcuts lead to misunderstandings.

Getting away from the differences in Station Pressure, Barometric pressure and Pressure Altitude and Density Altitude etc. ....here is the practical consideration.

If you have a reliable device that can accurately calculate Density Altitude at your exact location using direct sensory input, then you will likely be fine. However, that is not how it works many times. The input is often pulled from a signal station in some other location, giving inaccurate information to the shooter.

It is way too easy to get an instrument that gives absolute pressure and temperature at your exact location (I wear a G-Shock watch that gives both) to mess with possible errors in reporting DA. As has also been said, humidity is pretty insignificant. You can leave it at 50% and never notice the difference.

 

[LastShot300]

Using Density Altitude is a poor substitute for these individual measurements.

Not getting the argument. DA if properly done in any software, must yield 100% the same results as the usual atmospheric parameters. No reason at all to do not use or trust it, unless the software is of poor quality.

 

[User4302021]

Not getting the argument. DA if properly done in any software, must yield 100% the same results as the usual atmospheric parameters. No reason at all to do not use or trust it, unless the software is of poor quality.

Does your device calculate DA at your location, or does it pull information from somewhere else? If it malfunctioned or ran out of battery power, how would you calculate it then?

My watch gives me station pressure and doesn't need batteries. If my G-shock didn't survive, then I probably didn't either. A fob with a thermometer imbedded in it gives me temperature. If it somehow broke, I can guestimate temp to within about 5 degrees. I leave humidity at 50%.

It is the difference between relying on a device to do everything for you, which creates a single point of failure, and actually learning and calibrating your brain to the effect that each individual input has on the firing solution.

I shouldn't even have said anything. Seems nobody wants to actually learn this stuff anymore. They want to enter numbers into a magic box that tells them what numbers to dial.

 

[LastShot300]

Does your device calculate DA at your location, or does it pull information from somewhere else? If it malfunctioned or ran out of battery power, how would you calculate it then?

My watch gives me station pressure and doesn't need batteries. If my G-shock didn't survive, then I probably didn't either. A fob with a thermometer imbedded in it gives me temperature. If it somehow broke, I can guestimate temp to within about 5 degrees. I leave humidity at 50%.

It is the difference between relying on a device to do everything for you, which creates a single point of failure, and actually learning and calibrating your brain to the effect that each individual input has on the firing solution.

I shouldn't even have said anything. Seems nobody wants to actually learn this stuff anymore. They want to enter numbers into a magic box that tells them what numbers to dial.

We are talking different things here, you argue over versatility from guesstimating under field conditions and my post was about DA accuracy itself.

 

[985]

Example: RSI Ballistic calculator calculates that at sea level in 59 degree air and 29.53 BP a bullet with a .540 BC at 2700 FPS zeroed at 300 yards will drop 307.9" at 1000 yards.

The same program, same BP, bullet, velocity, zero, temprature, but change the altitude to 5000' and the bullet drop is calculated to be 315.8 with the same barometric pressure imput.

The differance is 7.9"

I always thought that the actual pressure and not altitude changed the bullets perfomance. I know pressure changes with higher/lower altitudes go hand in hand, but thought that altitude in and of itself had no effect on the bullet and just the pressure change with the altitude.

Any ideas?? Is the RSI program correct or is it a glitch?

Density altitude is non linear. Shooting Flat, or more noticeably, shooting from a 10,000' ASL ridgeline down to a canyon floor at 7500' ASL there is only one ballistic targeting software package capable of calculating this change. It is called, "X-Ring."

 

[Mram10us]

Been a pilot for 20+ years and can tell you high DA is bad for planes but great for bullets. Altitude is the big one with bullets with temp and humidity being small factors for normal ranges (less than 1k).

Examples: my 300 rum at 1000yds
Add 1000ft = <3"
Add 8 deg F = 1"
Add .10 baro = .2"

 

[dfanonymous]

Edit. Never mind. It's covered.

 

[TexSavage]

Getting away from the differences in Station Pressure, Barometric pressure and Pressure Altitude and Density Altitude etc. ....here is the practical consideration.

If you have a reliable device that can accurately calculate Density Altitude at your exact location using direct sensory input, then you will likely be fine. However, that is not how it works many times. The input is often pulled from a signal station in some other location, giving inaccurate information to the shooter.

It is way too easy to get an instrument that gives absolute pressure and temperature at your exact location (I wear a G-Shock watch that gives both) to mess with possible errors in reporting DA. As has also been said, humidity is pretty insignificant. You can leave it at 50% and never notice the difference.

Dog Rocket, which Casio model do you have? I called Casio to inquire about a model that gives SP and the rep could not tell me a model. That would give me a backup to my Weathermate Weather Hawk.

 

[villagelightsmith]

If you're in camp and your altimeter says your camp is rising, you can start singing "Looks like, we're in for nasty weather." (Bad Moon Rising)
The old sailer sez "The glass [Barometric Pressure] is falling, hour by hour. The glass will fall forever. But though you break the bloody glass, you still won't stop the weather."
Reduced air pressure at altitude is what makes high country hunting such a physical pull. And reduced air pressure means reduced air density/wind resistance/air pressure for your bullet.
Gravity, for all practical purposes, is still a drag on our lard-laden butts.

 

[FlyGuy11]

I didn't have time to read the entire post but what I did; wow you guys are over thinking it. Density Altitude is what the ballistic apps are accounting for which is simply a correction of non-standard temperature and barometric pressure. Without getting too deep in the weeds YES will cause a significant change in your ballistics. As will all long range shooting the longer the range the greater the effect. Also the longer you go the more factors come into play such as wind and coriolis effect.
Short Version: Use the apps with a weather device for the most accurate data.
If you would like a no kidding long version of understanding Density Altitude please let me know.

 

[dfanonymous]

I didn't have time to read the entire post but what I did; wow you guys are over thinking it. Density Altitude is what the ballistic apps are accounting for which is simply a correction of non-standard temperature and barometric pressure. Without getting too deep in the weeds YES will cause a significant change in your ballistics. As will all long range shooting the longer the range the greater the effect. Also the longer you go the more factors come into play such as wind and coriolis effect.
Short Version: Use the apps with a weather device for the most accurate data.
If you would like a no kidding long version of understanding Density Altitude please let me know.

What taking your pressure altitude and temp and using a DA chart?
Or mathing it and taking your pressure altitude and getting your ISA temp to get your ISA in ft then adding your pressure altitude and temp correction to get DA..?

 

[FlyGuy11]

What taking your pressure altitude and temp and using a DA chart?
Or mathing it and taking your pressure altitude and getting your ISA temp to get your ISA in ft then adding your pressure altitude and temp correction to get DA..?[/QUOTE
1. Standard pressure is Sea level, 59 degrees F (15 degrees C), and at 29.92" in of mercury. This is the scientific baseline from which Density altitude (DA) is measured.
2. As altitude increases barometric pressure decreases as well as temperature in a perfect world (which it is not).
3. Nature does not adjust for science it's the other way around. I would not recommend carrying around a bunch of tables, there are aviation apps that will calculate it as well.
4. My #1 recommendation is a Ballistic app with weather which does all of this for you. (Applied Ballistics by Brian Litz or Ballistic Arc). The poor man's way is to

- find your altitude above sea level that your shooting at
- write down the temperature and pressure when you're shooting
- know your actual velocity at time of shooting (use an actual chronograph not what it says on the box)
- run a JBM ballistic table
- estimate the environment you will be shooting in for several altitudes (JBM allows you to do this) Also a good weather forecast helps with this.
- verify Zero with the Ballistics table at altitude well before the hunt if possible
Now that we have done the proper pre-flight let's go fly (my bad pilot humor). Alot of Ballistics is the same or relatable to Aerodynamics; flight instructor is the day job.