Another Altitude/Barometeric reading question, Hornady Standard Calculator.

[Greg Langelius *]

I live at 4350ft ASL in SE AZ. My weather app is Wunderground, and it shows Barometric Pressures in the 29-30in ranges, as well as local altitudes. I have done some rudimentary research into density altitude, and it suggests that air pressures at my altitude are actually around 85% of Sea level, yet the Barometric pressures I'm reading off Wunderground also look like Sea Level Pressures.

So I run the calc using 4350ft for my altitude and both Wunderground Barometric pressures, and Barometric numbers corrected downward to 85% of that.

Obviously, trajectories and downrange velocities are very different. I don't know what to trust and how to get this right. At this point, I'm only shooting out to 300yd, and I'll be using the calc to get ballpark solutions for out to around 800-900yd.

I suspect that the Barometric pressures I'm reading off Wunderground are 'corrected' in some way.

Can someone clue me in, understanding that my math comes from a HS Grad level education?

Greg

 

[pbatesaz]

There is an IOS app called Density Alt+ that will give you the uncorrected barometric pressure. You can either use this, temperature and humidity to get your DA OR you can use the humidity, temp, altitude and your commonly available barometric pressure to arrive at DA. Do not use 85% of the commonly available barometric pressure ( also known as the barometric pressure corrected to sea level).

 

[Greg Langelius *]

Commonly available barometric pressure...?

Is this from my Wunderground App, or from an actual barometer? I suspect you are talking about the former.

To be clear, I'm really just looking for which sources to get the info to input into the Hornady Standard Calculator (with advanced functions) data fields. Is the Barometric Pressure number direct from Wunderground OK to use for the Barometric Pressure entry?

TIA.

Greg

 

[pbatesaz]

Ok. I should have looked at the Hornady calculator before to give you a better answer. I assume you are referring to what they call the BC Calculator. It wants Station Pressure, AKA “uncorrected barometric pressure”. This is probably not what you are getting from the Wundergound app. At least that is not what I am getting on the Weather Underground app on IOS. For example, it currently shows 30.18 in Hg for my location, but station pressure is 21.71 inHg. If you are on IOS, try Density Altitude+ app. I’m sure there are Android apps that can give you station pressure as well. If you are unable to get station pressure, at your elevation 85% of the Wunderground provided pressure will give you a reasonably good approximation of station pressure and you could use that value. For the Hornady app, that would be the value they want. Hope I haven’t confused the issue for you?.

 

[ZiaHunter]

Greg:

Most weather applications report corrected barometric pressure which will be different than station pressure which is uncorrected for elevation. I haven't used the Hornady app but most allow you to use either barometric or station pressure. To get the correct DA using barometric pressure you generally have to input the elevation of your location. I have an app on my iPhone called eWeather HD which allows you to select either barometric or station pressure. Rule of thumb is that station pressure will be 1.0 less for every 1000 thousand feet of elevation. Not exact but will get you close.

 

[Greg Langelius *]

Thank you both; you've cleared my confusion.

I had used the 85% and altitude values recently and gotten some very interesting extensions for Supersonic threshold (Nominally 1300fps) distances.

It would appear to convert the IMI 308 168 load into a 1kyd Supersonic load. But that's not the key point; what it really means for me is some additional confidence in computed trajectories within my own real world. I figure I can at least get impacts close enough to the computations to manage some observed impacts, and correct from there.

The Barometric Pressure I'm inputting at 25.50 as 85%-ish of 29.99 from Wunderground. 1in per 1000 (minus 4.35) gives a 25.64. So both methods are in the ballpark.

Thanks again, both of you, for taking the time to help.

O/T: This altitude makes my COPD come out to play often. I found out a year or so ago that Mt. Lemmon (9157ft ASL), near Tucson, is not a place for me. I have arbitrarily assigned the altitude of 7000ft ASL as my own personal altitude limit.

The good news is that the temperatures up here are nicer, especially after Sunset. It takes the edge off the Arizona Summers. This area in SE Cochise County, AZ is sometimes called "The High Lonesome". That pretty much fits.

Greg

 

[spife7980]

Just to sum up- Pressures reported on the news etc are 'barometric" pressures and are elevation corrected so as to match what they would read as at sea level. If you use your barometric news pressure you will also need to enter your elevation so that it knows how to actually account (uncorrect back to the raw number) for your elevation/pressure/density.

Station pressure is uncorrected, the raw data for the actual pressure where you are at. When using this you set your elevation to 0 so that it knows you arent trying to correct it for sea level.

Setting Station Pressure on Kestrel - Kestrel Meters

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More on Station Pressure. Really, the only people asking us about these are those using ballistic software....

Basically you need station pressure for ballistic calculations. Many applications assume though that you only know barometric pressure from the weather channel. So they want you to input the barometric pressure and your altitude. The program then calculates the station pressure from that. So if the customer’s program asks for both pressure and altitude, they most likely want the barometric pressure and altitude and then the customer would have to adjust for each time they move locations.

Typically for shooting, you will want your station pressure, not the barometric pressure. Station pressure is the pressure felt at that spot, without being adjusted for altitude. This is the same pressure being felt by the bullet when traveling in the air. When you adjust station pressure for your altitude, this is now Barometric pressure and typically only used to track weather patterns. In order to get the Kestrel to read station pressure, you should set your reference altitude to ZERO. This will then show you station pressure, every time, and there would be no need to adjust it if you move locations.

Unfortunately station pressure is not readily available if you do not have a Kestrel. Therefore many ballistics programs have the user enter their barometric pressure AND altitude. The ballistics calculator then calculates station pressure from this for the ballistics solution. This is because barometric pressure is widely produced by the local weather stations and easily obtainable. If your ballistics programs requires you to enter the barometric pressure and altitude, then you will need to adjust your reference altitude each time you change locations. You will also need to keep updating your reference barometric pressure on the altitude screen to ensure your altitude is correct.

If there is an option in the ballistics calculator that you are using to enter the station pressure vice barometric pressure, then please select this option and set your reference altitude to zero. If not, then you will need to enter your reference altitude each time you move locations to get the barometric pressure. Use this barometric pressure as your reference barometric pressure for the altitude and use those numbers for your ballistics solution.