JBM Calculator

[Old Navy]

I have been using the JBM calculator and found it is pretty close to get me on the target at 1K and one mile. I think I have been lucky to get the first shot on my 5.5X5.5' target at 1 mile. I notice a difference in the data depending on whether I check the "The Standard conditions at Altitude box or not. I have been checking this box. Which of the two modes is more accurate? Can someone explain this paragraph so a 78 year old senior can understand it a little better? Thanks. Frank

• Why doesn't the air density change when I change the altitude?
  The altitude is only used when you check "Standard conditions at Altitude". If this is not checked, the density is calculated from the Temperature, Pressure and Humidity you entered.

 

[KRP]

Entering actual pressure will be more accurate.

 

[jelenko]

It simply means that JBM has a set of conditions - pressure, temperature and humidity - that it will apply if you check that box.
You if know what the pressure, temperature and humidity are - or want to try different values - you don't check that box and enter what you want JBM to use for temperature, humidity and pressure.

 

[Old Navy]

I think I have it now. There is a weather underground station one mile from where I shoot at almost the same altitude to get all that data. There is another box titled "Standard Atmosphere at Altitude. I take it that that box is un-checked also?

 

[KRP]

I think I have it now. There is a weather underground station one mile from where I shoot at almost the same altitude to get all that data. There is another box titled "Standard Atmosphere at Altitude. I take it that that box is un-checked also?

If you have actual data leave that box unchecked, otherwise it will just input standard average figures for the altitude you enter. There is box for corrected pressure also, leave it checked if your weather station gives you corrected pressure. Station pressure would be the most accurate, then you'd want to uncheck the corrected pressure box. If you have station pressure you don't need altitude. Pressure is what matters, it just uses the altitude number to "SWAG" pressure if you don't input it or to adjust for corrected pressure data.

 

[Willie]

Get yourself a Kestrel and you'll be carrying station pressure, temperature, humidity, wind speed & angle and more in your pocket. It's not free like JBM but it does just as well and you can always have it at hand.

 

[divingin]

Standard conditions (for aviation anyway) assume 59F at 29.92"Hg and an altitude of sea level.

If you choose standard conditions for altitude, it does 2 things: tapers off pressure at whatever the weight of the air below you is (1" per 1K gain in altitude); and tapers pressure off at a rate of 3.5 degrees for 1K feet of climb.

Given that it's rare that your conditions match the "standard", you're better off (accuracy-wise) to enter the actual ambient environmental conditions.

For spitballing, standard conditions will probably give you what you want.