Long Range Shooting

demigodllc.com/icao.pdf

For the last three years, I have been using Density Altitude as the primary index to characterize atmospheric conditions.

From: Windmeters and Environmental Stations for Tactical Shooting (Guns & Ammo's Tactical Guns & Gear, December 2006):

   Environmental conditions are often described in terms of pressure, temperature, and humidity. Pressure is reported both as "station pressure", which is the actual barometer reading, or as a corrected number normalized to sea level based on the actual location altitude. With these three numbers, the shooter can cross-reference similar conditions in his log book, or he can use the parameters as input to a ballistic program to generate the drop values for his load.

  Keeping track of three independent parameters can be confusing. For a log-book, the shooter can end up with a lot of different environmental conditions, which are hard to match. If he runs data from one of the ballistic programs, he will have to sweep three variables to generate data for all likely conditions. There is also an overlap in pressure and temperature mapping to air density. For example, a cold 20-degree day in Denver will have about the same air density as a hot 95-degree day in Dallas.
 

One solution to this is a figure called "density altitude." Density altitude is one number that represents the density of the air, which is all the bullet cares about anyway, and it effectively replaces the need for the three original parameters.
 

The best way to utilize density altitude is to get the current value off one of the wind-meters that reports it, and use that as an index into the data-book or input into the ballistic program. With less technology, the shooter can cross-reference the location altitude with temperature to get a pretty accurate estimate of the density altitude.

My data cards are all indexed by Density Altitude. Simply choose the Density Altitude(s) at which you shoot, and then get a set of cards for those conditions. When you set out for a training session or match, simply look at the expected station altitude and temperature and select the right card from your deck.