Effects on Ballistic

Ambient air density

By ⋅ May 16, 2008 ⋅ Post a comment

Air temperature, pressure, altitude and humidity variations make up the ambient air density. Decreased air density will result in a decrease in drag, and increased air density will result in a rise in drag. Humidity has a counter intuitive impact. Since water vapor has a density of 0.8 grams per litre, while dry air averages about 1.225 grams per litre, higher humidity actually decreases the air density, and therefore decreases the drag.

Discussion

  • Rookie

    I have just started to get into long range shooting. One of the questions I had concerns air density changes caused by altitude. I know that temperature decreases approximately 3.6 degrees for every 1000 ft of elevation. Obviously, the density of air changes as elevation increases. How can you calculate for this change when you are changing elevations?

    On a side note… I find it interesting that the 3.6 degrees of temperature change is similar to the 3.6 MOA. 3.6 sure seems to be a common number for this sport. Almost like somebody designed it that way. 😀

  • http://longrangeshooter.com Sean

    The best way is to have a something that tells you the station pressure. Which is kind of like air density and altitude combined. Or you can use Barometric pressure but then you will also need to know the altitude. For example I live at about 2100 feet and the station pressure here is about 27.55hg most of the time and at 1,000 yards I have 21.8 MOA of drop. Now up at about 5,000 feet of elevation the mass air density is much lower now and the station pressure is 27.50 and my drop now at 1,000 yards is 21 MOA. So there is a 8 inch difference in drop just from station pressure and did not include Temp. or Humidity. If you get a Kestrel weather station it will tell you all the data you need to know. Remember that your muzzle velocity will also increase as the temp drops.