DO you correct for spindrift?
which kestrel do you use?

If you look in my example of a chart above you will see SD with a correction of L2. This means there is a correction of left two clicks (1/4MOA scope) for that range or a correction of approximately 5" for 1000 yards to compensate for spin drift.

I use the Kestrel 4000 specifically this model calculates for density altitude. One input in computer solution using DA vs. multiple inputs for temp, pressure, humidity or altitude.

Density altitude is the altitude of the standard atmosphere (sea level, 29.92 in/Hg, 59F) at which the air density would be equal to the actual air density at the place of observation. If you are standing at sea level with the standards described above and the temperature changes to a higher temperature, lets say 90F, your density altitude would then be greater than sea level or 0 ft.

I guess you could say DA is the altitude the bullet "thinks" it's flying thru using the standard as reference.

MMERSS, if I understand this, since I'm no expert mountain hunter, the pressure drops as altitude increases, so from your quote, if all things are equal at sea level, and temperature increases (to 90F), can one expect the pressure goes up? (just trying to understand this better)..thanks

if all things are equal at sea level, and temperature increases (to 90F), can one expect the pressure goes up?

lever-hed,

In response to your quote above, generally as temperature increases a lower pressure system moves in to the space and vise versa. Lower pressure assume hotter temps, higher pressure assume cooler temps. I’m not a weatherman but you can usually assume certain aspects regarding temperature and pressure.

Air density influences the predicted ballistics profile of a projectile in flight. The density of the air is related to the drag of a projectile in flight. An increase or decrease of the air density causes more or less resistance to the forward motion of a projectile. AIR DENSITY is the target we are after and can be calculated with the use of our weather stations and ballistics programs.
Density altitude is calculating the actual density of the air and then assigning the altitude at which that same air density occurs in the standard atmosphere. Air density calculators do the same and require input of four values common to ballistics programs; altitude or map elevation, barometric pressure, temperature and humidity. Most weather stations will provide the readings for the four inputs as stated above and A FEW weather stations will provide a density altitude option. Both options calculate air density.

For the long-range shooter we have to ask ourselves the question: "So what with air density?"

Below is data from two of my field charts for a density altitude of 3000DA (which is the standard air density of .91 kg/m3) and 7000DA (.59 kg/m3). I can say 3000DA and 7000DA on my charts but they equate to .91 and .59 kg/m3 of air density. My Kestrel 4000 does not have an output for air density but does for density altitude. Again, you can consider DA as having an air density value equivalent.

300 RUM Berger 230, DA3000, 10 MPH
Range ELV Wind SD
600 7.2 2.3 L1

300 RUM Berger 230, DA7000, 10 MPH
Range ELV Wind SD
600 6.9 2.0 L1

300 RUM Berger 230, DA3000, 10 MPH
Range ELV Wind SD
1000 19.5 4.3 L2

300 RUM Berger 230, DA7000, 10 MPH
Range ELV Wind SD
1000 18.6 3.7 L2

As stated in a previous post I have experiences as much as a 4000ft density altitude change in a given day. Looking at the charts with a range of 600 and 1000 yards ask yourself the question "so what?"

600 yard elevation MOA difference; 7.2-6.9 =.3 MOA or one click on a ¼ MOA scope. You should still hit a ground hog in the belly or head aiming center of mass at 600 yards with a .3 MOA correction on either end.

1000 yard elevation MOA difference; completely a different story. 19.5-18.6 = .9MOA or 4 clicks on a ¼ MOA scope. You are now approximately 10" in elevation correction error if not compensating for density altitude. Now combine this possible error with your velocity standard deviation!

I hope this post has helped some. Not compensating for air density at extended ranges can cause enough error for you to not achieve your intended outcome.

OK so ‘density altitude’ is simply the standard barometric pressure at the given altitude(s), I understand this part (shown in the chart in my previous post). Then the ‘real pressure’ or actual pressure is going to be plus/or minus that number depending on the particular weather system in the area. So if you are entering DA that simply gives you a standard pressure (and corresponding air density) irrespective of the weather system at your position, and, contrarily, if you are entering actual altitude, pressure, temp + humidity(from a kestrel for ex.) that is obviously better than just entering DA. So, essentially, your cards are only going to reference the standard pressure/air density for each elevation, which is simply your baseline, as I understand. Temperature and humidity will also have an affect on air density.
Thanks for the good explanation/rendition..
Lever

Last edited by lever-hed; 03-28-2013 at 12:32 PM.
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