does bullet speed change with altitude

[pressman]

i just spent one week in Sheridan Wyoming doing some long range shooting.
i live in southern Indiana and i was wondering a few things.
i have a custom 260
i zeroed it here in indiana 750 above sea level at 100 yards.
my light loads were a light 2706fps with 139 gr Lapua bullets
i was having trouble with a drop chart being correct when i arrived. the altitude their was 4230 above sea level and i was just wondering was my speed of my bullet different their than here in indiana?
out their this was my actual moa on my scope that i dialed.
500-8.5 moa
600=12.5 moa
700= 16.5 moa
800= 19 moa
1200=39moa
1400-=49.5 moa
My exball was really close but i was wanting to build a drop chart for here in indiana to mount inside my scope covers but i could not get it to match by putting in the data verses what i was actually getting in the field.
any help would be great.
also while i was their i was hitting a 3' foot target at 1975 yards ( with a 50 cal.)
we had a great week.

[Rijndael]

A change in temperature could cause a change is muzzle velocity.

What your experiencing is most likely air density changes. I generally track that with my Brunton (you can use some Kestrels), using a calculation called Density Altitude. If you just use the elevation and temperature, you're missing one big value .... pressure. Here is a write up:

Barometric Pressure and Ballistic Software

I generally shoot at 330'. It's not uncommon for my Density Altitude to read -500' in the winter, while getting to 2500' in the summer.

[eshell]

I'd agree with Rijndael.

I shoot through variable air densities that occasionally dip below sea level (certain days at Quantico) to air densities that approach 6,000'.

With your .260 (I shoot a .260 and a 6.5-284), it would not surprise me to see you experience 2-3 moa variation in impact at even 1,000 yards between extremes like this. Air density is substantially affected by altitude, temperature and barometric pressure and a certain combination of these factors could cause unexpected misses.

I too monitor Density Altitude, which I use in conjunction with ballistic tables run at various altitudes. I simply check the environmental factors and turn to the correct chart.