Author Topic: does humidity affect velocity? (Read 496 times)

thelaw · « **on:** September 08, 2004, 07:28:58 AM »

today i went and ran some loads thru both my .25/06 and .270. i've been loading the same loads for these rifles for several years now. my question is, does humidity affect velocity? the reason i ask this is because i live in southeast texas where it is very humid. i moved from northeast texas which was not as humid as it is down here near the coast. when clocking my loads today, which are of the exact same powder charge, oal, primer, case, etc., i got about an average of 75fps slower velocity out of all 4 loads i clocked. for example, my two different .270 loads normally clock 3078 (load1), and 3045 (load2). today they clocked 3010(load1) and 2973 (load2). if my humidity theory is accurate, do you think that when i go on my wyoming hunt in a couple of weeks, that i will experience higher velocities due to far less humidity. i know 75fps is nothing, i'm just trying to understand what is going on. any suggestions?

ricciardelli · « **Reply #1 on:** September 08, 2004, 08:36:28 AM »

Yes.

Standard testing is done at 78% humidity.

MUTTLY · « **Reply #2 on:** September 08, 2004, 08:49:14 AM »

elevations in Wyoming will affect it as well, as the air is thinner.

thelaw · « **Reply #3 on:** September 08, 2004, 11:16:56 AM »

so are you saying, the thicker the air the more resistance, thus slower velocity? it makes sense to me, but i don't know much technical stuff about ballistics and such like many of you do.

sand draw JAKE · « **Reply #4 on:** September 08, 2004, 05:59:31 PM »

Quote from: thelaw

so are you saying, the thicker the air the more resistance, thus slower velocity? it makes sense to me, but i don't know much technical stuff about ballistics and such like many of you do.

I think the proper term is air density, which is affected by a lot of things, including humidity, altitude and barometric preasure...I do about all of my shooting above 5000 feet, but when I visit my kids in Illinois, I shoot a little with my son-in-law, and my guns will shoot 2' to3' low at 100 yds, as they are at about 500 ft. and the humidity is always about 600 percect....

ajj · « **Reply #5 on:** September 09, 2004, 03:06:37 AM »

Raising humidity DECREASES air density.

Donna · « **Reply #6 on:** September 10, 2004, 06:58:21 PM »

Thelaw

I have a very good web site that can teach you about such things, in fact I have just posted two more pages, Standard Atmosphere and Trajectory Part 2. This sit shows you where the numbers come from and what to do with them.

http://www.aeroballisticsonline.com

Donna

onesonek · « **Reply #7 on:** September 12, 2004, 01:58:19 AM »

According to a couple of ballistic programs, humidity shouldn't affect velocity to the degree you are talking. On the other hand, air pressure and or air temperature very well could.
Dave

unspellable · « **Reply #8 on:** September 13, 2004, 12:41:29 PM »

There are two effects at work here, not to be confused.

The first is the moisture content of the powder. High humidity will increase the moisture content and vice versa. This effect will be very slow to produce changes as you generally keep a lid on the powder jug and the air circulation between the outside and inside of the cartridge is pretty restricted. More moisture in the powder will reduce velocity a bit.

When shooting, increased humidity in the air reduces the density and hence the drag on the bullet. A microscopic increase in muzzle velocity due to less weight of air to push out of the barrel ahead of the bullet for a given muzzle velocity a bit flatter trajectory at long range. But not enough to notice unless you are shooting for a one hole group at at 600 yards.