The weather component you probably should start taking into account

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mmahoney

mmahoney

Member
Feb 20, 2017
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Wyoming, United States
Which weather components do you need to worry about when shooting?

The wind direction and velocity is obviously the biggest factor. However, until a couple years ago, I meticulously monitored the temperature, altitude, air pressure, shot angle and humidity of my shooting sessions. I was reminded today while shooting in my backyard which of these is the only one (other than wind) that most shooters need to be aware of: temperature.

Temperature
I used to scratch my head wondering why my gun needed to be zeroed so frequently between shooting sessions. It took lots of trial and error to discover that the air temperature was the cause. If you zeroed your gun in 80F weather (like I did this summer) and decided to do some winter shooting in 20F weather (like I just did in my backyard today) expect your point of impact to drop a full inch at 50 yards. Without that knowledge you are likely to assume gun or scope error and constantly wonder why your gun's zero changes unexpectedly. This phenomenon is exponentially worse at 100 yards. I use a temperature sensitivity factor of 1.7 in Strelok to approximate the real world results I see. (Note Strelok says the factor is usually between 0.6 and 1.4, but that for airguns it is usually zero). My experience says otherwise. I landed on a factor of 1.7 through trial and error. If you shoot different distances with different pellets and velocities, your temperature sensitivity factor may be different. I haven’t experimented with other projectiles to find out.

According to Streklok, with a sensitivity factor of 1.7, shooting 25.4gr gr, JSBs at 890 fps at a distance of 50 yards, a 10 degree shift in temperature will raise or drop your point of impact by about 0.3 MOA.

chart1.1640922524.JPG


But what of the other factors? Let's look.

Barometric pressure
It takes severe air pressure swings to noticeably affect point of impact. An increase of 1 inHg (A very large jump) will decrease POI about 0.3 MOA at 100 yards. In all the times I’ve kept track, the air pressure always measured near the normal range of high 29s to 30 inHg. It never strayed enough from this range for me to ever worry about. 

chart2.1640922787.JPG




Elevation/Altitude
I was surprised to learn an Increase in altitude has almost no effect. Apparently, the air doesn’t thin at higher elevations as much as my mind assumed.



chart4.1640922976.JPG


Humidity
I figured POI would drop as the amount of water floating in the air rose. Nope. I had to use Chairgun to figure out the effects of humidity. It should have been a clue that the effect is minimal when Strelok doesn’t even have a place to enter relative humidity on the weather input page. Short story short: There is no difference between 0% humidity and 100% humidity. I don’t claim to fully understand this one.

Shot Angle
The bottom line is that minor inclines and declines are negligible. Unless the angle is SEVERE, such as shooting up in trees, there is very little POI shift. The change is only ¼ inch at 50 yards for a 5 degree incline/decline.

chart6.1640922907.JPG

So in summary, it is useful to know what temperature you zeroed your rifle at. If you find yourself scratching your head why the point of impact of your pellets has changed, there is a good chance temperature is playing a factor.
 
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Environmental conditions to include temperature, humidity, barometric pressure,etc. Have always had an effect on external ballistics, from pellets to 120mm tank cannons and up.

Temperature has always been the biggest factor. the others play, but compete with the shooter, which tends to be a larger factor in most cases.

Air is the propellant, and like propellant will act on the projectile differently when it is cold vs hot.

Then add Barrel temp, BP, Humidity.

Back in the day, when we would boresight the guns on our tanks, prior to gunnery, we would recheck our boresight if there was a 10 degree or greater change in temperature.

The ballistic computers would factor in all the Enviromental's as well. Even ammo temp.

Look at any ballistic calculator and how slight changes will affect POI (Point of impact)


 
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Hmmmm, this got me thinking. 

Does the projectile velocity drop significantly shooting in colder weather, causing POI shifts? Maybe that’s what is being inferred here as it relates to extreme temp changes. I.e., big velocity swings?

I recall shooting my first and only 2020 virtual Extreme Benchrest target in really cold weather, and believe my velocity had dropped unbeknownst to me at the time. I remember being surprised at how low my shots were hitting at 100 yards, and always wondered if the cold temperature lowered the velocity. 





 
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Just to clarify. I'm shooting from a heated indoor location out my back sliding glass door to a target 50 yards away outside. That means the gun is actually pretty close to room temperature whether I'm shooting in the hot summer heat or during freezing winter temps. The POI drop seems to be directly caused by the pellet slowing down in the cold air. However, I notice the same phenomenon at the range. If I sight in on a cool morning and then go back out shooting after it has warmed significantly during midday, the POI shift I describe will occur. The velocity from the gun is consistent (I rarely shoot without my chronograph).
 
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Just to clarify. I'm shooting from a heated indoor location out my back sliding glass door to a target 50 yards away outside. That means the gun is actually pretty close to room temperature whether I'm shooting in the hot summer heat or during freezing winter temps. The POI drop seems to be directly caused by the pellet slowing down in the cold air. However, I notice the same phenomenon at the range. If I sight in on a cool morning and then go back out shooting after it has warmed significantly during midday, the POI shift I describe will occur. The velocity from the gun is consistent (I rarely shoot without my chronograph).
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@mmahoney This is actually pretty interesting. What I’m curious about is this, when you noted the POI drops, were you shooting from a fixed rest? Like the type some people use when they zero scopes? I’m specifically referring to when shooting outdoors, but if you did while shooting from indoors I’m interested in your response to that question as well. I’m trying to wrap my head around this as it’s relatively new to me. 
 
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Laws of physics would dictate that air density increases with drop in temperature, among other things, too:

https://en.wikipedia.org/wiki/Density_of_air

I shoot from a heated room during frigid winter months in Minnesota. Its very common for me to experience POI shifts based on relative humidity and temperatures. Air guns are much more finicky than firearms. That's my observation, fwiw.
 
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Air is about 93 percent as dense at 86° f as it is at 50° f. So it should be about 7% easier for a pellet to travel through the air at 86°, then it is at 50°. Seems to me that in the case of a spring air rifle for example the rifle would be more efficient by something like 7% at 50°, then 86°, but I don't believe that's what experience shows or maybe it does. Just thinking out loud I believe I should have a look at my copy of the air gun from trigger to Target
 
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@Ezana4CE

I'm shooting from a Caldwell Stinger rest on top of a table I have modified to remove any wobbles from it. My scope is a March 10-60x56 HM scope. My gun is a .22 cal Taipan Veteran Long. The gun and rest are predicable and consistent enough to shoot perfect scores on National50 benchrest practice cards.
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Thanks that’s what I was wondering. The variables mentioned helps me to work on my longer shots. 
 
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@Ezana4CE

I'm shooting from a Caldwell Stinger rest on top of a table I have modified to remove any wobbles from it. My scope is a March 10-60x56 HM scope. My gun is a .22 cal Taipan Veteran Long. The gun and rest are predicable and consistent enough to shoot perfect scores on National50 benchrest practice cards.
Click to expand...

I have a .25 Vet Long and can’t shoot higher than a 235 average for N50. Would not mind seeing some of those ‘perfect 250 scores’ with your Vet. 
 
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I have to agree with your temp findings, but cant agree about humidity...my many years of airgunning has shown that high humidity also affects poi.. When humidity is high the velocity drops on my chrono which causes a shift in poi. Have made numerous tests over a chrono under various levels of humidity and found high levels have always shown lower velocities at least it has in my airguns.
 
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