Humidity and pressure will have little impact on your efforts. Rain will, but it depends on the rain.
While at Barnes Bullets a few years ago we did a rain test on 5.56 and 7.62 bullets in one of our indoor tunnels.
We we created a nozzle that emitted a stream of water droplets. We placed the stream in the path of the bullet, 10' in front of the muzzle with 100 yards to the target. The "targets" were sonic triangulation measurements of each shot recorded on a computer with a paper representation displayed on the computer screen. Each shot numbered in order.
With high speed photography we shot a number of bullets in each caliber. We could see which bullets hit a water droplet, and those that did not. The rifles and loads were 1 MoA without interference.
5.56 55 grn bullets were deflected up to 5". 7.62 150 grn bullets were deflected up to 2". Heavier bullets would be deflected less. Hitting a droplet further downrange would cause less deflection. Hitting two or more droplets on the way to the target would cause larger deflections.
The question becomes what is the likelihood of a bullet hitting a raindrop on the way to the target? Depends on how much rain is falling and the distance to the target.
If you shoot in the rain, it is one more variable for you to account for when trying to sight in a rifle. Load variations, sight problems or adjustments, and shooter skills all have an impact on groups and your points of impact.
I would not bother to try to sight in a rifle in the rain. Adds another uncontrollable variable.
BTW, we also did grass and twig variations of the same test with similar results. At times we noted key holing due to bullet instability.