Bright daylight vs. muted light conditions--scopes/iron sights??

[rockhound78]

When I used to shoot IHMSA, a number of very good shooters told me that differences in light conditions would change POI. If you were shooting in sunny conditions, and a cloud came over, you had to raise your rear sight to compensate for a lower bullet impact. Conversely , If shooting in cloudy conditions and the sun comes out, it would cause your bullet impact to be higher.

True or not? If true, does this apply to scopes and will the "power" affect any changes. Question came up at our local LGS this morning

 

[snert]

100 percent yes.

I shot or oversaw the shooting of over 600 woodchucks on one hillside over 25 years. From 8 Am to noon, use the normal setting, but start holding lower at about 11AM. From noon to 3PM we saw as much as a full minute of "artificial elevation" suddenly appear, sending bullets high over woodchucks. Those in-betweeny ranges of 350-425 yards were such a royal pain in rolling landscape. A click here and a click there, plus high sun and it was almost like we had no known zero. I have notes saying "down 4-6 clicks for noon-3 in full sun" in my log book for my Swift and 22-250. At about 4PM it started back toward normal, slowly, and by 6PM we were back to "normal" settings. The crazy thing? It seemed like every year we had to relearn that, despite killing all those groundhogs.

I do not know if it was mirage, or temps, or a combo, but I can say it got real fun when we had a faint breeze inducing a quartering mirage plus the elevation/sun issue and heat. I preferred a faint breeze, enough to suspend and move the dandelion seeds in the air consistently going one way.

Oh, and this was all thru scopes. My T10 was less effected than my 6.5x20...but then I wasnt "holding as tight" with the T10 either! But I saw it in all scopes. Ranges were 200-750 yards.

 

[Coyotefurharvester]

I believe it is a slight optical illusion due to mirage, the bullseye appears just slightly higher than it actually is. Results in reticle being rised and a higher bullet impact.

 

[1shot]

The old adage is "sun up, sights up" has been around forever, but others on here, are reporting the opposite is the case. Hmmm......

 

[jelenko]

I think the rule of thumb comes from shooting at round, black bullseyes with iron sights with a front post.
In bright sunlight, the edges of the black can become 'smeared' thus changing the point of aim when using a post front sight.

I think whether the point of aim goes up or down depends on how the sun is shining on the target and where on the target the top of the post is held.
When the sun is more straight overhead, it will eat into the top more than the bottom => if you're holding [apparent] center of mass, you'll hold higher than the actual center => need to move the sights down!

As the sun is more at an angle [lower in the sky], the blurring is more even around the black. Here, if you're holding 6 o'clock, you'll aim too high => sights have to go down.

 

[varmintshooter]

I like it, something else to blame when I miss

 

[Coyotefurharvester]

Now I know why I missed a prairie dog once out of a 50 rd box.

 

[tjtjwdad]

The other adage is 1 click into the direction of the light with open sights.