It's only a factor if it affects your ability to determine the focus point. Haze (as in brightening/whitening of the target) and darkness all lower the contrast between colours, which is how the eye determines focus and detail. It's easier to see the sharpness between yellow and black than muddy white and grey, or dark brown and black... if you can't determine a precise point of focus, your focus range will extend... where as you may have been within 1-2 yds, you may be within 3-5.
Temperature shift is different, even under good ranging conditions there is a definite change of focus point...
Low contrast problems can be helped by not focusing on low contrast objects... the face plate has to be about the worst for this... details such as the type on the nockover label can be good, but that can be shiny and be hard to get a definitive read on... i use single fibres of string fray on the knot. If I can see individual fibres splayed off, then i'm within a yard. Sometimes they're not there so i'll move to the edge of the base plate and look at bolts... and if i'm 100% certain, i'll look at the tree or mount as well. Straps i find in bright conditions can be shiny so i don't always use them.
Temperature can be combated by using the scope over varying temperatures on a known fixed range permanent target of the right detail for sure fire ranging. You need to be sure the target hasn't moved over the months you test it because you need to rule out POI changes of the gun making you think it's something else. You need to be very precise and very aware of the temperature of the scope, which can be 10-15 degrees away from what the expected temp of the day is... weather temps are shade temps... i've seen snow on the ground and the scope read 20 degrees before.
It's not easy, it takes time... but doing the work to know your kit is an edge you will have over another competitor that hasn't, or has just copied what everyone else has said.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep