A small head wind drops my MPH by at least 1. Car is an aero brick. I also use the DA calc from Wallace Racing when comparing runs. Weather underground has data from local weather stations. I use one that is very close to my track. It updates current conditions every 3-5 minutes.
The thing that surprised me, is the car was already over 1 mph slower in the 1/8, but still cut a itsy teeny bit of a better time. It lost a little over another mph the last 1/8 as well, loosing 2.3 mph total
On a side note, most cars were running about a half tenth quicker than what the dial in was in their window. It was about 72 degrees and humidity was low. Head wind was inconsistent, and a little bit shifty. Maybe 5-7 mph winds when it picked up, may 1-2 when it was low....?????..... sometimes hitting the passenger side of the car, but most of the time the flags were blowing straight down the track.