You can tell that it was quite cold that day because of the ice crystals. I was driving in Minnesota the other day, and as I drove further west, I noticed that the ice cystals were brighter and brighter. I kept checking the temperature outside, and saw that it was dropping about 10 degrees in just a few miles. Suddenly the ice crystals in the snow were gone, and I noted that the temperature had gone up by 10 degrees again. It must have been the location of a weather pressure in the area, pushing the temp down.