I cancel when it's below 20 air temp. I don't count windchill as our indoor is airtight enough that we don't get too cold. I make sure the horses have quarter sheets and/or coolers available and the kids are normally dressed pretty well. If they get too cold, we cut it short. I can't wait for it to warm up to 40 today! I'll be able to water and not have the footing freeze!
Life is hard. After all, it kills you. - K. Hepburn