MDSnow93

The two main things the hardening off process is for are sun exposure and wind exposure. As well as adapting to a broader range of temps. The sun part is obvious, but the stems and roots are really not equipped to handle any real wind. A calm day is best, and I also usually start with one hour on the first day. Then slowly add on a couple more hours before gradually adding time in slightly sunnier spots. It's tedious, but I put a lot of time and effort into my seedlings. So I make sure they all make it through the home stretch! Good luck with the next batch! At this point it seems like ground sowing might be a safe bet for a lot of things! But I know more about plants, and less about weather.

Radiant heat from your house perhaps?