There is no limit really. The baby is in warm clothes and in a sleeping bag during winter. Less clothes when its warm.

My children slept outside while it was below zero C. The only time they were not sleeping outside was while they were ill.. having a cold or something.

Edit: the limit is -10 C according to https://www.sst.dk/vidensbase/graviditet-og-smaaboern/barnet...

This is well known around here:

https://www.bbc.com/news/magazine-21537988