Hi Piz,
The paint being used should have the information you're looking for.Most paints I have run across use 45 degrees as the low.This time of year a painter needs to wait until later in the morning to avoid painting when the surface has moisture (dew)
that would become trapped under paint.You can touch base with your contractor for any of these concerns,but we still have a few weeks of painting weather left...I hope.

To add to what matches said...there are now low tempature paint formulas....your biggest problem is moisture(Dew,Rain). Also make sure that you stop painting about 1hr before sunset, this should give the paint a chance to set.