You can add a little solvent to the varnish to speed drying (for pettit varnish the thinner is T10) but there are limits.
The problem comes when night falls and you get condensation, and the varnish is still not dry. The moisture turns the varnish white and cloudy.
I would say 50F is the limit to get it dried out in one afternoon.
Bristol 31.1, San Francisco Bay