Nicholas,

I live in the tropics at 18 deg lat. The main thing that humidity will do is to slow down the drying time of the varnish. Temperature will affect this as well. The slowest coat to dry will be the first coat. It will take at least 24 hours, maybe more... or at least mine did with goldspar. However, the goldspar did dry fine and is still going after 8 months of heavy use.

The more humidity in the air, the slower things will tend to evaporate, hence the slower drying time, but once it's dry it's dry. Just make sure you don't try to put on one coat until the other is dry, or you can get cracking in the finish.

-Malcolm