This was always an issue with the hood tachs.
Moisture or condensation, I did notice mine has this problem and it's an after market hood tach, the condensation would go away after the engine heat would cause it to dry out. Mine has operated like this for the 15 yrs and still works today.
Last year I found that when I converted from points to electronic ignition that it would work erracticly or not at all, I found that I need to run a new 12v source to the wire that is hooked with the light terminal and it's working again, if moisture is a problem then mine wouldn't work at all, many times it was fogged up.
As far as condensation, I maybe wrong but it shouldn't affect it and if you are worried about the moisture they make a kit to keep it dry that was available on 1968 Pontiac's. I have seen it for sale, not sure where though.