If one plug is noticeably different than the others, you also need to look at that particular cylinder.
Best case: There is a problem with the plug wire to that cylinder.
Worst case: That cylinder is losing compression due to ring or valve wear, or some other mechanical cause.
When it is damp, like mentioned above, marginal plug wires or cap/rotor can be an issue. But the dampness usually means cooler air which on a carbureted car, can change the mixture slightly. Fuel injection self adjusts, a carb can't (though a feedback carb does a little).
If I were diagnosis this I'd check a few things with he engine first:
1) A compression test. If any cylinders are low, repeat with a wet compression test (add a little oil to the cylinder, if compression comes up rings are bad. If compression stays low it's probably the valves).
2) I'd check for timing chain stretch, which involves putting a breaker bar on the balancer bolt, turning it one way, making a note of where the rotor is pointing, then seeing how far you have to turn it the other way before the rotor moves.
3) I'd also look for slop in the distributor shaft.
Low compression or timing that is off can make it run rough. If everything looks good, I'd make sure the timing is set to spec, the carb is in good shape, the idle speed is set to spec, etc. And I'd put some good copper spark plugs, new quality plug wires and a good brass cap/rotor on it.