Making Fudge or Candy While it is Raining or Snowing

It's not an old wives tale; it's actually true to a point. I can't say it better than Exploratorium, Science of Cooking

Can weather affect candy making?

Oddly enough, it can. Cooking candy syrup to the desired temperature means achieving a certain ratio of sugar to moisture in the candy. On a humid day, once the candy has cooled to the point where it is no longer evaporating moisture into the air, it can actually start reabsorbing moisture from the air. This can make the resulting candy softer than it is supposed to be.

That’s why dry days are recommended for candy making, although the effects of humidity can be somewhat counterbalanced by cooking the candy to the upper end of the appropriate temperature stage.

Cool weather is also recommended for candy making, because—generally—the faster candy cools, the less chance it has to form unwanted crystals.

At The Fudge House on Fisherman’s Wharf in San Francisco, they like to make fudge on cool days for another reason: According to owner and candy maker Tom Lowe, people eat more fudge when it’s cooler.

Huge commercial operations are in humidity controlled buildings, but you can get home dehumidifiers too. Perhaps one in the kitchen could help.

This is actually (partially) true. It is more correct for fudge, not so much for candy overall. The texture of fudge and fondant depends a lot on the water contents where a 1% difference will matter and it will absorb moisture from the air during the cooling and beating period. So a high humidity will result in a runny fudge.

Source:
"On Food and Cooking", Harold McGee

If it was true at one time, modern HVAC makes it moot. Also, it is specific to where the saying came from: "cool" and "humid" mean different things in different places, to the point where dry in Kaua?i is more humid than wet in Indio.

If it's raining outside but the heater is on because it's also cold outside, it's probably very dry inside.

So the advice is very vague. It would be far better to make a statement about the relative humidity in the vacinity of the candy, which is not the same as outside and need not be the same as the kitchen in general.

Just as you can use a candy thermometer instead of relying on expert knowledge of indirect observations, you can get one or more cheap digital thermometer/humidity meters. You can position these (after making sure a cheap one is accurate via the salt water tupperware test) where you’re working, and collect precise advice on how the relative humidity and temperature where the bowl is affect the recipe.

Just did some research as I'm making caramels, which hate humidity. It said approximately 35% or less is best. If that isn't feasible run your A/C unit a day or two before. I would assume a good dehumidifier would work, too. Personally, I'll keep waiting for a low humidity day.