There are several factors that contribute to proper sugar crystal formation in candy, and temperature and moisture are 2 of the factors. So, yes it can be hard to make candy on a hot, humid day. But there are other things we need to make sure are done properly as well. Thermometers need to be properly calibrated before each use (place in boiling water; should register 212°F. If not, add or subtract to the final cooking temperature of the candy accordingly). Saucepans need to be the correct size for the correct concentration of the sugar mixture. Stirring or not stirring according to the recipe is important, as agitation will affect sugar crystal formation.