Humidity is the enemy of candy makers. The perfect candy-making climate has a relative humidity of less than 35 percent. But if you live in an area that's hardly ever that dry, you can always use your air conditioner or a dehumidifier for about a day or so before making candy.
Because excessive softness in candy is the result of high relative humidity, you can also cook hard candies, such as peanut brittle, to 2 degrees higher than the temperature your recipe calls for. The additional 2 degrees in cooking temperature will compensate for moisture in the air.