The humidity rating on the cards changes at the rate of 2-1/2% of the rated value per 10°F away from 75°F. That is, for a card that is rated at 30%, when viewing it at a temperature of 35°, the true value becomes 27%. (2.5 x 4 = 10% rating change, applied to a rating of 30% = 3%, 30% – 3% = 27%.) It works in the opposite direction for higher temperatures.