Joe Bollinger writes from Glen Burnie: "How often are temperature and precipitation averages revised? When is the period over, and do you think there will be a noticeable change?" The National Weather Service uses 30-year averages, recalculated every 10 years. Current readings are compared with averages from 1971 through 2000. In 2011, the weather service will switch to 1981-2010. The changes are noticeable. Last time, Baltimore's average annual snowfall dropped from 22 inches to 18.2 inches. Next time, I expect to see warmer, drier averages.