Chazz Ludwin of Rosedale wonders: "When and how were the original normal temperatures obtained? And does it take into account the weather patterns in the years/centuries before?" "Normal" suggests weather "as it should be." We should say "average," which implies variation is normal. For Baltimore, the National Weather Service uses a 30-year average of BWI temperatures, recalculated every decade. The current averages are based on 1971 to 2000. The next, beginning in 2011, will use data from 1981 to 2010.