Why is 30 years the magic number? This question came to my mind when looking at April precipitation figures for Washington, DC. The 1971-2000 "normal" that was used until recently seemed to show that April is a dry month in D.C., with only 2.77 inches of precipitation on average. In fact, adjusted for the number of days in each month, the 0.092 inch daily average for April during that period ranked dead last among the 12 months, narrowly edging out February. However, the new D.C. 1981-2010 A