I've just started looking at FEMA 320, "Taking Shelter from the Storm". http://www.fema.gov/library/viewRecord.do?id=1536
I stumbled and looked twice when I got to a statement on page 1, referring to a figure (map) on page 3. On both pages, values are given of the number of "EF3, EF4, and EF5 tornadoes per 2,470 square miles" during the period 1950-2006, which isn't an example of a "round number" in the denominator.
A little playing around with my calculator has convinced me that this data arises from a geographically oriented data field with 80 km grid spacings. So, that "2,470 square miles" is actually (80 km)^2. The squares are either 49.70 miles on a side or 79.98 km on a side, and I think the latter is more likely.
Grid sizes based on "round" numbers of kilometers are the norm in meteorology, for example in circulation models, storm modeling, etc.
Jim -- James R. Frysinger 632 Stony Point Mountain Road Doyle, TN 38559-3030 (C) 931.212.0267 (H) 931.657.3107 (F) 931.657.3108
