Found this on Digg. There are two graphics on there: 1) Time of day heat map, and 2) U.S. geographical choropleth map. Combined, this allows for a nearly instantaneous idea of when/where tornadoes are most likely to hit.
It would have been nice to see a month heat map here as well. I'm curious how many tornadoes, if any, hit in December for example. I'm also curious about the strength of a tornado vs time of day, but that was probably out of scope for this article.
In my opinion, this is an excellent data visualization. Instead of using rainbow colors, these graphs show variation by scaling a single color which is much more effective. Further, each map uses the same colors for the same purposes. Even more impressive, the colors tend to match what severe weather looks like (severe = dark, less severe = light). Lastly, although there are no illustrations within the graphs, they aren't needed here at all. On them, analyzing them, critiquing them, making connections, drawing conclusions, or finding http://writemypaper4me.org/ new