The Department of Atmospheric, Oceanic & Space Sciences at the University of Michigan has created this Google Map that shows the air flow to a number of Japanese cities. The purpose of the map "is to illustrate whether the air arriving in select locations has previously been near the crippled Fukushima Daiichi nuclear power plant."
To view the air direction record for a city you just need to click on its map marker and select a date from a calendar. Trajectory lines for the wind on that day are then displayed on the map. The yellow overlay represents the probability of an upwind region's potential to influence air quality at this location on your selected day.