Japan Radiation Watch app for iPhone and iPad
Displays the radiation present in the air from a location within Japan (Yokohama) with a large, easy to read display - including the amount in nanoGrays, and a gauge with clear "safety" and "escalated" zones.
The reading is "live" - updated every hour.
There is also a graphing option available which allows you to see a visual history of the radiation readings all the way back to 2012.
Now also visible on your Apple Watch!