I don't think the current radioactivity release is high enough in the atmosphere for weather balloons to detect? Not yet anyway. I think it's a pretty complex subject, dispersal of radioactive matter, depending upon a number of variables such as particle size, height of explosion, wind direction in the lower, middle and upper atmosphere, weather conditions (raining or not), etc.
Whoa - that really drives home just how many aftershocks are going on, doesn't it! Thanks for that link.