There is a realtime radiation data collected via the System for Prediction of Environment Emergency Dose Information(SPEEDI) and provided in a map. Let us note that the dose rate units are in nGy/hr. The unit Gy stands for Gray, a unit that cannot be translated directly to Sievert. From Wikipedia:
Although the sievert has the same dimensions as the gray (i.e. joules per kilogram), it measures a different quantity. To avoid confusion between the absorbed dose and the equivalent dose, the corresponding special units, namely the gray for absorbed dose and the sievert for the dose equivalent, are used. For a given amount of radiation (measured in gray - the plural of gray is gray), the biological effect (measured in sievert) can vary considerably as a result of the radiation weighting factor WR. This variation in effect is attributed to the Linear Energy Transfer [LET] of the type of radiation, creating a different relative biological effectiveness for each type of radiation under consideration. Per most government regulations, the RBE [Q] for electron and photon radiation is 1, and varies for other types of radiation (see Q value).
The Q-value can vary from 1 to 20 depending on the radiation being released in the atmosphere. The Q-value on Wikipedia has the following:
Here are some quality factor values based on ICRP 103 recommendations:
Photons, all energies : Q = 1
Electrons and muons, all energies : Q = 1
Protons and charged pions : Q = 2
Neutrons : Q is a continuous function of neutron energy
Alpha particles and other atomic nuclei : Q = 20
Here is the site in Japanese.