By monitoring precipitation on a regular basis, EPA is able to establish normal background levels of radioactivity and detect additional activity from man-made radioactive sources. Analyzing precipitation samples along with air filters helps estimate the amount and type of radioactive material that will be deposited on the ground.
It is very common for natural background radiation levels to rise during precipitation events such as rain, sleet or snow. Radon and radon decay products, which occur naturally and normally dissipate in the air, are held closer to the ground by cloud cover, significant snow coverage, pressure changes, and may also be captured in the precipitation itself and brought to the ground. This causes the gamma radiation levels to rise in some measurements.