Rainwater can be highly radioactive due to high levels of radon and its decay progenies Bi and Pb; the concentrations of these radioisotopes can be high enough to seriously disrupt radiation monitoring at nuclear power plants. The highest levels of radon in rainwater occur during thunderstorms, and it is hypothesized that radon is concentrated in thunderstorms because it forms some positive ions during thunderstorms. Estimates of the age of raindrops have been obtained from measuring the isotopic abundance of radon's short-lived decay progeny in rainwater.