Atmospheric γ-radiation has been measured since 1999 and recently at three elevations 220m from the first site to ascertain position dependency and optimal elevation for observing γ-rays from radon and radon-progeny found in precipitation. Radiation from time-independent and diurnal components was minimized in order to ascertain the reliability, accuracy and practicality of determining precipitation rates from correlated γ-rates. Data taken with 4-12.9cm~3 NaI detectors at elevations above ground of 9.91, 14.2, 15.7, and 21.4 m were fit with a model assuming a surface and/or volume deposition of radon progeny on/in water droplets during precipitation which predicts γ-ray rates proportional to the 2/5 and/or 3/5 power of rain rates, respectively. With mostly surface deposition and age corrections for radon progeny, the correlation coefficients improved with elevation and reached a maximum at 0.95 around 20m. Atmospheric γ radiation enables monitoring precipitation rates to 0.3 mm/h with time resolution limited only by counting statistics. High γ-ray rates, decreasing with 40-minute half-life following lightning may be indirectly due to ions accelerated in electric field.
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