An increase in the ionization near the ground due to the fall‐out from a radioactive cloud formed by a nuclear explosion will increase the conductivity and lower the potential gradient in the lower atmosphere. Records of atmospheric conductivity and potential gradient from the Tucson Magnetic Observatory are compared with records of the deposition of atomic debris on the ground following the Nevada tests. The observed changes are not inconsistent with values computed from theoretical considerations. Most of the effects are confined to a very shallow layer, within a few meters of the groun