The negative of the vertical component of the atmospheric direct current (DC) electric field is referred to as the atmospheric electric potential gradient (PG). The PG depends on the actual ionospheric potential, local electric fields, and the electrical conductivity of the air at the place of the measurement. These factors are more or less directly connected to meteorological conditions. The overall state of the global network of large-scale electrical currents in the Earth-ionosphere system can be inferred from the PG when the weather is locally calm. This meteorological condition is traditionally referred to as “fair weather” and is characterized by allowed ranges of specified meteorological parameters. This is why information on the actual weather condition is supposed to be an inherent supplement of PG datasets. This dataset contains PG data recorded in the Széchenyi István Geophysical Observatory (NCK, 47.632°N, 16.718°E), Hungary from 1962, when the regular measurements were started, up to 2009. Throughout this time period, data were collected using practically the same instrument at the same location. The PG was measured by a locally developed radioactive apparatus which equalizes the atmospheric potential over the lowest 1 m thick air layer so that the potential difference between the sensing and grounded electrodes at ground level is the PG itself. Zero signal offset was determined daily and the instrument was calibrated in the ±250 V/m range weekly whenever it was possible. The instrument has a measuring range of −300 V/m to 300 V/m. The data were recorded photographically by a sensitive galvanometer. Hourly averages were then obtained from the photographical records via manual evaluation with an uncertainty of ±10 V/m. Hourly averaged PG data was included in this dataset when valid records from more than 30 minutes from a given hour were available. Records in the original dataset marked as unreliable or saturated have been omitted. Detailed characteristics of the instruments and the applied ...