The diurnal variation of the gross alpha (alpha) radioactivity in the air near the ground and the gamma (gamma) radioactivity emitted from the ground have been monitored in North-eastern Greece. Meteorological information comprising air temperature and humidity has been simultaneously recorded. Over a period of the 24h of a typical day, the variation of alpha-radioactivity reaches a peak in the morning followed by a remarkable decrease, rising to a second peak in the afternoon. Furthermore, its significant dependence on the air temperature and humidity is confirmed, rising with an increase in humidity and decrease in temperature. The variation of the ground gamma-radioactivity follows that of the air alpha-radioactivity. A mathematical model has been developed to describe the diurnal variation of the alpha-radioactivity in the air near the ground in terms of the above meteorological variables and ground level gamma-radioactivity.