Parkhomov published decay rate measurements of 90Sr/90Y and 60Co beta decay sources with Geiger-Müller counters which showed annual cyclic deviations with less than 0.2% amplitude from a purely exponential slope. He investigated instrument instability induced by environmental parameters, yet did not find a clear coincidence with local temperature, atmospheric pressure, and relative humidity. Parkhomov hypothesised that gravitationally-focussed 'slow' cosmic neutrinos influenced beta decay. In the current work, environmental conditions in the Moscow area at the time of the experiment are presented. There appears to be a resemblance of the shape of the annual 90Sr/90Y decay rate anomalies with the inverse of the absolute air humidity, albeit with an apparent time shift of 0.05-0.15 year. Humidity may have influenced the range of beta particles in air, as well as geometric and electronic properties of the detection set-up, however causality could not be unambiguously demonstrated. The instabilities in the 60Co data were more difficult to correlate with environmental data, except for some similarities with temperature and external dew point.