Unusual peak

Yesterday I got a strange peak in readings counts went up from about 16:30Z until 19:30Z, then dropped off until about 22:00Z. Any ideas? There were no significant meteorological events. Another counter, about 1ft away also showed an increase, but not as marked, this is a similar type counter but with some extra beta shielding.

No moving of sources at all. No rain here for weeks, although last week we had very strong winds. We have also had a number of forest fires in the district, although there was no change in readings on Saturday when we had smoke from one of them, but that lasted all day.

I do get a diurnal variation that I associate with radon, but this was a far bigger excursion than I have seen before. I have had a number of peaks in the past but was able to identify causes, principally due to movement of the probe relative to my sources.

I think this effect could be explained by considering air pressure changes. If the soil is dry, radon gas deep down would seep upward with low surface pressure. This may contribute to the diurnal variation seen by many outdoor stations. With geographic variations in Radon levels, a higher-Radon area may experience this seepage, then the wind could carry it a bit before it dissipates. I've imagined a study to measure this effect, but it would require I think a dense grid of sensors.