Atmospheric pressure plays a significant part in radon levels underground.
During the summer, on average, there are high pressure systems and outside air temperatures above those of the cave air. This has the effect of keeping the cooler air pressed down into the cave. With less air exchange with the outside, the radon can build up.
During the winter months there are more low pressure systems and the outside air temperature frequently falls below that of the cave air. The low pressure sucks the air out, drawing fresh air in. The cold surface air sinks in to replace the 'warm' cave air that is rising out.
That is why you generally get more radon in the summer than winter.
However, each weather and pressure condition could potentially occur at any point during the year, so what the radon levels are on one day can be quite different to the next. Also, a very deep low pressure any time of year can draw even more radon from the rocks and sediment in the cave and lead to very high levels. Even the wind speed and direction can no doubt affect it. It's really complicated! We can make the generalisation that radon levels are typically higher in the summer, but in reality, the actual weather and pressure of the day has a large effect on the actual concentration of radon you will encounter on any given trip.
I've had 21 'working' hours in there so far this year, which is estimated to be about 16% of my annual allowed dose of radon. By contrast I have clocked over 150 work hours in Cwmorthin, which has only consumed 4% of my 6mSv allowance.