Increase in CPM during downpour

I've heard it is radon decay products that are embedded in dust particles (like PM2.5) that get washed out of the air with rain. At least that's the most common explanation short of a nuclear excursion.

Yes, Radon decay products. They are only active for an hour or two. Good fun to wipe your (wet) car down with a kitchen towel afterwards and test that - it concentrates the activity as the water evaporates a little
Good example from Tom Watson (antiprotons on YouTube)

I tried this today after a heavy rain; I swiped my car with a cloth and the readings instantly raised 2.5 times with my Geigertube.
Really interesting, I will record a spectrum with my Scintillation detector and compare it with my previous background recording

Click for graph

Hi All,

I'm new on this forum having joined this day, the 25th of July 2017. And, I must say I see a great deal of interest here, having been monitoring background gamma across the UK for the last 31 years since the Chernobyl disaster. This is the first thread I noticed and I am fascinated as it addresses elevated background readings during the onset of rain events. I felt compelled to comment, as I have been observing such events over the decades at some 20+ stations, less now I have often heard talk of radon washout as the cause of the increased gamma, but, I can also add that the highest elevated readings during rain events notably occur during occluded weather fronts. I am no meteorologist or physicist, but I understand occluded fronts come with high altitude atmospheric turbulence. It has been suggested that these rain events are likely washing out long lasting radioactive fallout of old, that is still floating in the upper atmosphere from the atmospheric nuclear bomb tests of the 50s and 60s, not to mention the contaminants from coal fired power stations ... but I am less certain of the latter. It is certainly during the onset of many typical rain events that the readings are at their highest, dropping back down within a couple of hours, despite the rain continuing long after.

In the mid-90s we set about monitoring the acidity in rain alongside gamma background as the more radioactive rain was seen to be causing fish kills in the river Humber. The fish kills were reported by local fishermen whenever such significantly radioactive rain events resulted in washout from the North Yorkshire moors reaching the river. Whilst we developed the technology to remotely monitor these parameters, it turned out to be too costly to continue the research as we have largely operated through voluntary input and Crowdfunding was not on the cards back then !!

I was fascinated to see AstroW on the forum thread here has a scintillation detector. I have to ask, is this detector being used to provide an energy spectrum able to analyse the component radionuclides ?

BTW Please check out the Argus Project's new website at ArgusProject.net

Graham.