I was flying south in UK around the Luton area in T shirt weather one May. I was at 3'000 basking in the sun at around I would say 20C in the cockpit. Within seconds of flying under quite a normal looking Cu I was in a snowstorm on instruments and I would say the temp dropped 15 degrees instantly. After at the most 30 seconds I was back in the sunshine and the cockpit temp was back up to where it was previously. Nothing was forecast so I passed a Pirep to the ATC unit I was working.
Can't beat the good old British weather.