I remember this particular May Day 1990 very well, because at the time I was an observer at Kinloss and was due on the night shift, taking over from John Sutherland who had been on the day shift. He told me the story of how a southerly foehn had kicked in during the early afternoon and increased the temperature by at least 10°C. All morning rotors from the strong southerly wind at 2000 feet, coupled with a sea breeze effect, had been keeping the surface wind in the north or northeast, blowing of a cold Moray Firth (fig 1), but eventually the southerly burst through and the temperatures soared from around 15°C at 12 UTC to around 27.2°C within the hour. I can’t be too precise about the exact times or magnitude in the rise of the air temperature, suffice it to say that John was not particularly happy about getting a phone call from Bracknell asking him to correct his 13 UTC observation! I think the maximum that afternoon which had been a smidgen higher at nearby Lossiemouth at 27.4°C (fig 3), will still rank as one of the warmest May Days on record in Scotland, if not the UK.
One day I’ll have to pay the archives a visit in Exeter, dig out the old obs book if they have it, and jog my memory! I only have six hourly SYNOP reports for that particular day, but here are the 12 and the 18 UTC plotted charts for posterity and for John, who was quite a character.