A Goldilocks moment for snowfall over Reigate this Sunday / Monday

February 7, 2013 — Leave a comment

Snow is very fussy. It requires particular conditions throughout the depth of the atmosphere to form. Any one of these conditions not met, and it will fall as sleet or rain. For example, snow requires particularly cold temperatures (obviously!) extending right the way up through the atmosphere to allow deposition of ice crystals instead of water droplets: indicator temperatures are lower than -27°C at 5500m, lower than at least -5°C at 1500m and surface temperatures at least below 2°C, so it doesn’t melt on the way down.  Also, a dew point below freezing is a pre-requisite for snow to fall. On Sunday and overnight to Monday not one of these conditions is met over Reigate, so snow in the current model forecast is not likely: 5000m temperatures are too warm (-25°C), at 1500m the temperature is a balmy +1°C and the dew point is +5°C through most of that period.  The only period when all the snow-forming conditions are met over Reigate (as things stand currently) is on Monday afternoon / evening.  Unfortunately, at that stage the LOW will have drifted off and taken much of the precipitation with it.  So… does this discount snowfall for Reigate over the weekend? Not at all, snow is still very possible because of nocturnal cooling and evaporative cooling and any slight change in the track of the LOW, but this is another story for later. Stay tuned!

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