Archives For snow area

The impending LOW approaching from the NW this weekend is a real teaser!  It will track SE across the country and the centre of the LOW will pass to to the SW of Reigate sometime over Sunday night. It will then “stall” for a day or two and drag in cool winds from the East.  Fronts associated with this LOW will bring every type of precipitation known…probably!  A weakening warm front slowly drifting east across the country on Saturday is not likely to reach Reigate until the evening. Saturday will be cloudy, calm and cool but hopefully remain dry. The leading edge of the warm front could bring some light snow by Saturday evening.  Sunday will be the BIG ONE… wet, cold and miserable, all day. North of London there could be heavy snow.  Reigate is on the VERY EDGE of the forecasted snow area (see map). Temperatures will remain above freezing at up to 3-4C, too warm for snow. However, despite this, heavy rain could turn to snow any time.  From Sunday afternoon onwards a process called evaporative cooling could cause snowfall. Some raindrops always evaporate on their descent from clouds, even in really cold air. Evaporation (turning a liquid into a gas) uses energy / heat (think of how chilly you feel stepping out the shower; the cause = evaporation!). Now imagine millions of tiny raindrops evaporating, for every evaporating raindrop some energy (heat) is taken out of the atmosphere and lowers the air temperature. In marginal snow conditions this temperature drop can be sufficient to turn HEAVY RAIN (the more raindrops the more cooling) into SNOW, even if surface temperatures are above freezing.  So, the question for Reigate this weekend is not “will it snow on Sunday?” but “will it rain hard enough to snow on Sunday?”  Enjoy the precipitation anticipation!