Archives For airmasses

Reigate: decently warm and very pleasantly “springy” through to mid-week but then a significant fall in temperature and showers to end April!
Cloudless, blue skies bathed Reigate in sunshine all day today and brought temperatures up to nearly 15ºC: perfect for cricket!  However, the air flow was a  cold easterly which meant that it felt cool in the wind, more like 11ºC, but in the sheltered sunshine it was pleasant. London Marathon Day 2013 will start at a chilly -2ºC and will cloud over later, reach max 13ºC and stay dry with only a slight risk of a light shower later; low humidity perfect for marathon runners.

The big temperature graph shows temperatures at 1500m above the surface (at 850hPa), used by forecasters for predicting temperatures because it is a height which smooths-out the complicating influences of surface diurnal (day/night) temperature changes and surface features like sea, land, cities, hills and vegetation, so making overall temperature trends and changes in air mass easier to spot.  This week it’s easy to spot the cool air mass currently over the region, the warming mid-week temperatures and then a fall-off to next weekend.  Three different airflow directions are responsible for these changes, so here’s some detail…

reigate midweekLOW to the north and HIGH to the south of the UK: this pressure pattern will squeeze breezy westerly winds across the UK as a result of the pressure gradient between the HIGH and LOW pressure. A lively jetstream will be directly overhead of the UK and drag an increasingly mild upper westerly / south-westerly air flow from the Atlantic which will bring the risk of the odd shower on a few weak fronts reaching Reigate now and again early in the week but nothing much.

The HIGH creeps up more from the south over Reigate on Wednesday and this should bring warmer and sunny conditions, possibly up to 17-18ºC in much lighter SW winds: so making for a “warmer” HIGH than the one we are currently experiencing which is dominated by a cool easterly airflow over Reigate.

End of next week and next weekend:

It looks like it’s all downhill after mid-week: temperatures are due to take a plunge with HIGH pressure building in the Atlantic with an upper level trough (LOW) digging down over the UK bringing a cold Polar air mass down to Reigate through into next weekend.  Unsettled April showers, some possibly thundery, may break out at the end of the week and over the weekend as this cool northerly air mass meets the warmer surface air and increases lapse rates and instability, encouraging convection.  Next weekend could well struggle to reach 10ºC daytime max , with some showers during the day and nights turning cold.

This is all early days and things could change, so do keep updated on twitter for Reigate weather @RGSweather.

Where are bluebells flowering NOW? Find out here nature’s calendar

YES, we are in for another cold snap here in Reigate!  It might seem a rather tenuous link, but a severe weather warning for Eastern Canada today is critical to the build-up of our next cold snap, arriving this week. Temperatures for Reigate are due to go cold again and it is linked to what is happening right now in Halifax… and that’s Halifax Nova Scotia, not Yorkshire!  Nova Scotia is experiencing a fierce LOW pressure and unusually WARM conditions as a storm charges North up the Atlantic. This warm air is associated with the northward limb of the jetstream which is dragging warm air up from the Tropics to the Pole. This vigorous feed of warm air will help to build the HIGH pressure in the Mid-Atlantic which will block out warm westerlies from reaching the UK and open the door for a brisk plunge of Arctic air to reach the UK on Wednesday. The southward limb of the jetstream encourages a trough of LOW pressure to build over Europe and this could usher in even colder Polar easterlies to Reigate by the end of the week.  Arctic air usually dumps most of its snow over Scotland and rarely brings much snow down to sheltered Surrey (see recent post on Arctic airmass).  Whilst heavy snow is forecast for the NW and North there probably won’t be much snow getting to us in the SE this week, at least at first.  BUT!… we need to concern ourselves with those continental easterlies emerging at the end of the week as LOW pressure builds over Europe: these frigid winds could bring snow to Reigate but this will depend on the precise pattern of LOWS which are rather hard to predict right now. Check back later for updates on what is likely to be another exciting week of Reigate weather!