Archives For January 2013

Rainfall is usually measured in millimetres but it is hard to imagine how MUCH WATER this actually means. It is easier to use the idea that 1mm of rainfall is equal to 1 litre of water falling on every square metre of ground.  Friday is going to be a wet day with 10mm of rainfall expected.  But how much water is actually going to fall on the school during this wet day? Here’s how to find out:
1. Use Google Earth to plot a polygon.
2. Right click on the polygon file in the Places list on the left hand side of Google Earth and COPY it.
3. Find
4. PASTE the polygon file into the empty box mid-page.
5. Select sq metres for the area units and click “view on web page”.
6. This brings up the AREA of your polygon in sq metres.
7. Now multiply the area by the expected or actual amount of rainfall in mm.
8. The result will be the litres of rain falling on that area.
On Friday we can therefore expect about 200,000 litres of rain to fall on Reigate Grammar School. This is about the same as 11,000 buckets or 650 car fuel tanks being emptied onto the school grounds during the course of the day.
Weather advisory: bring an umbrella!
Update: in the end, the actual rainfall only delivered 6mm of rain onto RGS… the LOW pressure didn’t pep up to form the awful winter storm forecast earlier in the week. Friday turned out to be wet in the morning and then cleared up to be a nice day.  Nevertheless it still added up to 6000 buckets of water thrown over the school!

It looks like February could see a return to winter conditions for Reigate (2 small colour charts) and maybe even cold enough for some snow; though with the sun being that bit stronger it is unlikely to be as chilly as last week. The jetstream, currently right over the UK and bringing our stormy conditions and unseasonably high temperatures (12°C today at midday in Reigate), is likely to be shifted south during the second week of February. A blocking upper ridge in the Atlantic and building HIGH pressure over the North Pole will combine to swing winds round to a cool northerly direction over the UK. A LOW over Europe will add to the chill by bringing in periods of cold Easterly winds over the UK as well and it is these which often bring significant snow to the South East.
Meanwhile, this week’s extraordinary weather is not over yet: a deep depression (see big synoptic chart) is forecast to rapidly swing up out of the SW on Friday. Depending on it’s precise track over the UK it could bring heavy rain and strong winds or EVEN some snow. The models disagree by a critical hundred miles or so: MetOffice has the LOW tracking south of Reigate= easterly winds = snow; GFS has it tracking north of Reigate = westerly winds = rain. (Update: UK Met Office model is now on it’s own with snow in the south; all other models take the track of the LOW further north leaving strong SW gales and rain on Friday but no snow for Reigate – snow will fall further north across the UK; so the synoptic chart above is probably WRONG already!!).  In both scenarios the back of this LOW will see nippy northerly winds bringing temperatures right down on Saturday, but at least it should be bright.

Today illustrated how observing changing cloud types can help predict an approaching front and rain. If you look up, you can often have a good shot at forecasting the weather several hours away. Today, a warm front approaching Reigate rapidly from the west was heralded by a series of cloud formations. In case you missed the drama, here is a round up of the best bits:

A bright blue sky and a frosty calm morning quickly gave way to a milky sky with the appearance of high cirrus cloud building into a thicker blanket of milky cloud called cirrostratus. These indicate increasing moisture in the upper atmosphere, which is often a sign of deteriorating weather and frontal rain approaching. A “halo” might appear around the sun with thickening cirrostratus: again, a sign of deteriorating conditions aloft. Cirrostratus clouds continued to thicken and lower into a blanket of middle altitude clouds called altostratus. These turn the sun into a fading white disc. Altostratus clouds are rarely thick enough to cause rainfall. As the front approaches low level stratus clouds appeared scudding above Reigate Hill in the 50mph winds aloft. Persistent rainfall started in Reigate mid-afternoon when thicker featureless nimbostratus clouds moved overhead, marking the arrival of the front proper. The temperature rose throughout the day, from 0°C at 8am to 10°C by 10pm, another indication of an approaching warm front.  The front travelled 200 miles in about 4 hours, a 50mph race across the UK and, despite the gathering wind and gloom, brought a wonderful lesson in meteorology, hope you had a moment to enjoy it!

Snow cover picks out the high ground over South East England prior to the big thaw on 25 January 2013. Note the snowy North Downs and a thin thread of snow along the crest of the South Downs, in contrast to the lack of snow in the London Basin and the Thames Valley. Contrails radiating from London airports are also visible. Note the lack of snow near coasts, showing how the sea retains sufficient heat even in January to turn snow to rain or melt it.  The English Channel sea surface temperature off Brighton is still over 8°C.

A powerful extra-tropical storm is hovering between Iceland and the UK for the next few days. The central pressure of this LOW has fallen to an astonishing 930mb, one of the deepest ever recorded in the Atlantic. Wind speeds of 90mph and 50 foot waves are being recorded. It has been named “Jolle”. It will not directly cross the UK but is forecast to send several fronts and “baby” storms our way: the potent fronts corssing Wales and delivering heavy rain and thunderstorms tonight are part of this new weather pattern associated with a strong jet stream. Warm, wild weather approaching: hang on to your hats!

Beautiful satellite image of UK covered in snow

A very deep depression (936mb) to the south of Iceland is whipping up 60 mph+ westerly winds in the Mid-Atlantic. Fronts with rain attached to this LOW pressure will rapidly cross the country overnight tonight and should clear Reigate tomorrow morning leaving a bright showery day. The winds in Reigate will be blustery with maximum gusts reaching over 40 mph tonight and calming down to ~20 mph Sunday, but do spare a thought for ships in the Atlantic where waves could exceed 40 feet (13 metres on map) in height in sustained violent storm force winds (Force 11).

An unusual winter pressure pattern is building in the early part of next week: a HIGH over Spain and NW Africa and deep LOWS over the Atlantic to the NW of the UK will combine to create what some meteorologists call the “blowtorch”. The relative position of HIGH and LOW pressure will funnel Tropical air from the Azores direct to Reigate possibly raising temperatures to 12°C on Tuesday (the big map shows temps for today, Saturday). This effect is sometimes called the “BLOWTORCH” (or hairdryer) because it can bring unseasonably high temperatures and melt snow rapidly across NW Europe (this blowtorch could reach as far as Russia where snowfall has been extreme). Reigate snow will be long gone by Tuesday but the warming effect could still be marked and a big contrast to a week ago when Tuesday temperatures fell to -6°C. Don’t break out the flip-flops just yet! Unfortunately, the blowtorch is unlikely to last more than a day or two at most and, in any case, it will be accompanied by rain and wind which might make it feel fairly unremarkable! Watch the video clip below which shows temperatures rising across the continent and the cold being beaten back as BLOWTORCH blasts through Europe!

Update@Fri 4.45pm: main fronts making slower progress from the west: JUST rain arriving Reigate later this evening, some heavy, overnight. Blustery tomorrow, still feeling cold until after more rain overnight Sunday. Rain cleared off 7am Sat.

Weather fiends will be fixed on the big change afoot from Polar easterlies to warm Atlantic westerlies taking place this weekend. The change is best illustrated by early morning temperatures in Reigate like this…
Friday 6am: -4°C
Saturday 6am +2°C
Sunday 6am +9°C (warm!)
The first of many fronts and storms marking the gradual transition between cold polar and warm tropical air will arrive in Reigate on Friday afternoon / evening  around 6pm as the HIGH pressure is quickly pushed off into Europe with gradually increasing southerly winds which could be gusting 25mph+ for a while overnight. Heavy precipitation overnight Friday could be sleet, or snow or rain and probably a mix of all three at times: it’s a very close call! Temperatures will hover around 1 or 2°C, rising slightly towards midnight and then falling slightly by dawn on Saturday.  There is the added complexity of a little sister LOW developing on the southern end of the trough in the Channel (see map). This baby sister could develop and be responsible for heavier precipitation and a subtle shift of wind overnight in the SE to a more easterly flow which might push temperatures down just enough for us to wake up to significant snowfall early Saturday morning: a final sting in the tail! Saturday could have an amazingly snowy start if this sister low kicks in. Anyhow, despite a chilly start Saturday temperatures will rise to 5°C and see a thaw of any remaining snow. Heavy rain is expected overnight Saturday to Sunday as a big classic Atlantic depression with warm and cold fronts sweep through. Properly breezy westerly winds on Sunday which will bring in increasingly mild Atlantic air which we haven’t experienced for a good while: feel the atmosphere and light change! Temperatures could rise to a balmy 11°C this week in a weather pattern called a “Blowtorch”. More on this later.  Longer term we are expecting a return to wintry weather in early February: winter may not be over yet!

Can you spot the differences between these two weather charts? Two similar looking synoptic charts but hidden within them are 5 subtle differences that are set to bring totally “opposite” weather to the UK.

Friday 18 January brought “SKYFALL”: heavy snow across much of the country which worsened a wintry cold snap lasting a over week.

Friday 25 January is predicted to bring (eventually) warm and wet weather (preceded by some snow) across the entire country.

Two totally different outcomes from very similar looking charts. So… what’s the difference? Your answers welcomed in comments! n.b. there are more than 5 differences, but 5 is a good start, especially if you can explain them!