Nice early June: good effort in the UK, but warmer in Northern Iceland!

May 31, 2013 — Leave a comment

Good news! HIGH pressure is building across the UK for early June and should last well into next week. The “Azores High” is moving our way and this will bring settled, calmer, warmer, drier, brighter and occasionally sunny weather to Reigate and much of the UK. Some rain is possible as fronts brush across the west and a trough over Europe brings a low risk of thundery showers later next week for the SE but this is not certain at this stage. The weather is looking mostly good for early June with temps reaching Tmax 20ºC.  

SSTs late May 2013Despite this the SE will remain average or cooler than average for the time of year due to being on the cool side of the HIGH: winds will be from a cool east or NE crossing the North Sea (still cool 8-9C) as the HIGH migrates to the North and East of the country.  

high and low pressure diagram

High pressure is where air sinks over a wide area. Sinking air is caused by cooling either from Tropical air chilling over comparatively cool oceans, like the “Azores High”, or air cooling as heat is lost from frigid continents in winter, like the continental HIGH over Asia./ Siberia.  Subsiding air inhibits convective updrafts of air (thermals) which create cloud and rain, so HIGHS are usually dry. HIGH pressure systems, also called anticyclones, also BLOCK the passage of low pressure cells pushing frontal systems away to the edges.  So, HIGHS are more or less the “opposite” of LOW pressure which has dominated our weather for most of May. In LOW pressure systems (cyclones or depressions) air rises, cools and condenses to form clouds and rain.  

air flows from high to low!Air flows from HIGH to LOW: that is, winds blow from areas of HIGH pressure to LOW pressure.  Due to the rotation of the Earth wind is deflected to the right in the Northern Hemisphere.  This is called the coriolis force and creates the familiar pattern of spiraling winds circulating anti-clockwise into LOWS and clockwise out of HIGHS in the Northern Hemisphere (it’s the opposite in the Southern Hemisphere).

 
fohn effect #1

fohn effect #1

The HIGH is forecast to move to the NE of the UK and this will bring in cooler NE and E winds to the SE: but light winds always. Remarkably, on Tuesday the relative location of HIGH pressure to the East and LOW to the SW of Iceland will mean Northern Iceland could be warmer than Reigate! The reason is an unusual weather phenomenon called the Föhn Effect.  Iceland has big ice sheets and mountains in the south and central region,including Vatnajokull, which is the biggest ice cap in Europe and has the highest mountains in Iceland.  The southerly / SW wind building over Iceland, created by the movement of the HIGH to the east and movement of a LOW to the South, will roll up and over these mountains and ice sheets and create LOTS of rain and snow!  On the windward slopes, in the teeth of these SW winds, saturated air cools as it is forced to rise over mountains.  However, as it it cools there is a release of latent heat due to condensation.  On the leeward side of the mountains and ice sheets the comparatively dry air sinks and warms up at a faster rate than it cooled. This is because the descending drier air warms more rapidly than the ascending cool air cooled (due to that latent heat being released)! (Still with me?!)  The result is a warm, dry wind streaming down from the mountains of central Iceland and bathing Northern Iceland in temperatures over 18ºC. It is rather like an “extreme” rainshadow effect experienced to the east of mountains in the UK, for example.  Föhn winds are associated with most high mountain ranges which experience stable saturated airmasses being forced to rise over them, such as the Alps and the Rockies.  In the Rocky Mountains, the Fohn wind is called the Chinook or “snow-eater” which melts snow rapidly across the plains at this time of year.

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