The Librarian
03 April 2012 14:44:50

Originally posted by Matt Hugo (MVH) March 2012

The current settled and quite exceptional Spring conditions across the UK is associated with a large area of persistent high pressure.  However, if you remember a week or two ago high pressure was dominating the weather but it was often cloudy with it, so why is this high pressure different?...


The continued unbroken sunshine that is affecting many areas of the UK is down to the fact that the air mass across the UK is very dry, unlike the other week.  Meteorologists use atmospheric soundings to show conditions within the upper atmosphere and I have attached below a sounding for Leeds during the course of this afternoon (Tuesday 27th March).  This sounding is particularly impressive because it indicates a large amount of dry or very dry air across the region which is prohibiting cloud development; 



The above information should help to explain why at the moment the UK is basking in unbroken sunshine and why even during the warm afternoons not even a few scattered cumulus clouds can develop.  This very dry environment is not only evident throughout a large portion of the middle and upper atmosphere, but it is also evident at the surface.  Those with a weather station may have noticed some exceptionally low humidity and dew point readings in recent days, well this once again helps to highlight just how dry the air is at the moment and why clouds just simply can't develop.


Now just for comparison's sake, the weather is set to change later in the week to cooler and generally cloudier conditions as our high pressure declines to the west of the UK and allows for a cooler north-westerly air mass.  The below atmospheric sounding for Leeds on Friday afternoon does look very different compared to the above image; 



So, in summary; When high pressure becomes located over the UK it is often extremely important to use and look at atmospheric soundings to gauge what the atmosphere will be like above surface levels.  Atmospheric soundings are an extremely important "tool of the trade" when it comes forecasting.  So if you ever see an atmospheric sounding present like in the first image, think plenty of sunshine with very little or no cloud.  However, if high pressure is evident but the atmospheric soundings look more similar to that of the second image, then whilst it may well be mainly dry, cloud is likely to be an issue with perhaps cumulus clouds developing during the day or perhaps stratocumulus clouds persisting in large sheets, as was evident the other week.


If anyone is interested in viewing more of these specific forecast and actual atmospheric soundings, then they can be found here; - Just use the scroll down menu on the right hand side.


Regards to all, 








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