As many will have noticed over the last few days there have been some very heavy showers evident during the afternoon  period, with some locally torrential downpours with hail and even some scattered thunderstorms as well.  In a previous blog I placed some emphasis on how atmospheric soundings can be used to gauge cloud amounts in anticyclonic (high pressure) conditions.  Well, equally and also very importantly, atmospheric soundings come into their own during times of instability and particularly between the months of April and September...

The aim of this particular blog is to shed some light on how atmospheric soundings, which are freely available on the internet, can be used to highlight the risk of instability which can lead to heavy showers and thunderstorms in the coming weeks and months in particular. 

The first image of interest here is an atmospheric sounding from Leeds at 1600BST on Wednesday 11th April.  I have used this particular sounding as it was one of the best to highlight instability.  Now as a quick recap, the red line represents the air temperature up through the atmosphere, whilst the blue line is the dew point temperature.  Now when the air becomes unstable, on these particular charts, an additional line appears and in this instance this is the dotted line I have highlighted within the image.  In essence what this line represents is that it signifies that the air mass is unstable and that the air can freely rise.  Clearly when air rises it brings the development of clouds and if the air can rise enough it can lead to significant cloud development with large towering cumulus clouds and cumulonimbus clouds (thunder clouds). 

The next image, highlights more specific and important features; 

The image should highlight and explain some of the primary features that are of interest.  I have overlaid the picture of a cumulonimbus cloud which places emphasis on the level of instability present within this atmospheric sounding and that with clouds potentially rising up to 24,000ft easily leads to the development of cumulonimbus clouds and hence, intense and perhaps thundery downpours at  the surface.

Now moving on to take a look at one primary important variable and that is CAPE.  This stands of Convective Available Potential Energy and is a measure of the instability present within the atmosphere.  CAPE is derived from the space present between the temperature line (red line) and the dotted line I have highlighted on the images.  The larger this space the greater the CAPE value is and hence the greater the instability also is.  You will note that on this specific image the CAPE value is provided at the top beneath the date and time.  In this instance the CAPE value is 394j/kg (joules per kilogram), which is a reasonable amount of CAPE.  

As a general rule of thumb from a UK point of view the following should give an indication as to the risk of heavy and perhaps thundery downpours; 

CAPE VALUES: 

100-300 = Marginally unstable, risk of some heavy showers, but thunderstorms generally isolated or scattered.

300-600 = Moderately unstable, heavy showers/downpours a distinct possibility, thunderstorms also probable.

600-900 = Highly unstable, heavy showers, downpours and thunderstorms a significant likelihood. 

>900-1000 = Extremely unstable, torrential, thundery downpours a high probability, risk of severe convective weather. 

CAPE values are a changing variable and for example across parts of the USA CAPE values can reach and exceed 2500j/kg, which is clearly an extreme level of instability and because of many variables those kinds of values are never achieved in the UK.  But from a UK's perspective the above values should give a reasonable indication as to the level of instability that is forecast. 

So, in summary, if you hear that thunderstorms are forecast and see atmospheric soundings similar to the ones used within this blog, along with large CAPE values then envisage the risk of heavy and perhaps thundery downpours.  The forecast soundings used within this blog can be found at the following website; 

http://rasp.inn.leedsmet.ac.uk/RASPtable.html

Regards to all, 

M.

NB: I did a post on TWO a number of years ago with some additional information on convective variables, perhaps the link to this post is still active somewhere and can be added to this thread as an additional source of information...

Edited by user 25 April 2012 08:58:46(UTC)  | Reason: Not specified

Great post - that's really helpful.

Originally Posted by: lanky

Thanks Matt - interesting post

I do remember your thread (quite a while back) on variables used in connection with rising air masses and perhaps someone will have kept a copy as it will have been deleted from here long ago

Edited by user 12 April 2012 08:08:04(UTC)  | Reason: Not specified

Excellent explanation - Cheers Matt

Matt,

Thanks very much for this very useful post.

Just one thing - the Netweather link above also mentions Lifted Index which should be used along with CAPE when looking at storm risk.

What sort of negative values can we relate to slight risk, meduim risk and high risk that the CAPE will be 'tapped into'?

Thanks for taking the time to post this Matt, a great help to someone like me.

It would be a shame to lose this valuable info. any chance of it being moved to the library please?  

Yes, I'll move it to the library next time it drops off the main page

Originally Posted by: Sevendust

Originally Posted by: Marigold

Yes, I'll move it to the library next time it drops off the main page

Thanks Chrissie

Hopefully it may be useful this weekend