‘Loaded Gun’ Atmospheric Sounding

7 Jun

The term ‘loaded gun’ may well seem quite a strange one, but is quite an appropriate one for this particular kind of sounding. Clearly a loaded gun can “go off”, just like the atmosphere can under the right conditions and of which the below illustrates;

sounding 1

Atmospheric soundings, or tephigrams as they are also known, have many uses, but one of the important uses during the summer is to gauge potential instability within the atmosphere and to discover whether there is a risk of some heavy and thundery conditions possible from large cumulonimbus clouds.  In this instance, I use a forecast sounding for Exeter and I’ve highlighted two sections of the sounding which are of primary importance;

A = This area of the sounding is indicating that this portion of the atmosphere from approximately 6,000ft up to nearly 30,000ft is unstable.  When I say unstable I simply mean this area of the atmosphere is cable of allow large clouds to develop and air to rise and given the height at which clouds could well rise to (30,000ft) then these would be very large cumulonimbus (thunder clouds). However, for the clouds to properly develop the whole of the sounding needs to become unstable and there is an important part of this particular sounding that is stopping convection and the large clouds from developing…

B = Whilst this region of the sounding looks quite insignificant, it is in fact very important. You’ll notice on the image that near 4,00ft to 5,000ft the temperature (red line) increases slightly after decreasing from the surface. This is known as an inversion and whilst it looks insignificant this is prohibiting convection from occurring. Think of being in a room with the central heating on and all doors and windows are closed.  The heat from the radiators rises and will continue to rise until it reaches the ceiling and then can’t go any further, the heat is essentially ‘capped’ at the level of the ceiling, this is the same principle in this case. The warmth and humidity at surface levels is attempting to rise but is unable to rise past that level near 4,000ft to 5,000ft due to the inversion.

So, how do we get past this problem and allow the atmosphere to become completely unstable and for those big thunder clouds to develop?

Well there are a number of ways, but one of the primary ways is from the surface and that is with further warming through the afternoon. Additional warming of the surface will increase the temperatures and will then essentially override that inversion and once that is the case, the entire atmosphere then becomes unstable, as is evident on this forecast sounding for an hour or so later:

sounding 2

Note how the dashed line is now visible from approximately 3,000ft all the way up to beyond 30,000ft.  This is a particularly impressive atmospheric sounding in terms of the instability. Temperatures at the surface and humidity have increased enough to allow complete instability and that ‘capping’ near 5,000ft has been removed from the equation and all the heat and humidity within the lower layers of the atmosphere, that has built up, is now free to rise rapidly and significantly, in essence the “loaded gun” has “gone off” and more often than not, it would usually go with a ” bang” in the form of heavy and likely thundery showers.  I’ve also highlighted, at the top of the image, one variable known as CAPE (J). CAPE stands for Convective Available Potential Energy and is a measure of the potential instability within the atmosphere.  In essence the larger the value the more unstable the atmosphere is and in this instance a value of 992 is evident.  Anything over 1000 from a UK’s perspective is pretty impressive and does signal some possible significant convective activity. Of interest CAPE is derived from the area between the dashed line and the solid red (temperature) line on a forecasting sounding.

So if you ever see soundings like the above images posted on twitter or from other sources then think convection, heavy showers, cumulonimbus clouds and possible thunderstorms. The next 3 months are the prime time of year to experience significant convection across the UK that more often than not leads to some very lively weather.

Regards to all,

M.

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