On last nights developments a thundery low from Spain moved north with the result of producing thunderstorms over us

But the BBC explanation on their website for the Spanish Plume and on their weather bulletins prior to the thunderstorms over us was of Atlantic air from the west overriding hot air over us from the south with the collision of air masses sparking storms

So what actually happened ?

And just as in winter you can get severe snow events with a collision of air masses - Atlantic vs cold pool like January 1982 or snow directly from a cold pool with no collision of air masses like Jan 1987 or Feb 1991

Which is the more common method to produce widespread storms over us

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The answer in this kind of situation is both.

We have a situation where high pressure has receded to our East and low pressure sits to our West.

In the absence of the two forces (Coriolanus force and atmospheric angular momentum) that make air masses spin around in eddies in the mid latitudes, air would always travel from high pressure to low pressure in a straight line (barotropic flow). However, this doesn't happen outside the tropics and instead we get so called "baroclinic flow", ie the tendency of wind if unimpeded to flow around high and low pressures parallel to isobars.

At high altitudes the flow of air around a depression is almost completely baroclinic, meaning it generally runs parallel to isobars. So high level winds from that depression to our west are travelling in a South Westerly direction in line with isobars.

At lower levels the air encounters frictional drag from the surface so it is less baroclinic more barotropic. So low level air has been moving from a more Southerly direction, spiralling towards the low pressure centre.

In a Spanish plume like this weekend' see therefore get a relatively stable (because cooled from below as it travels North), but warm and moist lower air mass coming up from the South, and a cooler, drier Westerly flow from the Atlantic aloft. This creates instability and convection in the atmosphere not at low levels but starting at mid levels. Hence we get high based storm clouds, cloud-cloud lightning (often closer to find the reverse ionisation in a neighbouring cloud than the ground), spectacular light shows as a result but often rain evaporating before it hits ground.

I say "both" because the upper air instability itself tends to develop into gently cyclonic flow, hence the "low pressure from Spain".

Most memorable Spanish plumes I've experienced we're first week of June 1996 (remember that one?) and in Western France in July 2003, which triggered a huge gust front and tornadoes.

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Thank you Tim.  Although I didn't understand all of what you wrote, it did make sense and confirms what I thought, that the storms must be higher than usual, as we could see the lightning from a long way off.

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Yea I think that explains things pretty well Tim.

Here's a copy of the blog I have posted on my website and to the local press for my own local populous of the events over the last few days.

"Over last week the area became bathed in very warm air coming North all the way from Spain. At the same time Low pressure and cooler air in the Eastern Atlantic was pushing East against this hot feed of air and creating instability within the atmosphere. This is a feature known as a 'Spanish Plume' which is a feature of Summer weather which occurs more commonly than you might think here in the UK. More often than not the resultant thunderstorms quickly displace the hot air and make things much fresher once the storm passes. What is more unusual this time round was that no displacement of hot air occurred hence why two consecutive nights saw similar thunderstorm events in terms of timing i.e when most of us were trying to sleep.

As already mentioned the product of all the energy from these two air masses coming together over the area late last week and the weekend resulted in some spectacular skyscapes with a couple of nights of quite strong thunderstorms when there was an unusually high amount of frequent lightning and one or two 'too close for comfort' type lightning strikes especially in the small hours of Saturday morning. This was also accompanied by heavy rainfall locally and Radstock saw 35mm of rain from the event on Saturday which is approaching 1 and a half inches.

While this may seem a very exciting and spectacular pyrotechnic weather type for weather enthusiasts I am well aware of many people who are completely petrified of thunderstorms, a conditions known as 'Astraphobia' and it may surprise you I suffer from this slightly too. The sheer awesome power of lightning and the spectacular nature in which it displays itself is one aspect we can all witness and hear in wonderment or fear but we must also respect it for it's destructive and damaging capabilities. However, despite the listed 'do's and don'ts' listed below the chance of lightning striking any particular structure is small even in the severest storms but there is a risk and for the general public there are several rule of thumb do's and don'ts when it comes to safety in thunderstorms and I have highlighted these below.

When you here thunder you are pretty much in the range of the area in which lightning strikes could occur so take heed of the following statements.

1. Do Not shelter under tall trees as you can become the catalyst that attracts lightning.
2. Do not use umbrellas as the tips on on an upheld one can be an attraction to lightning.
3. Avoid areas where you will be the highest object such as open fields and seek shelter under a low lying hedge and never a tree or where overhead wires are above you.
4. Suspend field sporting activities in a thunderstorm such as football and especially golf where golf clubs and people can be an attractive target for lightning.
5. Stay off open lakes in a boat and where water courses are nearby such as a beach
5. In the home disconnect electrical equipment such as the TV in a storm and never use the telephone as lightning can travel down the leads from outside.
6. When indoors you are relatively safe but should be advised against viewing a storm from an open window or porch.

These simple rules are not meant to sound alarmist when thunderstorms are around and are more a note of caution but the fact is lightning can and has killed someone before in all of the situations above so these basic safety features should be taken seriously.

So with the children breaking up for the Summer Holidays this week and many of us going on Summer outings and holidays you will probably be asking is the incidence of thunderstorms likely to be repeated again through the remainder of Summer 2014. Well my own feelings are 'Yes' as the current pattern of weather is not looking like changing anytime soon with a lot of sunshine and very warm weather likely to remain close by for some time to come and off and on well into September. So with high temperatures and humidity there will always be the risk of a heat of the day storm as well as the risk of a repeat of the 'Spanish Plume' type weather such as we experienced late last week. That said we should all enoy plenty of summer warmth over the next 6-8 weeks and perhaps witness some more resultant thunderstorms.

 

 

 

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Thank you Martin for the blog and post. As a member of your populace being up the hill and round the corner ,,, I shall  read it again in The Guardian...

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