What can act as a thunderstorm "trigger"?

[Nick L]

This is a follow up to my guide regarding CAPE and LI, which can be seen here:

 

http://forum.netweather.tv/topic/80249-what-is-capeli/

 

As I alluded to previously, CAPE and LI aren't the only factors needed to be favourable in order for thunderstorms to develop. We also need a source of lift. There are numerous methods, so I will go through them one-by-one, assuming an unstable atmosphere in each case.

 

Surface Heating

 

This is quite a simple mechanism. During the daytime and mostly sunny conditions, the sun warms the ground. This then heats the air immediately above the surface, and so becomes less dense than the air above it (warm air is less dense than cool air), which causes it to rise. As the daytime heating builds, this process repeats, with air at the surface constantly being heated and thus being enabled to rise. If we have favourable LI, this initial nudge means that a heated air parcel will accelerate vertically upwards until it is no longer warmer than its surroundings. As it cools it is liable to condense into clouds. This acts as a "fuel" for convection, and can lead to heavy showers and thunderstorms. Pre-existing cloud cover is often a much hated cause of "bust" days for UK storm enthusiasts as it limits surface heating.

 

Convergence Zone

 

A convergence zone is an area where opposing wind directions "clash". When this happens, through a process known as mass continuity, the air is forced upwards. Another fairly simple mechanism, and quite a common cause of thunderstorms in the UK.

 

Frontal Lift

 

Weather fronts (both cold and warm) are also a common source of lift. Let's first look at a cold front, with a cross section shown below:

 

 

The cold, dense air behind the cold front causes the warm, moist air ahead of it to rise. As this moist air rises, it can cool and condense into convective clouds. We see the opposite mechanism but the same result with a warm front:

 

 

The warm air behind the warm front is forced to rise over the cold air ahead of it, which provides potential for convective development.

 

Dryline

 

This is a phenomenon most often seen in the plains of the US during storm season, but we'll go through it anyway. First of all, what is a dryline? Technically it's not a front but behaves very much like one. It is a boundary separating hot, dry air (such as the desert air of the US southwest) and warm, moist air (such as a moist flow off the Gulf of Mexico). There can be quite extraordinary differences in moisture either side of the dry line, as can be shown below with the dew point values from two stations just 10s of miles from each other:

 

 

So how can this enhance convection? Well, it's not a frontal boundary technically, this is due to the fact that the density difference either side is minimal. The density difference that would be evident with the temperature differential is offset by the quite marked moisture gradient. Moist air is less dense than dry air. So if density differences don't play a part, what causes the dryline to cause some quite severe storms at times? We need to look at the surface wind directions. You can see by the wind barbs in the above image that we have a south-westerly wind to the west of the dryline, and a south-easterly wind to the east. This creates a convergence zone (which we have explained above). This, combined with the presence of warm, moisture-rich air to the east, can trigger quite rapid convection and severe storms in the US. It's not a phenomenon we usually see in the UK as we don't get such marked moisture differences between two air masses.

 

Sea Breeze

 

If you're not familiar with the sea breeze, our very own John Holmes has put together a guide on the subject:

 

http://forum.netweather.tv/topic/38567-the-mechanism-behind-a-sea-breeze/

 

They can initiate convection in two ways. Firstly, over a peninsula such as Florida (or even Cornwall!), sea breezes from opposite sides can meet in the middle of the land mass, causing a convergence zone. Here is one such example from Florida:

 

 

As well as this, the very nature of the sea breeze is down to the fact it is a density current - with the cool, dense air from the sea under-riding the warmer air over the land. This can initiate convection much in the same way as fronts do, and the sea air provides plenty of moisture for any thunderstorms.

 

Terrain

 

The final common trigger for convection is due to orography. Air can be forced up over hills and mountains, condensing and causing showers/storms. As the air descends down the other side of the elevated terrain, it sinks back to the surface, causing a rain shadow on the lee side of hills and mountains.

 

Well, I hope you've made it this far! That covers most of the main triggers, and in the almost certain event of me making a mistake in here, please point it out

Edited July 18, 2014 by Nick L
Correction in dryline section: "Moist air is less dense than dry air"

[Chris K]

Fantastic guide. Very useful with lots of detail that was easy to understand.

Thank you

[Nick L]

Cheers guys

[Lauren]

Thanks Nick. It certainly puts into perspective how much more than good CAPE and LI is needed to light a storm up and explains why we are often frustrated when everything looks good on paper.

Thanks Nick. It certainly puts into perspective how much more than good CAPE and LI is needed to light a storm up and explains why we are often frustrated when everything looks good on paper.

 To add, I'd love to see more of these guides as they are easy to understand compared to many other guides.

[Nick L]

Thanks Nick. It certainly puts into perspective how much more than good CAPE and LI is needed to light a storm up and explains why we are often frustrated when everything looks good on paper.

 To add, I'd love to see more of these guides as they are easy to understand compared to many other guides.

 

Well if there are any topics people want to learn more about I'll do my best to accommodate any requests! 

[knocker]

If I remember correctly Nick the A30 was always regarded as the the convection zone for the meeting of sea breezes down here in Cornwall.

[Nick L]

If I remember correctly Nick the A30 was always regarded as the the convection zone for the meeting of sea breezes down here in Cornwall.

 

I remember in a lecture a couple of years ago being shown a satellite picture illustrating the effect brilliantly! Shame the storms aren't as potent as in Florida though

[knocker]

I remember in a lecture a couple of years ago being shown a satellite picture illustrating the effect brilliantly! Shame the storms aren't as potent as in Florida though

 

The last convection event at Boscastle will do to be going on with thanks very much.

[Nick L]

Solar winds even cosmic rays? Thunderstorms sure are fussy at times :)It is said that lightning never strikes twice. But scientists have long been puzzled as to how lightning even strikes at all.The laws of physics mean it should require a huge electrical current – much bigger than that in a storm cloud – for a lightning bolt to reach the earth.Now British scientists believe they have the answer, and it may enable forecasters to predict when a lightning storm is due.Scientists agree that lightning is caused by ice particles colliding within a storm cloud to create an electric charge.But they have been divided on how that charge creates a bolt big enough to reach the ground.Now researchers at Reading University have suggested that a stream of high-energy particles pumped out by the sun, known as a solar wind, creates a ‘pathway’ needed for lightning to travel.They say that the solar wind – in which the particles are propelled from the sun’s atmosphere at around a million miles per hour – charges the air, meaning a lesser current is needed for a lightning bolt to strike.Cosmic rays – another source of charged particles from exploding stars on the other side of the Universe – are thought to be another trigger for lightning.The results could prove useful for weather forecasters, since solar winds synchronise with the rotation of the sun, sweeping into the Earth’s atmosphere at regular intervals.As these streams are also tracked by Nasa, it offers the potential for predicting the severity of dangerous storms weeks in advance.Dr Chris Scott, of Reading University’s department of meteorology, said: ‘Getting a lightning bolt to jump through the air requires a much bigger charge than that in stormclouds. Laboratory tests show that it would require about 150 kilovolts per metre (kV/m) to generate lightning, but you would expect to generally see no more than 30 kV/m in a stormcloud.http://www.dailymail.co.uk/sciencetech/article-2628758/Unlocked-secret-lightning-strikes-Scientists-reveal-high-energy-particles-pumped-sun-act-pathway-bolts-hit-ground.html#ixzz31n0oKkPNCertainly more bits of the jigsaw to add - a phenomena that still baffles both amateurs and scientists alike.

 

Chris Scott is my dissertation supervisor

[Biggin]

Top post Nick. Cheers for that. Perhaps you could do one on skew t  diagrams, although they dont  appear that easy to simplify. i.e I still dont understand them Even after reading some explanation

[Nick L]

Top post Nick. Cheers for that. Perhaps you could do one on skew t  diagrams, although they dont  appear that easy to simplify. i.e I still dont understand them Even after reading some explanation

 

I can do a Skew-T tutorial That will take a while though and I'll have to leave it until after exams. Give me a month or so

[Biggin]

I can do a Skew-T tutorial That will take a while though and I'll have to leave it until after exams. Give me a month or so

Please Nick that would be great.  I have found JHs simple guide here http://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

[Nick L]

Please Nick that would be great.  I have found JHs simple guide here http://forum.netweather.tv/topic/16002-a-simple-guide-to-understanding-skew-t-diagrams/

 

Oh of course, JH did one! Well if there is still demand for me to do another one I'll happily put it on my to-do list

[knocker]

Oh of course, JH did one! Well if there is still demand for me to do another one I'll happily put it on my to-do list

 

To be honest Nick the guide that John did is excellent and I would have thought perfectly adequate. No reflection on your good self of course.

[Nick L]

To be honest Nick the guide that John did is excellent and I would have thought perfectly adequate. No reflection on your good self of course.

 

Yeah I think John's guide is great. A while back there was a bit of an issue with not all the images working, but seeing as though they are there doesn't seem to be a need to

[Greeny]

Yeah I think John's guide is great. A while back there was a bit of an issue with not all the images working, but seeing as though they are there doesn't seem to be a need to

 

I wouldn't think it a bad thing to make another tutorial but told in a different way (obviously the same drawn conclusions) cause everyone is different and may need a different approach to it to understand it better. Even though it seems a lot clearer now it did take me a while to get to grips with it. Perhaps it's the technicality of it.

[knocker]

Regarding sea breeze the Florida panhandle sea-breeze boundary showing up well on Visible satellite imagery

 

 

[chrisbell-nottheweatherman]

Bit late I know, but, in the description accompanying the diagram showing cooler, drier air undercutting warm moist air, shouldn't "warm front" be "cold front"? 

[Nick L]

Bit late I know, but, in the description accompanying the diagram showing cooler, drier air undercutting warm moist air, shouldn't "warm front" be "cold front"? 

 

Indeed it should! Edit made, thanks