Lightning

[capnsensible]

Today I took the family out for a run around Portsmouth harbour...more than anything to check everything was OK prior to a longer trip next week.

I always check the wind and tides before i leave port (obviously) and I knew rain was possible with potential for short sunny spells. I admit i didn't pay particular attention to the risk of thunderstorms but found myself smack in the middle of one with lightning to boot. I suddenly felt a bit foolhardy.

I wondered if others pay particular attention to the chance of thunderstorms? Would you stay in port?

The wind was light and the rain, torrential as it was, didn't bother me but I sent the family down below and only half jokingly told the wife she would have to steer if I got fried.

Some great pictures here:

13 great pictures of lightning above Portsmouth and south Hampshire in the overnight thunderstorm

 

[AntarcticPilot]

In fifty years or so, I can't think of an occasion when I have been out at sea in a thunderstorm, though I have endured/enjoyed a number while in harbour. Would I set out when a storm was in prospect? Almost certainly not, though it is only in recent years that forecasts have been that good.

Yes - I've only been out once in a storm, and that was more than 50 years ago! But you're also right about the difficulty of predicting thunderstorms; the real problem is that while you can forecast conditions where storms are likely, individual storms are quite small in extent - often only a few miles across - and so you can't say "a storm will happen here", only "Storms are likely in this area".

Not at sea, but I remember one incident that impressed on me the local nature of thunderstorms. We were driving from Albuquerque airport to Los Alamos, and as we left Santa Fe, we could see a massive storm over the Pajarito Plateau on which Los Alamos is situated. The storm clouds towered high into the sky, but it was very clearly confined to a small area and had dissipated before we reached it. The topography and clear air of the region gave me a lasting impression of a powerful but very local phenomenon.

 

[RobbieW]

A useful feature of GRIBs is to look at the CAPE data as well as other more usual data...
https://study.com/academy/lesson/co...energy-cape-definition-use-in-forecasting.htm

 

[Giblets]

The weather was not what was forecast, we moved from a pontoon to back onto our swinging mooring in Portsmouth hbr and 2 minutes after we cast off we were drenched, but it was the lightening which was the fun bit....... one of the stikes was i guess less than a mile away. All part of the great British summer.

Errm, the Met Office were putting out yellow thunderstorm warnings for the whole of southern England from Friday!

 

[AntarcticPilot]

I think the major danger of being out in weather when thunderstorms are forecast is that you will be exposed to strong winds that change rapidly in direction. Thunderstorms are small, intense convection cells, so the wind round them will blow towards the thunderstorm, and may be very strong locally. So, the potential for being taken aback or an accidental gybe is greatly enhanced in such conditions.

 

[Graham376]

Alas that is not true, it heads for a conductor that has built up to the highest opposite voltage and follows the path of least resistance to it. It also heads towards the nearest point where is can find an isolated conductor. So earthing the mast head to avoid a build up of exccess charge reduces the chances of being struck, that is why you often have near misses. It's only once lightning has struck an object that it looks for a direct path to earth. Most old times like myself store any plastic cased electronics in alloy foil, or metal box.

My information came from a physics professor who was well in to the subject and his advice was not to ground the mast unless fitting a dissipator. I think I'll stick to his advice, having had strikes into the water near the boat but not being hit - yet .

 

[oldharry]

Its cold comfort, but lightning is totally unpredictable in how it will behave: lightning conductors on buildings are just that. They are designed to ensure lightning actually hits that specific point, so it can be carried safely to earth, thus avoiding damage t other parts of the structure. On a boat the one thing you so NOT want is to attract lightning! After that its largely guesswork.

The upward leaders which 'reach out' to connect with the downward leader are known to form more easily from spikey or pointed objects, so most lightning protection is designed to discourage their formation, by providing multiple points to diffuse the ground charge, or a rounded surface to discourage the formation of the leader. The lost likely prtection seems to be to climb the mast and put a welly over the top of it!

Providing a 'safe' route to earth for the discharge by linking the rigging to the sea by the anchor as suggested above is pretty dodgy too. 1. how will your forestay stand up to many thousands of Amps? Cable to carry that current safely would be many times thicker, so the likelihood is your forestay will be vapourised.....

Also, studies have shown that lightning will go the most direct way to ground, so down the mast, jump the couple of metres to the metallic mass of the keel is likely with a heavy discharge. (Considering the charge has already leaped perhaps a couple of miles or more - another couple of metres and through a skin of a few Cms of GRP is no obstacle to it!)

Robbie W is right. A big lightning bolt 150m away can still fry electronics by EMF, even if not connected up. One thing that has been proved to work is to put them all in the oven. This provides Faraday cage and saves them. At least, thats the theory. On a previous lightning discussion on the Forum some years ago, someone reported having had quite severe shocks from the guard rails and pushpit during a nearby strike that had missed the boat.

But this is the problem, its nearly all theoretical. The topic comes up quite regularly on the Forum, and I reckon I have followed nearly every one of them.... There have been various tales of woe from people unfortunate enough to have been aboard a boat that was struck, each with a lesson to be learned. One man had his electrics fried, including an old style Seafarer echo sounder. he sent it back to Seafarer, asking if it was repairable. They duly replaced the innards and returned it with a note 'we strongly suggest you do NOT connect the unit to the mains again...!' Another man was 30 miles in to the delivery trip of his brand new Westerley when it was struck, caught fire and sank..... Thankfully the RNLI got there in time to save the crew.

The cumulative forum advice seems to be: 1 Put a welly over the top of the mast. 2 Wear wellies to insulate yourself. 3 Do not touch metal parts of the boat. 4 Put all the electronics in the oven, 5 Make sure you have a supply of wooden plugs to replace blown out skin fittings, and 6 wear a condom! (prevents leaders forming where it might hurt it was suggested....!). But above all, dont set off when thunder is in the forecast. Cumulative wisdom on the Forum has been that there really nothing you can do to that is effective in preventing lightning strikes.

 

[capnsensible]

Having been in a number of electrical storms at sea, I can say that crossing ones fingers has worked for me so far.....?

 

[Praxinoscope]

We had a similar situation last Friday (23rd) we had planned along with another boat to sail down to one of our local 'picnic' beaches,... but the Met office forecast was consistently showing thunderstorms centred over the area between 13.00 and 15.00,
It was disappointing but we all agreed sitting around in a thunderstorm was not our idea of a pleasant days sailing and picnic so we gave up and went home.
I should add that if we had decided to go we could not get back to our mooring for about 10 hours when there was sufficient water in the harbour.
For the rest of the day I was a bit like a bear with sore head, partly due to missing what we were hoping was a good day sailing, but more by the total absence of any form of thunderstorm or even a spot of rain for the entire day. All we got was a lovely sunny day with winds force 3 - 4.

 

[JumbleDuck]

Providing a 'safe' route to earth for the discharge by linking the rigging to the sea by the anchor as suggested above is pretty dodgy too. 1. how will your forestay stand up to many thousands of Amps? Cable to carry that current safely would be many times thicker, so the likelihood is your forestay will be vapourised.....

On the other hand, a lightning bolt usually only last for a fifth of a second or so, and it's possible to carry very high transient currents in suprisingly thin wires. In a lab I worked in we used to put 30 kA through standard coax ... briefly. For all that, I wouldn't give much for the hopes of the forestay.

Robbie W is right. A big lightning bolt 150m away can still fry electronics by EMF, even if not connected up. One thing that has been proved to work is to put them all in the oven. This provides Faraday cage and saves them. At least, thats the theory.

An electromagnetic pedant writes: an oven is not a Faraday cage[1] and a Faraday cage doesn't protect against magnetic pulses anyway. That's not to say an oven is a bad idea, but really just to provide a path for induced currents around the stuff in it.

[1] A continuous conducting surface into the interior of which external electrical fields, including the electrical part of electromagnetic fields, cannot penetrate.

 

[FairweatherDave]

I've written this story on here before. No electrics to protect, just human beings in a Wayfarer. We were sailing on a lake in the SW of France and saw the weather rapidly changing and simply headed to the side of the lake through the reeds and sat about 30m from the boat. Monumental bang. A direct strike to our mast. The shockwave from 30m was what I imagine a bomb blast would be like. I am totally convinced we would have all been dead had we stayed on the boat ,,,,,and made a very sad Brits on holiday tragic accident story. Thankfully we were fine and so was the Wayfarer apart from the burgee that blew off. It probably is pot luck and not much you can do if caught out at sea but if you can avoid it do. A couple of years later in the same part of France the beach lifeguards ordered everyone out of the sea and off the beach to shelter ....... obviously they are more used to the risks, for a reason.

 

[capnsensible]

?

 

[GrahamD]

I have the app Lightning Alarm on my iphone and iPad which shows where the the lightning is occurring, in fairly close to real time. It will also monitor in the background and give an alert if lightning occurs within 30km of your location.

Having been caught in massive and unexpected storm in Greece I keep a very good eye on the app if I think that lightning is around, and plan accordingly.

 

[AntarcticPilot]

I have the app Lightning Alarm on my iphone and iPad which shows where the the lightning is occurring, in fairly close to real time. It will also monitor in the background and give an alert if lightning occurs within 30km of your location.

Having been caught in massive and unexpected storm in Greece I keep a very good eye on the app if I think that lightning is around, and plan accordingly.

Good thought. I have the Blitzortnung app on my phone, which does much the same. It's useful to look at the history, as it gives a clear picture of how storms are moving, too.

 

[Rappey]

From NOAA,s take it seems you are in just as much danger from lightning not from above

Lightning doesn’t strike the ocean as much as land, but when it does,it spreads out over the water, which acts as a conductor. It can hit boats that are nearby, and electrocute fish that are near the surface.

 

[johnalison]

I think the major danger of being out in weather when thunderstorms are forecast is that you will be exposed to strong winds that change rapidly in direction. Thunderstorms are small, intense convection cells, so the wind round them will blow towards the thunderstorm, and may be very strong locally. So, the potential for being taken aback or an accidental gybe is greatly enhanced in such conditions.

There seem to be two kinds of thunderstorms, those with strong winds and those without. I remember an article in YM that explained the sort of frontal conditions that will give rise to wind but I don't remember the details. We were in our last boat anchored off a lee shore (mud) when we heard a forecast of 60knot winds, when it was too late on the tide to seek shelter. In the event, the wind did arrive and was measured locally at 55kn+, which was quite exciting at the time and the Essex mud held out well.

 

[JumbleDuck]

Lightning doesn’t strike the ocean as much as land, but when it does,it spreads out over the water, which acts as a conductor. It can hit boats that are nearby, and electrocute fish that are near the surface.

Standard 1st year electromagnetism question asks students to work out the potential difference as a function of radius from a tree struck by lightning, and come up with a safety recommendation. Which is "If you must be near a tree, stand facing it or away from it, so your legs are equidistant from the current feed"

 

[JumbleDuck]

There seem to be two kinds of thunderstorms, those with strong winds and those without.

There are indeed two sorts, convective/thermal and frontal. Convective ones are what you get when a summer cumulus goes mad and turns into a cumulo-nimbus with tremendous vertical winds in the upper section and consequently tremendous inflow around the base. In a frontal thunderstorm the vertical movement is started off by a cold front wedging in under and lifting warmer air up. That's enough to get unstable convection going, but not as viciously as in a cu-nim.

Good descriptions of both, including diagrams, at Thunderstorm

 

[Rappey]

Do steel ships often get hit by lightning?
I can find quite a few accounts of pleasure boats being struck but not one article on a boat with lighting conductors getting hit.
I quite like TNLI,s explanation about meta v grp and static voltages

 

[AntarcticPilot]

There are indeed two sorts, convective/thermal and frontal. Convective ones are what you get when a summer cumulus goes mad and turns into a cumulo-nimbus with tremendous vertical winds in the upper section and consequently tremendous inflow around the base. In a frontal thunderstorm the vertical movement is started off by a cold front wedging in under and lifting warmer air up. That's enough to get unstable convection going, but not as viciously as in a cu-nim.

Good descriptions of both, including diagrams, at Thunderstorm

Thanks.