Windsurfing in a Thunderstorm

you get really beautiful blue glow at the tip, and a nice harmonic hum. we thought we were imagining it until the lightning struck a few kms away and the hum and glow stopped. I presume the masts were just one of many candidate earthing points

Well the car is insulated from the surrounding ground by rubber tyres so (in theory) the charge state in the ground should induce an opposite charge state in the car and that will be the same as the outside of the clouds where the lightening comes from, ie negative. One would expect the ion path to initiate at points of high positive charge on the ground. I understand pointy features connected to the ground fill the bill. Cars are not like that.

Then there's the Faraday cage. Charges travel on the outside of conductors (because like charges repel) so if you have a cage that is a conductor the current will zap around the outer surface without affecting the core. I saw a bloke sitting in a cage like that while it was zapped with an artificial lightening bolt at the Munich Science Museum. It was deafening. The air was filled with ozone and whisps of smoke and the bloke in the white coat said a few words in the reassuring tone of an airline pilot with an engine failure to show he wasn't dead. It was a remarkable demonstration.

There may be more to it but I will continue to feel safer in a car than most other places in a lightening storm.

Hard to know if the car is safer than a building... A decent lightning strike could easily blow a hole in the steel roof of a car as Matthew said. The simulated ones you see on shows like topgear are usually pissy whispy things that scare more than damage.

A friend of mine used to repair TVs and was visiting a customer who's house got hit by lightning. It took out most of the appliances, but more worrying was the bathroom mirror that had exploded with shrapnel embedded into the bricks on the other side of the room. The chased-in copper water pipes had also been blown out of the walls in other parts of the house. Doesnt sound too safe to me...

As for tyres protecting you... bah. They do nothing if they are wet, or if there is a blade of grass or anything getting near the car. Or if the air is humid. The charge just has to bleed out somewhere- not particularly fast, and that can be through the air.

Normally the clouds are negatively charged at the base and positive at the top. But they can be the other way around. A negatively charged cloud will repel electrons from the ground making it positively charged in the area under the cloud. This also sets up a big voltage gradient. Sharp conductive objects in this area are then exposed to a very high intensity electric field (due to the small radius), which past a certain point causes the air to ionise due to the dielectric stress on the air molecules, and it becomes conductive in that area- the result is a brush discharge- St Elmo's Fire (car aerial, roof racks...). Similar to what you see coming off the top of a tesla coil (sparks radiating out into the air).

If the general voltage gradient between the cloud and ground is high enough in the area, the discharge can grow upwards to the cloud, drawing electrons from the surrounding air to feed the arc channel and keep the air ionised. This is the leader strike. When it reaches the cloud (which has an abundance of freely available excess charge) the main strike occurs suddenly, dumping a portion of the cloud's charge through the conductive path formed by the leader stroke. It becomes more conductive the more the air is ionised so its an avalanche effect. This is the main strike- the bright noisy bang. Several discharge points can be initiated once the main strike starts, since the potential of the cloud is brought closer to the ground and other forming leader strikes get a shortcut to the cloud's energy through the main stroke. Thats when you see the main lightning strike fork out. Somtimes those other leader strikes are too late, and the bulk of the energy has already been drained before they get a chance to join in. The initial process can occur within 1/1000th of a second so you cant see the individual stages before the strike occurs. The strike can then last a few milliseconds.

scary stuff.

I think about Matt's comment when Matt Holder and I were sitting in the carpark at brighton when a massive thunderstorm kicked off. A rather chunky lightning bolt pounded the tree behind our cars maybe 20m away. Hmmm Mitsubishi isnt known for thick body steel. Surrounded by cranes, ob city, and the 5 level apartments 100m away. Sharp things are more likely to get hit, but the path the lightning takes is constrained by optimal conditions that are quite localised (wind, rain etc).

As for feeling a jolt when lightning strikes nearby, I've had that happen before... Its not fun. I suspect that this is the sudden equalising of charge causing the voltage gradient under the cloud to change rapidly. Since the sail and mast and all of the air in between has its own slowly equalised charge leading up to the strike, the rig charge also needs to be equalised, and it does so through a jolt of current travelling partly through our feet and into the boom through our arms. Having an Aluminium mast increases the peak current.

I'd be pretty sure you're safer sitting in a car during a thunderstorm than you are sitting in a car zooming down the road at 100 kph.