Lightning of Panama
first appeared in Ocean Navigator magazine March / April 1992 Issue No. 45
A flash of lightning broke the calm. Moments later, thunder rumbled around me. Renaissance, my Southern Cross cutter drifted towards a doldrums storm. The name doldrums conjures images of clouds and calms for sailors making trans-equatorial passages, but as I discovered, as I slowly crossed the Gulf of Panama, the doldrums did not test my sailing or navigational skills; it tested my preparedness - for lightning.
The doldrums, also known as the Intertropical Convergence zone (ITCZ), is where the trade winds of the northern and southern hemispheres converge and cancel each other out. As the winds converge and rise they send warm, moist, tropical air into the cooler regions of the upper atmosphere and create areas of rain and thunderstorms. Satellite photos show the doldrums to be like a serpentine waistband of clouds worn by mother Earth near her equator. This waistband migrates seasonally and regionally. In some places and times it is north of the equator and in others, south. At sea level, the doldrums can be up to 150 miles wide. In order to reach the trade winds of the South Pacific, I had to pass through this windless barrier.
Since my departure from the Panama Canal a day and a half before, I had made good only 50 miles out of 800 on the way towards the Galapagos. I was nearly becalmed in wide band of the doldrums. Because the next reliable source of diesel was in Tahiti, 4,000 nautical miles away, I used the motor as little as possible. Ghosting along at one or two knots, I entertained myself by looking to the sea and watching the sharks, sea snakes, fish and dolphins which seem attracted by the small garden I was inadvertently growing on the boat's bottom. Looking to the sky, I entertained myself by watching the lightning on the horizon flash and pound like the fourth of July. The bright flashes outlined mountains on the Panamanian mainland which were otherwise obscured from this distance through the hazy, humid air.
After a day and a half of fortuitously avoiding the lightning storms, Renaissance slowly began to converge with an area of particularly intense electrical activity. I gauged my distance from the storm by counting the interval between flash and the sound of the thunder. The light from the flash, which travels across 186,000 statute miles every second arrives instantly. Sound, in comparison, travels slowly, a mere 0.20 miles every second or one mile in five seconds. I counted the seconds between the flash and the thunder and divided by five. The thunder lagged 25 seconds behind the flash: five miles and slowly closing.
The rain was heavy; the glassy sea hissed from the impact of huge rain drops. Intervals between flash and boom were now down to about 10 seconds, or two miles, and this meant the storm was too close for comfort. In the interest of preserving the boat's electronics, I took time out from bending over a bucket of laundry in the cockpit to go below and remove the antenna and power leads to the SSB, the SATNAV, the loran, the depth finder and even, for good measure, the stereo. A nearby lightning strike could damage sensitive electronic parts. A direct strike could blow them to bits.
As I unscrewed the antenna connection of each unit, electric shocks lightly burned my finger tips and made them twitch. At the same time, I noticed a popping, cracking and hissing sound coming from the mast. It was like the sound of expansion in a heating pipe, or the sound of a hot engine cooling down on a winter day.
The two occurrences were symptomatic of the same event. Electricity was accumulating at the masthead and discharging. Had it been nighttime, this discharge would have been visible as an eerie aura of electric blue: the fabled St. Elmo's fire, a phenomenon that seamen of yesteryear believed, ironically, to be a harbinger of good fortune. They believed that the worst of the storm had passed and now the corposant (from the Latin corpus sancti meaning "the body of the saint," and referring to the patron saint of the sailor, St. Elmo) had arrived and all would be well. Not always the case!
What causes lightning? In the atmosphere, free charges commute between atmosphere and ground all the time, but in some circumstances an imbalance develops. In my situation, the warm, moist tropical air rising to the upper levels of the atmosphere cools and the water vapor it carries condenses. This causes and electrical charge to be "squeezed off" the molecules. As a result, the lower and central parts of the clouds become negatively charged, while the region of the earth below the cloud becomes positively charged (opposite to the usual charge associated with earth). When the electrical potential between ground and cloud (or between clouds, or within a cloud) is great enough, electricity begins to arc.
When this event is analyzed with high speed photography, the photos show a tiny wisp of electricity emerging from the cloud as a progression of increasingly lengthy steps with a 30 to 50 microsecond pause between each step. This "stepped leader" paves the way to the earth. When it approaches the ground it meets with a rising arc near the surface and POW! The pathway has been paved and a one - inch column of electricity , carrying up to 1,000,000,000 volts at 54,000 F, replies as the return stroke to discharge the imbalance. As the high voltage flows along the column of air, invisible electricity becomes visible as the gasses in the air fluoresce. And the heated air expands faster than the speed of sound, thus producing the sonic boom we know as thunder.
If residual electrical imbalances remain, they may at this time take advantage of this nicely paved (ionized) pathway to send down the stepped leader's direct traveling (no staccato motion) cousin, the "dart leader," which evokes another return stroke. The exchange between dart leader and return stroke may recur up to 40 times (known as a multiple stroke flash). A multiple stroke flash occurs over and average interval of 0.2 second.
Okay, but what about that great big mast that looms so close to the black clouds above? Will that attract a strike? Yes and no. Lightning (electricity) takes the easiest path when it connects the sky to the Earth, so it is not going to go far out of its way to seek out a mast. If charge has built up and the mast is in the area of the strike, then yes, it will strike the mast first. Lightning likes to strike the highest, most pointed objects. However, if we consider the small height of the mast in proportion to the distance a lightning bolt would have to deviate to reach it, a mast is quite small and insignificant.
Back on Renaissance, the thunder became deafening. The dolphins which had earlier in the day been jumping and playing near the boat had long since disappeared. The storm was upon me.
Am I safe out here? Have we done everything we can to protect myself on my little fiberglass boat? All the metal (stainless steel) standing rigging on Renaissance is connected together below decks with battery cable. (Chapman's recommends wire with a conductivity to number eight gauge non-insulated wire.) All the seacocks are bonded together with copper wire. All of the above is bonded together and meets at a porous and electrically conductive thru-hull grounding plate. What good is that? One benefit is that as electrical charge accumulates, the bonding system will bleed off some of this building charge - St. Elmo's fire - from the mast and therefore keep at bay a threat against an electrical overload.
The second benefit of a bonded rig is to take advantage of the skin effect. The skin effect is the tendency of high voltage electricity to pass to ground along the outside of a conductive material instead of through it. The skin effect is brilliantly demonstrated at the Boston Museum of Science's electricity exhibit. Next to a two story electron accumulator, called a Van de Graff generator, a museum staff member enters an all metal bird cage analogous to the stays on my sailboat. The cage has been, like on my boat, well grounded. Inside the bird cage, the staff member has full control of the generator and the cage. After charging the generator up to one million volts, the staff member raises the cage until the cage is close enough to the generator so that an arc leaps between generator and cage. The audience jumps at the big noise that results, but the staff member is completely safe within the cage. The electricity has traveled over the outside of the cage and along its merry way to ground.
My mast and grounded rigging serve like the bird cage or a lightning rod to conduct a lightning bolt safely to the sea ( ground). Steel - hulled vessels offer the greatest safety. Below decks, you are within a shell of steel around which the lightning flows.
Captain James T. Cook while in the Pacific in 1768 on his ship the Endeavour, protected his vessel during a lightning storm by hoisting a copper chain in the rigging with one end overboard. This system was a crude lightning rod in the wooden rigging. A bolt hit his vessel and the copper chain guided the lightning into the sea. A companion ship to the Endeavour, less than two tenths of a mile away, without this protection, had her mast split by a strike.
The chain is a good backup; a bonded rig is a step better but, realistically, no amount of protection can save you if the strike is vicious enough. It's just one of those forces of nature that remind us of our insignificance.
I stood in the cockpit counting the flash to boom interval when all of a sudden there was a blinding flash and ripping boom which, like and invisible hand, delivered a blow that knocked me off my feet - I had been hit by the shock wave. I could smell burnt air.
I pulled myself together and scanned for damage. I looked out the companionway and scanned me for damage. I forgot to isolate the electronic speedometer and its screen was blank There was no fire, nothing charred. A giant fist slammed a mighty blow, but stopped shy of our destruction. Were we hit? Maybe, maybe not. The important thing is that we were prepared, but since nature will have its way, we were lucky, too.
The grand strike came and went without encore. The storm's vengeance seemed to be satisfied. With unprecedented suddenness, the storm dissipated. The flash was our grand finale, the 1812 overture in our departure from the doldrums. Soon thereafter the wind began to freshen. We had reached the southern hemisphere trade winds.
first appeared in Ocean Navigator magazine March / April 1992 Issue No. 45
modified Tuesday, August 1, 2006 1:55 AM