Getting a charge out of thunderstorms

Squally weather in the Kimberley, NW Australia.
Looking over marina in Sabah, Malaysian Borneo, at a thunderstorm building over the South China Sea.
Looking over marina in Sabah, Malaysian Borneo, at a thunderstorm building over the South China Sea.

When I was seven, during a family vacation in Maine, we got caught out in a summer thunderstorm. We were parked beside a pond, and I remember my father telling my siblings and me not to worry as long as we stayed inside our car. That’s my first memory of a thunderstorm; in the time since, there have been many more. In a year, the continental U.S. sees an average of 25 million lightning strikes. “Don’t stand out in an open field in a thunderstorm.” “Don’t be the tallest object when lightning is around.” “Every year, people get struck on golf courses.”

Similar advice is given to boaters: “Don’t be the tallest thing around.” The greatest concentration of boats is near land, where most thunderstorms happen. With a sailboat in a marina or at anchor, it usually means cozying up to a boat with a taller mast so they’ll get hit first.

But what if you are caught on the open sea? Nothing else around to draw that dangerous bolt? At moderate boat speed, it’s hard to move away from a storm, and experts say you can’t prevent a lightning strike if you are in its vicinity; you can only protect yourself from damage. It’s not that lightning seeks out a sailboat mast or a powerboat; it takes the shortest route from the bottom of a thundercloud to the ground underneath, and anything that stands up shortens the distance.

Thunder and lightning are the audible and visible effects of cloud activity in the atmosphere. Clouds are formed from water. The simplest cloud forms when warm moist air rises from the planet (either earth or water) as the surface heats up and reaches a height where the atmosphere is cool enough to cause it to condense into water droplets. Morning is a good time for this to happen, after the sun rises. If conditions are right and the cloud keeps building, eventually the water may become heavy enough to fall back out of the cloud as rain. If there’s no underlying condition causing the cloud buildup, after it loses enough moisture, the cloud can dissipate, as we see in its simplest form in sun showers. But if the atmosphere is unstable and the buildup continues, the top of the cloud may grow, reaching a height where the air is cold enough to change the state of the water droplets.

Between -15 and -25 degrees Celsius, supercooled droplets, ice crystals and soft hail can form. Movement of these particles within the cloud causes them to rub against each other and become electrically charged. If the charge continues to build up, a thunder cloud forms, and the cloud rises high enough to hit the stratosphere where it can’t rise any higher and starts to spread out, forming the classic anvil shape, which we recognize as signaling a thunderstorm. At the base of the cloud, the charge accumulates until it has enough strength to jump to the closest object, which can also carry a charge. If you have heard that lightning also goes from the ground up, this is technically true because when the “step leader” from the cloud approaches the ground, streamers of opposite charge flow up, and when one or more of them meet the leader, the connection is complete and the lightning channel discharges.

Squally weather in the Kimberley, NW Australia.
Squally weather in the Kimberley, NW Australia.

Open ocean thunderstorms are most likely to occur in the tropics where the water is warm, and as a cruising sailor, I have been around numerous thunderstorms. While land thunderstorms are most common in the afternoon, out to sea they generally happen at night, when the air cools but the water stays constant. One of my most memorable thunderstorms came after Tom and I dropped anchor off an unnamed reef in the south end of Tuvalu to decide whether to continue north into that island country for cyclone season. Eventually, we decided not to, and it was after we turned south for Fiji that hours later, a thunderstorm found us. It was a dark night and we didn’t get any warning until it was on top of us and a bolt of lightning crashed out of the sky directly above our mast, lighting everything in an eerie fashion. But either it didn’t hit us, or it did and was harmless because we didn’t suffer damage.

Proximity to land greatly increases the chances of a boat being struck by lightning. Common boater’s wisdom suggests stowing sensitive electronic equipment in a metal oven, a type of faraday cage that blocks out harmful electromagnetic radiation, and clipping chain to the side stays. Stowing electronics in the oven is based on the same principle as safety in a car, and the chain improves the chances that lightning striking the top of the mast will take a direct route to the water, not through the boat’s interior where it can strike sensitive electric and electronic equipment. Boats on land are by no means protected from strikes, and I have on several occasions seen holes blown in hulls struck while sitting in yards. After we sold our Peterson 44 Oddly Enough in Borneo, she was hit by lightning that blew out a hole.

When we cruised the South Pacific, we had relatively few electronic devices on board that could be damaged by lightning strikes. All our GPS units were portable, and we did indeed try to get them into the oven when a thunderstorm was nearby. Our fixed mount VHF and SSB radios were vulnerable, but we carried a handheld VHF. I had made the switch to digital charts for harbors but used a laptop computer for viewing them. It wasn’t connected to any other system on the boat, and we had paper charts for everywhere we went. Lightning struck Oddly Enough in a marina in Darwin, Australia, while we were back in the States. Neighboring boaters saw the lightning arc to the powerful, vertical Maxwell windlass mounted on the bow. When we returned, we found the electric motor destroyed. It was a pain to replace, as it was in the anchor locker, a tight space which I fit into better than Tom.

The thunderstorm that burnt out our windlass came during Darwin’s wet season, when we returned to the U.S. to work and escape the rain that inundated the region from November until May and then shut off like a tap. During the dry season that followed, we cruised the wilds of the Kimberley. At the end of the season during the northern buildup, we experienced dry thunderstorms, which still need moisture to form clouds, but the rain falling out of them evaporates before reaching the ground. At night in anchorages on our way back to Darwin, we could see the storms flickering just inland from us.

Which brings up the issue of distance, and the old saw about counting the time between a bolt of lightning and clap of thunder. According to the National Weather Service, “Since you see lightning immediately and it takes the sound of thunder about five seconds to travel a mile, you can calculate the distance between you and the lightning.” Counting the seconds between flash and thunder and dividing by five gives the distance in miles from you to the lightning.

Thunder is a sound wave created by rapid expansion from the heat of a lightning bolt and the quick contraction of cooling air behind it. Air in the bolt may reach 50,000 degrees Fahrenheit, five times hotter than the surface of the sun, and the sound wave can be heard up to 10 miles from a lightning strike. Sometimes we heard thunder in our Kimberley anchorages, sometimes not, but when we did, we were within striking distance of a storm and sitting ducks as we couldn’t move out at night. But the dry storms never came any closer.

So what can a boater do? Solutions vary depending on location. If you keep your boat in a marina, you can’t move to a location where your mast is lower than other structures around it, but you can add surge protection to your shore power; Marinco makes shore power pigtail adapters.

Leading edge of cold front over Wickford, RI, on Narragansett Bay.
Leading edge of cold front over Wickford, RI, on Narragansett Bay.

Boaters on the water either at anchor or underway have a different set of issues. Given that you have to assume if there is a thunderstorm nearby and close enough overhead, you will get hit, the first and most important line of defense is making sure to give lightning a safe path from the highest point on your boat to the water. On a sailboat, this would likely be the VHF antenna at the top of the mast, and an adequate system of conductors and grounding needs to be in place to isolate that antenna so the lightning can use the mast as a conductor, along with the stays, to go to the water. There are varying opinions on whether an encased keel acts as a good ground, which I won’t go into here. On a powerboat without a mast, or on a sailboat with a carbon fiber mast, it’s necessary to design a grounding system that may include a grounding plate on the hull. Again, Marinco makes a variety of plates, and it’s worth putting some thought into placement.

An evolution of thinking in lightning protection has been going on since ships became steel, started carrying engines and were electrified. For yachts in the days of wooden hulls, oil lamps and sextants, even though a sailboat’s mast would be the tallest thing around on the open sea, a lightning strike couldn’t damage sensitive equipment. When I cruised on Oddly Enough, we had electrical and electronic equipment that needed protection mostly because it made our lives easier and wasn’t crucial to survival.

Now on Ora Kali, a Sabre 30 coastal cruiser, I’ve made the leap to setting up a boat that is startlingly dependent on integrated electronic systems, which makes her vulnerable to the effects of lightning strikes and surges. Luckily, Tom and I sail Ora Kali in Maine, which ranks 41st in lightning casualties and accounts for about 60,000 lightning flashes out of the tens of millions across the U.S. each year.

Surge Protectors
Most modern boats are at least as dependent on electronics as Ora Kali. Our Simrad GO7 XSE chartplotter has come to replace paper charts almost entirely. I still find the only way to get a big picture of a cruising ground is through a paper chart, but if lightning struck and destroyed the chartplotter, we would have no detailed navigation capacity. We also wouldn’t have a depth sounder because that is connected up through the plotter, nor most likely a Raymarine Cockpit Mk II Wheel Drive autopilot, nor the Halo20+ Simrad Radar, which is connected into the system, as is the Standard Horizon VHF radio with AIS receiver. In other words, one well-placed lightning strike could destroy all the systems. Battery management systems for lithium-ion battery banks like those made by Battle Born can also be damaged, along with electronic control modules that regulate engines for efficiency and emissions reduction.

It’s interesting in light of this interconnectedness that discussions of lightning among boaters focus on products to prevent strikes, which is not proven to be possible, and grounding solutions, ignoring the how-to’s of protecting all these electronics. People generally have surge protectors for electronics in their houses. Granted they are easier and cheaper to set up in a home, but products have existed for some time that can be added to a boat system and act as surge protectors. Transtector makes a direct current TVSS, or transient voltage surge suppressor, for marine or outdoor use designed to protect sensitive electric and electronic equipment from transient voltage. This semiconductor device suppresses lightning-related voltage spikes and is widely used in the telecommunications, wind generation and avionics industries.

According to an article by James Coté in BoatUS Magazine from 2016, TVSSs can be thought of as fuses that react to voltage instead of current. They remain an open circuit as long as the supply voltage feeding the equipment is in the normal range. However, “if a lightning strike causes a momentary voltage spike and puts, say 1,000 volts on a 120 volt device, the TVSS will ‘clamp’ or short circuit 880 volts and convert it to heat.” The excessive heat would most likely damage the TVSS, but better to destroy a $250 surge arrestor than an Inmarsat broadband terminal that costs thousands of dollars. Bad Wolf, a veteran-owned American company, makes TVSS devices. LSP and Transtector are both Chinese companies and also make the devices.

Towering clouds of the ITCZ.
Towering clouds of the ITCZ.

ITCZ Thunderstorms
Thunderstorms are not common in vast stretches of the oceans, but they do occur frequently in regions popular with coastal and offshore sailors. Sailors in the Caribbean Sea and the Pacific and Indian Oceans encounter the Intertropical Convergence Zone, or ITCZ, near the equator where trade winds of the Northern and Southern hemispheres come together. It’s a remarkable place, with towering clouds that reach far higher than anything in temperate zones.

The intense sun and warm water of the equator heat the air, raising its humidity and making it buoyant. The convergence of winds and air masses increases instability and helps the buoyant air rise. The height of the troposphere over the equator can be 11 to 12 miles, which accounts for the cloud towers. As the air rises, it expands and cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms.

Florida is home to lots of boats; it also has hot, humid weather and thunderstorms, the frequency of which lends it the distinction of being the lightning capital of the US. While hot, humid summer weather is already a recipe for storms, Florida’s geography, a peninsula jutting out into warm water, adds to the mix. The land mass heats up much faster than the ocean during the day, setting up a cycle where cooler, denser air from the ocean pushes inland under the less dense, warm air, which flows out to sea over it. The air movement of the denser air at the surface is a sea breeze, and on a typical summer day, it moves inland from the ocean, which surrounds Florida on three sides, colliding in the middle of the state in mid-afternoon, leading to the creation of often intense thunderstorms.

Thunderstorms and cold fronts
The sea breeze cycle in Florida offers an example of a front, which is defined as a boundary between air masses of different temperatures and air densities. Cold fronts are also indicators of changes in weather. They move fast and push a tongue of dense, cold air under warm air, lifting it quite abruptly. Out on the water, boaters who encounter cold fronts may experience thunderstorms even outside the warmer weather regions if the air is moist and unstable enough. Cold fronts can lead to squall lines of multiple storm cells, which extend laterally for hundreds of miles and persist for hours. On Oddly Enough, we encountered a squall line north of the Bahamas that was deep and wide enough to force us to turn back and wait to head north to the U.S. coast until it passed.

Like sharks, lightning and thunder can foster a knee-jerk fear in humans. Looking back at my cruising logs, my writing when thunderstorms were in the area suggests foreboding and conveys relief when the storms pass. The storm on the night we left the reef at the edge of Tuvalu was followed by days of squalls with wind and and hard slogging into high seas. It’s the constant repetition of thunder and lightning that most sets the scene.

“Continuous growl of thunder. Can’t see lightning. Heading NW to get away.”

“Went through patch of cells w/thunder but no visible (here I drew a lightning bolt).”

“T-storms all around. Sat foto shows nothing. Wouldn’t know which way we could go.”

“Lightning to south, and dark mass behind.”

“Yuck! T-storms.”

“Spent last three hours dodging t-storms.”

“For a while we just didn’t know where to go.” n