14 minute read
max ebb: squall tutorial
My only qualifi cations are age, creaky joints that don't like foredeck work, and the fact that I own my own copy of the Expedition routing software. That was enough to get me signed on as navigator on a big boat racing to Hawaii. That and my willingness to write some checks to cover some of the expenses. The result was that I had surely been promoted to my level of incompetence — but that would be my secret. I do know how to drive Expedition. Routing software, however, is no help in dealing with squalls. Lee Helm turned out to be our inspector. As usual, she had volunteered to do some of the safety and equipment inspections for this race, hoping to get a crew spot. Of course, we both knew that the boat would probably fi nish about a day sooner if she were navigator and I were doing the inspection. Lee's job was relatively easy this time because the owner had prepared the boat very thoroughly. All that she could fi nd defi cient were lifelines not tight enough, according to the little fi sh scale she carries, and an out-of-date battery in the Lifesling light. There were also some lifeline stanchions that were an inch and a half too far apart, but she let us slide on that one because the wire was a size up from the required minimum. Most surprisingly, she didn't make us go through her usual live demo of the emergency rudder, probably because the owner had built it to Lee's own design. "Lee," I asked as we sat down at the chart table to go over the required paperwork, "What can you tell me about
Basic squall thermodynamics: The temperature profi le of the air mass determines the vertical stability for moist or dry air.
Advertisement
dealing with squalls?" "Squalls sail on starboard tack," she shrugged. "Except, like, when they don't." "That's not much help," I said as I cleared a pile of tools from the chart table lid so that I could open it without spilling them off the table. "You need to read Will Oxley's book, Modern Race Navigation," she continued. "Is it a good squall tutorial?"
"No, but it explains why you shouldn't have a chart table with an opening lid and storage inside," she said. "Actually, the book is sort of a detailed user guide to Expedition. Only available as an e-book, but a must-read for anyone navigating their fi rst major ocean race." "I'll check it out," I assured her. "Now where did the owner put that folder with the liferaft certifi cations...?" Lee had a good point about the chart table. It was full of the crew's wallets and key rings, last year's tide tables, plotting instruments that we never use, a rusty splicing fi d, spare batteries of unknown condition, a few almost-empty rolls of sail repair tape, a single wool sock, a big crescent wrench, and a soggy paperback novel that appeared to be about young women surviving a simultaneous storm at sea and a shark attack in a liferaft while wearing almost no clothing. At least, that's what the cover art seemed to indicate. "None of the stuff in here has anything to do with the navigator's job," Lee pointed out, "and every time someone needs something it will interrupt your work." I finally located the folder with the raft info, and asked Lee again about squall strategy and why I needed to know that the squalls usually sail on starboard tack. "Upper air follows the isobars because the coriolis forces are so strong," she said. "Surface wind is slowed by friction, so it defl ects in the direction of the pressure gradient from high pressure to low pressure. But, like, all you really need to know is that upper air, compared to the surface wind direction, is defl ected to the right. So the squalls, moving mainly with the upper air, are also defl ected to the right, and blow downwind as if they are sailing on a starboard tack broad reach, typically about 165 degrees off the true wind."
"H ow do I use this info?" I asked. "Simple. If you want to get out of a squall you're in, or into a squall when you're out of one, or out of a clear spot between cloud streets with not much wind, jibe to port. You'll be on the opposite tack from the squalls. Port tack is for change. Starboard tack is for status quo." "Finally," I said. "That's the level of simplicity I need." "It gets worse," she warned. "But fi rst you have to understand how a squall works." "Okay," I said cautiously. "I know it's driven by vertically unstable air." "But do you know why the air is unstable?" she asked. And before I could answer, we were in thermodynamics class. "Let's say you have a cubic meter of air, and you push it up a thousand meters," Lee began. "Pressure drops by about 11%, to 89% of sea level pressure. What happens to the volume of this chunk of air?" "Even I can remember this from school," I said. "The gas law states that P V equals N R T. N is the number of molecules of the gas, R is a constant, so
Squalls sail on starboard tack, usually broad reaching at about 165 degree true wind angle (in the Northern Hemisphere trade winds).
SQUALL TUTORIAL
MAX EBB
Determining exactly what effect a squall cloud will bring is diffi cult, if not impossible. It helps to understand basic thermodynamics.
if pressure times volume has to equal a constant, when pressure goes down by 11%, volume has to go up by 11%. Right?" "Bzzzt! Wrong!" said Lee. "That's only true if temperature stays constant. In our thought experiment, we're lifting the air much too fast for temperature to equalize. That requires that the lifted air get colder, because as the air expands, it's doing work on the surrounding air. This is called adiabatic expansion, meaning no heat energy goes in or out; the only energy change is the mechanical energy lost due to the expansion. And this is also an isentropic process, no change in entropy, so it's, like, fully reversible if the air moves down again.
As much as my memory of the gas law had inspired a moment of confi dence, the mere mention of the word "entropy" took it all away. From my undergraduate days, I could never quite grasp "entropy" as a quantitative parameter. I was in for a rough ride. "Have no fear," Lee continued, "we can calculate the temperature of the air after it's moved up a thousand meters. We need the ratio of constant-pressure specifi c heat to the constant-volume specifi c heat, usually represented by the letter
As shown below, surface wind from the squall's downrush column radiate outward. Squalls often kick off a new column of rising air ahead of the downrush column, sucking in surface air.
gamma. For air this ratio is 1.4. And to calculate approximate temperature after adiabatic expansion, the formula is:
T2/T1 = (P2/P1)(gamma - 1)/gamma
So for a thousand meters of lift from sea level, we have:
T2/T1 = 0.89(0.4/1.4) = 0.967
Thankfully Lee spared me the derivation, but calculated the new temperature after a thousand-meter altitude change as 96.7% of the initial temperature. This of course is based on the absolute temperature scale, so if the air starts out at 15 degrees C, it ends up at 0.967 (273+15) - 273 = 5.5 degrees C. "Now things get interesting," she warned. "If the surrounding air is at constant temperature, same temperature at all altitudes, the air that moved up becomes a lot colder than the air around it, so it sinks back down. This is an air mass that could be described as vertically stable. In fact, even if the air mass is colder up high than down low, as long as the rate of temperature drop with respect to altitude — also called the lapse rate — is less than the adiabatic temperature drop, the air mass will still be stable. But, like, if the temperature gradient in the air mass is more steep — very warm near the surface and very cool up higher — then a chunk of air that moves up a thousand meters might fi nd itself surrounded by cooler and heavier air, even after adiabatic expansion, so it keeps fl oating up and keeps getting more buoyant relative to the air around it." "Like a lava lamp?" I suggested. "Close," Lee allowed. "But, like, now
The resulting surface wind around a dipole squall, with wind shifts exaggerated. This explains why fast boats jibing across the squall front often sail into a header when it's time to jibe back. It also shows that there can be a lefthand shift when the squall center is passing to starboard — but it might still be worth jibing to port to avoid the back of the squall system.
it gets even more interesting. Suppose, instead of just dry air, we have a mix of air and water vapor. You know, like the kind of air you usually fi nd in the trade winds near the surface of the ocean. As the air goes up, it has less capacity to hold water vapor, so when the relative humidity hits 100% the water vapor condenses out into liquid drops of water. This releases all the energy of vaporization — 600 calories per gram! Basically the reverse of boiling. So a lot of heat gets added to that chunk of air as it rises, and instead of cooling off by more than 10 degrees C for every thousand meters of lift, it only cools off by about fi ve degrees for every thousand meters." She paused, as if expecting me to fully grasp the signifi cance of this last fact. "See what that means?" she prompted. "As soon as the air becomes saturated at 100% humidity, it becomes much more likely to be unstable, because the wet adiabatic lapse rate is a lot less than the dry adiabatic lapse rate, so it's a lot more likely that the environmental lapse rate is greater than the wet adiabatic, so a chunk of air that moves up is more likely to become more buoyant in the air around it and will keep moving up." Evidently my facial expression failed to convey that sublime fl ash of new understanding that Lee was aiming for. "That's why there's more turbulence in clouds!" Lee practically shouted, "and that's why towering cumulus clouds form over the ocean in the afternoon. And now, the best part: Remember that this process is isentropic, therefore reversible? That means that if the air starts moving down, it will also be unstable in the down direction. Stable air from a high altitude that starts moving down will become heavier relative to the surrounding air as it falls, and then fall down faster and faster. Water can have the same destabilizing effect on the way down. If there's liquid water in the air, as in cloud droplets or rain, then the water evaporates as it sinks and keeps the falling air cool. That keeps the air from warming up as it's compressed by the increasing pressure. That makes it even colder and denser than the air around it, so it sinks even faster. We get a squall!" "Okay, that all makes sense," I said. "It certainly explains why we have squalls at night." "And it explains basic squall behavior. You get the blast of air from the downrush column of a collapsing cumulus cloud, with rain if it's a powerful one. And you get the wind shift to the right, because it's bringing down wind from the upper air where the wind direction is shifted to the right. It explains why you have to be ready to jibe on that clockwise shift if you're on starboard and you want to get south. And it explains the calm area in the wake of the squall, because the wind from the downrush column fans out in all directions around the column, so behind the squall it's subtracting from the trade wind instead of adding to it."
I asked, "Does that also explain why the standard advice is 'exit stage left?' Is it because we want to get away from the back of the squall where the wind is light, and we can do that faster on port if the squall is sailing on starboard?" "You got it. But, like, there's just one more very important complication." "Which is?" "The ocean surface can still be a lot warmer than the air at night. And remember, these vertical motions are isentropic and therefore reversible. New columns of rising unstable air can form ahead of the squall's central core of air collapsing downward." "How does this affect squall tactics?" I asked, trying to get right down to the practical advice. "A fast boat can reach back and forth across the front of a squall and stay in some pretty decent wind for hours at a time," Lee explained. "And, like, if the wind pattern is radiating out from the center of the squall, you would expect a lift as you get to the edge, which would be the cue to jibe back across the front of it. But instead, it's common to sail into a big header just before sailing out of the squall." "How do you explain that?" "Strong squalls are dipoles," she said. "There's a strong cell of collapsing air, but right in front of it there's often a cell of rising air that sucks a lot of the falling air back in toward the squall's axis, to replace the air that's going up. The instability is probably triggered by the cold air from the downrush column forcing its way under the warm air near the surface, kicking off the new unstable thermal of rising air. You get a wind pattern on the surface that looks like this..." Lee fumbled around for something to draw on, but the only bit of blank paper within reach was the inside back cover of the pulp novel. She drew the non-squall trade winds as a uniform fi eld at about 15 knots. Then she added the downrush of air from the squall's main cell, radiating outward from the core. Then the cell of upward instability right in front of the main downrush column, sucking air back into it. The diagram was a mess by the time she had added up all the various local wind effects, but it was clear why a boat might get headed just before reaching one side of the best breeze. "This is still a simplifi cation," she warned. "Think of squalls as multicylinder engines. Some pistons are going up and some are going down, and the real art is fi guring out which is which in time to call the necessary jibes to stay in the good wind. It really helps if you're set up to jibe without having to wake up any crew from the off-watch." "Ain't that the truth," said the owner, who had just arrived in time to provide some of the missing documents for Lee. "Last time I called an 'all hands' to jibe at three in the morning, I had to call another 'all hands' to jibe back at 3:45. They were not amused, and as it was I waited way too long before jibing back. This time we're set up for two-pole jibes, so the watch on deck ought to be able to jibe early and often, almost capriciously, without extra help."
Lee ran through the squall tutorial again, for the skipper's benefi t. "Maybe that's where the old ditty comes from," he said before reciting an ancient rhyme:
First the rain and then the wind Tops'l sheets and halyards mind First the wind and then the rain Hoist your tops'ls up again
"Yikes, call the rhyme police!" exclaimed Lee. "I mean, 'wind' and 'mind?' Followed immediately by 'rain' and 'again?' But, like, it does have a basis in fact. A really big squall will often have a rain cell in front of it because of the rising air it kicks off. So if the rain comes fi rst, there's probably a big blast of air behind it. If the rain comes after the wind, the squall is past. But that's for a classic two-cylinder dipole squall. Wait 'till you sail into a V-8..." Lee and the skipper concluded the paperwork, and left the cabin. I examined Lee's sketch again. But the front cover of that soggy paperback was hard to ignore. I decided that I needed to take the book home for further study.
