Max Ebb on Squalls

Revised: Tuesday, November 27, 2012
Ebb, Max

 

Squall Strategy

Re-printed with permission from Latitude 38, July 1994.
Copyright Max Ebb Publications and Latitude 38, 1994

 

 

"What's the event tonight?" I asked the bartender. "I had to drive all the way back to the second overflow parking lot to find a space!"

But the bar was almost empty. Just a few people I didn't recognize buying drinks, and a member serving.

"It's the weather briefing for the race to Hawaii. Standing room only in the dining room."

"Ah, that explains it. Big entry list this year. Who's speaking?"

The speaker was a well-known name in Transpac navigation, all the more reason for the full house. So I ordered a gin and tonic for myself and wandered towards the dining room, to see if I could squeeze in for the rest of the presentation.

  "If you're on a sled," advised the speaker, "you can actually sail as fast as the squall is moving. VMG downwind will be about half the windspeed, with no upper limit."

He was referring to a table of numbers projected on the screen, showing the tacking angle, apparent wind angle, boat speed, and VMG for a large ultralight.

"The squall typically moves at 15 knots, but the wind in the squall is around 30. That's enough to get you down the wind fast enough to stay in the area of strongest wind, so you can actually jibe back and forth across the face of the squall many times. Remember, the object is to stay *in* the squall"

This generated a chuckle from the audience. Having seen the entry list, I knew that most of them were cruisers before racers. The next chart flashed up on the screen, the numbers for a much older and heavier 40-footer.

"But in a slower or smaller boat, one pass is usually all you'll get. How you exit the squall is critical."

This was interesting. I walked along the back perimeter of the darkened room, navigating between the packed bodies and re- arranged tables and chairs. No seats, but it seemed like it might be worth standing and listening for a while.

"On a slow boat - and that means anything but a sled - you should generally `exit stage left' when the squall ends. That is, leave to the left, on port pole. Never get caught right on the ceterline of the squall's track behind it, or to the right. And especially not just before dawn! On the average, the squall wind will be a starboard-tack lift, for reasons that I explained earlier. So you'll probably have jibed onto port pole anyway. Once you leave the squall, you want to get away from the light air behind the squall as quickly as you can."

It sounded like I had missed something important.

"Rule of thumb:" summarized the speaker - "Once you're out of the squall itself, if you want the wind conditions to change get on port tack. If you want them to stay the same, sail on starboard. This is also the rule of thumb for increasing your chances of staying under the 'wind stripes' that you'll see as lines of clouds during the day."

"Why?" I thought to myself. I had definitely missed something important, walking in in the middle of the talk.

"Another rule of thumb." he continued. "If you're in a sled and are trying to jibe back into the squall, jibe as soon as you get headed. It sounds crazy to jibe on a header - but I've repeatedly observed the wind direction aimed inward towards the track of the squall. The header is often the first indicator that you're getting to one side of the strongest wind, and it's time to go back."

 Just about everyone in the room was taking notes furiously, copying down a diagram showing wind speed and direction around a squall cloud.

"I don't know why this happens," he confessed. "You'd think that the strongest wind would just radiate outward from the center of the squall. But for some reason, the wind is directed inward like this."

He emphasized the wind arrows on his diagram, drawn in a "toed-in" orientation on the left and right front corners of the squall.

  As my eyes became adjusted to the low light level, I could pick out many of my sailing friends in the crowd. There were a few of my competitors from the YRA fleet, and the handful of yacht club members that had berths on the Hawaii race were scattered in the crowd. There was my sailmaker, over by the wall on the right. And even naval architecture student Lee Helm, who sometimes can be persuaded to crew for me, was over on the left side of the room taking notes along with all the others.

"Maybe you can explain this phenomenon," the speaker asked the sailmaker.

All eyes were on the sailmaker, himself a veteran of many successful races to Hawaii. Lee noticed me standing along the back of the room, and she waved acknowledgment when we made eye contact.

"Oh no," answered the sailmaker. "I know enough to not guess at questions like this, especially in this crowd!"

"Anyone else?"

Now the room was silent, and I saw the sailmaker looking at Lee. I looked at Lee to see if she would respond. She looked back at the speaker. The speaker looked back at Lee. I looked at the sailmaker again. He shrugged. Lee shrugged. The speaker shrugged. And for good measure, I shrugged.

"Let's take a 10-minute intermission," announced the speaker. "After that we'll cover wind stripes, effects of tropical storms, and best approaches to the finish."

The house lights went on, and a large number of people made for the bar. I made my way over Lee Helm's table, borrowing a temporarily vacant chair.

"I'm surprised you didn't offer an explanation for that wind shift question," I said as I sat down.

"Like, I'm just auditing this class," she joked. "Collecting those `rules of thumb.'"

"Do you have a spot on the race this year?"

"I wish. But like, I really have to finish my thesis this summer. So I'm on the beach again. I mean, I'll be up for it next year, though. Especially if you know someone doing the race to Tahiti..."

"I'll be on the lookout for a berth for you, Lee. But I still can't believe you don't have an explanation for that wind shift."

"For sure, there are ways to explain it. The main thing is to think of systems of convection cells, instead of just a single source of wind from an isolated downrush column. When one convective cell is collapsing, it's almost certainly triggering new ones ahead of it. So a squall system often resembles a kind of dipole, a source-sink pair with the strongest wind right between he two."

"Ah ha! Of course!" exclaimed a racer who was sitting at the same table, until now absorbed in studying the notes from the previous part of the lecture. "If you simperimpose a dipole flow field on the surface wind - taking into account the veered upper flow - you get exactly the wind shown on that last diagram!"

"Wait, wait, back up," I said. "What on earth are you talking about?"

"Okay, Max," said Lee patiently. "I'll try to explain this for the differently clued."

She turned over the page on her yellow note pad, and drew a graph showing temperature versus altitude.

"The new word you need to know is *lapse rate*. This is the observed vertical temperature gradien in a column of air, and it's typically about 3.5 degrees Fahrenheit per thousand feet of altitude. It's like if you sent a thermometer up in a balloon, and measured temperature with respect to altitude as the balloon went up, you like get this line."

"Okay, I'm with you."

"Cool. Now, there are two more lapse rates to deal with, the *dry adiabatic lapse rate* and the *wet adiabatic lapse rate.* These refer to the rate at which an imaginary piece of air would change temperature if it's moved up or down without any heat being allowed to flow in or out - hence `adiabatic.' A typical value for dry adiabatic lapse rate is like 5.4 degrees per thousand feet. So you start with a container of air, move it up a thousand, let the container expand to match the reduced pressure, the temperature will be drop by 5.4 degrees. That's for dry air - if the humidity in the air is 100% when you start, then some of the water vapor will be forced to condense, because at the lower pressure the air has less capability to hold water vapor. This involves a change of state for the water vapor - from gas to liquid - and the heat of vaporization is released into the air when the water condenses. This keeps the air warmer, so the wet adiabatic lapse rate is less than he dry rate - typically 3.2 degrees per thousand feet."

She drew two more lines on her graph, representing the two additional rates of cooling.

"Now, the good stuff. Suppose it's a typical day, and the actual measured lapse rate is the average 3.5. If the surface temperature is 75 degrees, what would happen if you took some air from near the surface and raised it a thousand feet?"

"Dry air or wet air?" I asked.

"Good question! Let's say dry, for now."

"Okay, you said the dry rate was 5.4 degrees per thousand feet, so it cools 5.4 degrees, and end up at just under 70, at - let's see - 69.6 degrees."

What's the temperature of the surronding air at that height?"

"I get it," I said. "The surrounding air is 3.5 degrees cooler, according to the lapse rate curve, so the surrounding air is at 71.5 degrees. The air that we lifted up is now cooler than the surrounding air, so it would sink back down."

"Very good! We have stable air. No convection cells here. You remember from the last time we went through this, huh?"

"Okay, but what does this have to do with squalls?"

"Hang on. What if you have air that's fully saturated with water, and raise it a thousand feet?"

"Use the wet lapse rate," prompted the other racer at the table, when I hesitated."

"Thanks," I said as I did some more arithmetic in may head. "Now the air only drops in temperature by 3.2 degrees, to 71.8."

"And?" asked Lee.

"Compare with ambient," suggested the racer.

"Oh, I see. It's a little warmer than the air around it this time, so it would rise up."

"So?"

"So, I guess it would keep rising."

"Which means the air is unstable. That's why cumulus clouds billow up - the moist air keeps rising, water vapor adding heat to the rising column of air. Whenever the lapse rate - what you measured with the balloon - is steeper than the adiabatic rate - wet or dry, as appropriate for the altitude and the amount of moisture - the air will be unstable. Push some air up, and like, it keeps going up. Push it down, and it keeps sinking."

"Last time we discussed this you mentioned the lava lamp." I noted.

"Right. If heating near the ground causes the measured lapse rate to be steeper than the dry adiabatic lapse rate," added the other racer at the table, "even dry air will be unstable, and boil up in a column of rising air, or a thermal. I know all about this stuff because I fly gliders. When the air reaches `cloud base,' pressure drops to where the water vapor saturates the air, then the wet lapse rate takes over, and it becomes even more unstable. The thermal in the cloud is generally stronger, and more turbulent."

"Okay, back to squalls," said lee. "The ocean surface is heated slightly by the sun during the day, but it doesn't change temperature nearly as fast or as much as the air. At night the upper air cools, but the air near the surface stays warm. The actual lapse rate becomes very steep - relatively warm at the surface, much cooler a little way up - so the air is unstable, even the unsaturated `dry' air right at the surface is unstable. Rising columns of air form. But the air doesn't have to rise very far before it becomes saturated, causing it to cool at the slower, wet adiabatic rate, which makes it even more buoyant relative to the surrounding air, which makes it rise even faster, which makes it even more buoyant still. You have a humongous towering cumulus cloud, transfering hot air up from the surface, trying hard to bring the temperature gradient in the atmosphere back to normal."

Lee was gesturing with her hands as she spoke, trying to depict clouds rising to the stratosphere.

"And it works the other way too." she continued. "When the moist air at the top of these clouds cools down, it's ready to start falling. Extra liquid water in the form of cloud droplets and rain are re-evaporated back into the descending air, effectively refrigerating it as the pressure increases. The `downrush column' picks up speed, and as long as the lapse rate of the surrounding air is steeper than the wet adiabatic lapse rate, the downdraft air just keeps sinking faster as it falls through the cloud. If there's rain falling out of the cloud, then the wet rate applies right down to the surface, because there's still water evaporating into the air."

"That checks with my experience," I noted. "Cold rain in squalls."

"That's the standard description of how an isolated squall works," said Lee.

"Right," added the racer. "They build all evening, and start collapsing later at night or in the early morning hours. The biggest squalls of all are the ones that hit just before dawn."

"So you'd think," Lee continued, "that the wind field around a squall would be a strong outward flow of cold air, from the downrush column hitting the surface and spreading out in all directions. I mean, you have to add to that wind pattern the existing trade wind field, so the two winds reinforce each other in front of the squall, and cancel out behind it leaving you becalmed. You also have to add the wind component from the motion of the squall cell itself, which will be deflected to the right relative to the surface wind. That's because the upper air follows the isobars, but down low the wind is slowed by surface friction and tends to be distorted along the pressure gradient, away from the center of the high."

 "That explains the usual starboard-tack lift in a squall," said the racer. "At least it's a starboard tack lift slightly more than 50% of the time."

"I'm not sure I got that last part," I allowed, "but everything else agrees with what the books say. The squall behaves like a strong downrush of air, fanning out from a point right under the clouds, adding to the average wind in front and subtracting from the wind in back."

"Except when it doesn't," said the racer/pilot. "The shift is to the right most of the time, but how do you explain the times when it shifts the other way? And how do you explain those toed-in wind arrows on that diagram?"

  "There's a few more things going on," said Lee. "First off, you really don't know where the convection cell is in its cycle of developing and collapsing. That affects wind speed and rainfall. But more important, squalls hardly ever exist as isolated cells. The night air is unstable, and when the downrush air turns horizontal and flows out in front of the squall, it forms a cold wedge, like a miniature cold front, that lifts a lot more warm air up from the surface. This sets off a new convection cell of rising air immediately in front of the squall."

"So the air from the downrush gets sucked right back up into the new thermal?" I said, as the idea finally sunk home. "Now I see why the wind direction turns inward!"

"Not exactly. I don't think the downrush air is going to warm up quickly enough to power a strong upward convection. So it's not a true dipole flow field, in that sense. But I think there's converging flow into the new cell right above the downrush air, and this tends to deepen the layer of cold air, making it converge into a narrower band of wind like the diagram shows."

"Interesting theory," said the other racer. "It suggests that the strongest squalls would be double cells like that, to get the effects of downflow and upflow combined."

"Does it explain the old rhyme about wind and rain?" I asked.

"What's that, Max?"

I recited a rhyme I remembered from an obscure book about nautical folklore:

  "First the rain and then the wind,

 Topsail sheets and halyards mind.

 First the wind and then the rain,

 Hoist your topsails up again.

  "Owe! That's a rhyme?" Lee scoffed. "It uses `wind' and `mind,' then `rain' and `again.' I mean, call the rhyme police!"

"It must be very old," noted the other racer. "Those words might have sounded okay as rhymes a thousand or more years ago."

"But the amazing thing is," I said, "it seems to be true!" The strongest squall winds come right after the rain. According to the usual description of a squall, it always seemed to me that the wind should hit first."

"It checks with my dipole model," said Lee, "despite the dorky rhymes. Like, you'd expect to find rain under the new, strongly driven convection cell in front of the main squall cloud. Then the strongest wind would follow the rain. But if the rain comes after the wind, then the squall is already over and you're in for a period of calm in the squall's wake."

  "So how can we use all this to advantage during the race?" we asked.

"By understanding that some clouds are going up, and some are going down. Some people like to think of cumulus clouds as pistons - when one collapses in a squall, it pushes another one up. Then there's the gravity wave theory. Researchers have actually identified big atmospheric waves, like water waves, causing new convections cells to appear near existing cells. So it gets pretty gnarly."

 Meanwhile the room was filling up again, but fortunately whoever had a prior claim to my chair had found a better view from another part of the room. Lee flipped her pad to another new sheet of paper as the lights dimmed.

"Is any of this actually going to help us get to Hawaii?" the racer asked again.

"For sure," said Lee. "Just use all the rules of thumb!"

  max ebb

  RULES OF THUMB FOR SQUALL TACTICS:

1) "Incoming lane" for squall: squall should be just abaft the beam on starboard tack.

2) Jibe on headers to stay in the squall (sled only).

3) Always exit stage left.

4) Don't be anywhere near the last squall just before dawn. Wind dissipates at first light.

5) Best lure: blue & white feathers with silver head.

File Attachment:

MORE IN THIS CATEGORY

Driving

John Jourdane ,
, Kame Richards
Ebb, Max ,
, Kame Richards

Sails

John Jourdane ,
, Kame Richards

Weather and Environment

Ebb, Max ,