Microbursts and Multihulls

[GordMay]

A squall line is a line of severe thunderstorms that can form along and/or ahead of a cold front, whereas a microburst is a very localized column of sinking air.

More ☛ Microburst Handbook

[clockwork orange]

Dave,in both cases i have experienced the day was hot and sunny and humid,the first a drifter,the other a nice sailing breeze, 5-10 knots.The first time the signs were there but we were too stupid to recognize them,we were looking at the cloud in awe when a larger racer which was well ahead of us got knocked down,thats when we clawed everything down,trouble is we didnt have time to get things tidied up so we still had too much windage,in both cases we did feel the chill but it would have been to late to respond if we didnt have the visual clues. I dont know if these thing officially meet the criterior of microburst but i cant think of a better description as they are very localised and very violent for a short duration then are gone.The first for me went from virtually zero to 70(local news) to nice manageable 20-30 in no more than 15 minutes from the first sign to sailing again.The second time when it passed we were back to aboat the same breeze as we had before,both of these were over 20 years ago but im pretty sure there was no rain involved in either case.On the racing cat Great White which i mentioned earlier it was dark and if they felt the front coming they probably wouldnt have had time to respond. With sails down and properly contained i dont think most cats would have any problems.Being able to strike your sails quickly and completly is paramount.On the 34 we had a furler so rolled that in quickly but the main was a bolt rope type and we were not able to contain it in a very tidy manner quickly enough so still had a lot of windage,roller boom would be nice.
Steve.

[Laidback]

Originally Posted by John A

Laidback,
Please read my post above explaining micro-burst. They can occur anywhere that the wind blows, even in clear air.
regards John.

Hello John, Understood the contents of both your posts.
My question was "In Which Latitudes have so called Micro Bursts been OFFICIALLY recorded? " In other words is there a source for such information?
EG: THE BRITISH MET OFFICE or the NOAA, other than news media sources.

[sandy daugherty]

I was on the NTSB Go-Team that investigated Delta Flight 191 in Dallas. This was the accident that prompted the FAA to install dedicated Radar that detects rapid changes in wind direction at many airports in the US, and led to similar installations at airports all over the World. Dr. Dennis Fugita had just published his seminal work on micro-bursts, and it was remarkable that we found the textbook example when the textbook was still warm from the presses! Weather Science has benefited from these NEXRAD systems that have documented close to a thousand cases since then.

Micro-bursts act like a a balloon filled with flour and then popped. There is a fairly localized but rapidly falling column of air that hits the ground or water and spreads in all directions at high speed but short duration. It will knock down buildings, blow cars off the road, and tip tractor trailers. Velocities may be as high as 60 knots.

Micro-burst occur in thunderstorms rather than blue skies. There will be a noticeable change in air temperature DURING the burst, not ahead of it. If you feel that rapidly dropping temperature your are in it, its not an advance warning.

Flight 191 crashed because it flew through the micro-burst; as it exited the far side, the air was moving in the same direction as the airplane, which stalled as its airspeed dropped too low.

Our advance warning is often only an ominous dark wall of cloud. If the sky ahead of that wall was clear we would see massive rapid overdevelopment, huge climbing and roiling cloud tops. More often than not, we can't see that.

Micro-bursts weren't invented that day in 1985. Sailors have always known that a line of thunderstorms could hold that kind of violence. Since so many are associated with frontal activity, a rapid drop in atmospheric pressure is reason enough to take precautions, and a squall line is the last wake-up call. Reef, close up, start engines to maintain heading, Life jackets over foulies, and all for a minute or two of maelstrom. (I've always wanted to use that word!)

This is not a blue sky development: there has to be a massive imbalance of energy to cause it. A micro-burst comes from a major thunder-bumper, but it usually hides in a squall line. They can occur in any lattitude, and at any temperature. While surface terrain contributes most to the development of storm systems, sea surface temperature variations can do the same thing.

[Captain Bill]

I was hit by a microburst two years ago in Spiraserpula. There was a very non-threatening looking cloud with a light shower coming out it. I didn't even consider dropping canvas because it really looked like a thousand other light showers I had been through. One could easily see through it without any trouble. It looked to be a mile or so in diameter. It was moving off the shoreline in the Neuse river in NC near the ICW. Suddenly the wind changed direction at least 90 degrees, maybe a bit more. We had been close hauled in about 10 knots true and suddenly we had 50 knots+ from about 120 relative. The boat started to heel, maybe 15-20 degrees, which feels like a lot in a cat. I released the sheets which relieved the heel and made a rapid turn into the wind. As I was doing so the wind made a rapid 180 degree direction change which put the wind off my stern again blowing 50+. The boat gybed but the boom brake kept the main from going over too hard, but the camberspar jib went out of control around the forestay going past 180 degrees relative before hitting the stopknot on the sheet. Since the boat was still in a turn the jib got backwinded and because the stop knot on the sheet wouldn't let it go any further the something had to give and the camberspar bent about 30 degrees. 20 seconds later it was all over and we were back to 10 knots. I looked up and there was clear blue sky and not even a remnant of that cloud, it had completely disappated. I would guess that the whole thing was over in less than a minute, certainly less than 2. Since then I have of course replaced the camberspar and I have a new stopknot in the jib sheet that prevents the jib from going past about 120 degrees.

I'm pilot as well as a sailor so I am well aware of microbursts. There was no lightning associated with this cloud at all and I wouldn't have thought twice about flying through this shower in my cessna, the bottom of the cloud wasn't even very dark, though perhaps a little darker in the center than around the edges. What I did learn is that microbursts do not require thunderstorms and can occur in even very benign looking rainshowers. I'm sure glad I had not stepped away from the helm and had the boat on autopilot. If I had not been able to release the sheets we certainly would have been in for a wild ride if not a knock down.

Sorry Sandy, but these things don't just happen in thunderstorms. If I had flown my cessna through this at the same time your buddies at the NTSB would have had an interesting time finding all of the parts.

[Captain Bill]

Forgot to add we did not get hit by any rain in the incident.

[Joli]

Two years ago during a race here the breeze went from the low 20's to 60~70 with little warning. Lot's of knockdowns and shredded sails, one F27 flipped and another came very close. It seemed to affect the entire fleet spread out over 15 miles and lasted 30 seconds to a couple minutes and then the breeze died out completely.

Here is an FP Tobago that went over but the article doesn't say how quickly the breeze came up. Note: article from Latitude 38.


Surviving a Twice-Flipped Cruising Cat

April 20 - Off the West Coast of Florida
It's rare for a cruising cat to flip, but even rarer for one to be flipped twice. But that's what happened to Paradox, a cat most recently owned by Tom and Stanna Galbraith of Durango, Colorado, which started life in '96 as an F/P Tobago 35. We're not sure how the cat came to be upside down in Belize's Rio Hondo in '01, but it's our understanding - from some fascinating video on the couple's Web site - that using a small tug, they managed to right the cat. Having been upside down long enough for there to be growth several inches long throughout the interior and deck of the boat, it was one disgusting mess. Nonetheless, the Galbraiths obviously saw possibilities, and after what had to be endless months of dreadful work, they ended up with a cat, stretched to 38 feet, that looked damn good. And based on other photos on their Web site, they had a ball cruising the western Caribbean.


Paradox looking sweet after being flipped right side up and being restored.
On April 11, however, the couple were sailing from Key West to Tampa when a squall suddenly increased the windspeed from 11 knots to 48 knots. The cat flipped immediately. Either just before or just after, 60-year-old Tom grabbed his wife, and knowing that the cat wouldn't sink, pulled her into the hull where they had tools and wetsuits. While it had to be creepy inside the overturned hull, they knew the cat wasn't going to sink and that there was plenty of oxygen. Having heard only one ping from their EPIRB, Tom realized that the signal wasn't getting out. So the next day he drilled a hole in the bottom of the hull - which was now above their heads - and stuck the EPIRB antenna out. The EPIRB immediately started pinging away. Coast Guard Miami got the signal and launched a search plane at 5 p.m., and found the overturned cat an hour later some 171 miles southwest of Tampa. A rescue helicopter arrived on the scene at 8:30 p.m. and hoisted the couple aboard. Neither Tom nor Stanna had suffered injuries and both declined treatment. On the other hand, they apparently don't have any interest in restoring the cat a second time.

Tom Galbraith hanging out on Paradox in Honduras.
Photos Courtesy Paradox
A word to the wise: All other things being equal, as cats increase in length, they become much, much more stable.

[Dave]

This was a squall line incident that I mentioned once before in a different thread (Can't find it now..so if you've read this before I apologize..) This could also fall into the Sailing by headsail alone thread...

Aug 21, 2006. ~30 miles South of Grand Caymen, 2 AM, Thunder heads had been forming all day, no stars, no moon, wind ESE 12 to 15, sailing North under full Genoa, tightly sheeted (Main was torn and stowed, topping lift acting as a back stay)
Out of nowhere (so to speak) got hit from the NE with ~60 Kt wind and heavy horizontal rain. 38 ft Cat immediately vaned stern to the wind and sat, basically, hove-to backwards, sheet to the wind, making about 4 to 5 kts dead downwind. No helm. The sail stayed nicely filled, no flapping. Lasted about 30 to 40 min. then returned to previous conditions. Somewhat like running before a storm without a rudder. The motion was strangely comfortable despite thinking the jib could shred at any moment. I was very surprised at how fast the helm disappeared and the boat spun off the wind. Maybe there's something to be said for a heavily unbalanced rig on a cat in gusty conditions. :-)
My gut tells me that if the wind had hit harder and faster that either the boat would have spun off the wind faster or the jib would have blown out.

With the main up and a balanced rig, it would have been a very different story. That would be a lot of power..something has to give. I don't really want to find out what goes first.

Dave

[Tropic Cat]

If there is anything good to be said about squall lines, it's that you can see them coming a long way off. When I get caught, my procedure is to lower sails, start the engines and keep the bow into the wind. This works up to 60 knots of wind in my boat, beyond that the boat can not maintain forward speed as she's not very aerodynamic.

[Robertcateran]

I was hit by a microburst while in PNG. From beating in a light breeze to wondering if the bows were going under as they went faster and faster from wind astern was a matter of seconds. The burst went before the bows went under. This may have come off a steep hill. This is why I am going with an unstayed rig so I can let the rig weather-cock in similar situations

[oscar]

I'm pilot as well as a sailor so I am well aware of microbursts. There was no lightning associated with this cloud at all and I wouldn't have thought twice about flying through this shower in my cessna, the bottom of the cloud wasn't even very dark, though perhaps a little darker in the center than around the edges.

If you would have been airborne you may have seen more of the cloud above. Lightning is hard to see in the daytime. Flying through showers you can't see the cloud they came out of in a light aircraft without wx radar however is always a bad idea. You never know what goes on in there, or what it looks like above. Some of the most bumpy rides I've been in was around weather that was barely in the 20's.....

(23 years in big airplanes)

[annk]

We were hit by one, Mid Atlantic. No warning, at dusk. Strong down draft that felt as though somebody had opened the oven door. Boat had top spreaders in the water, she's 40 feet weighs 17 tons. Snapped the reefing lines, snapped sacrificial tube on Monitor, wind instruments stopped working at 60 knots.

Wind constantly changing direction, heading into wind just kept us in the centre. Took decision to turn 90 degrees to port, all over in 10 minutes. At its height couldn't read the instruments from the wheel, maybe 8 feet. Clipped on with two tethers and still bouncing off bench to the height of the webbing.

very scary, imagine worse in multihull as no time to spill the wind before you would be over!

[Captain Bill]

Originally Posted by oscar

If you would have been airborne you may have seen more of the cloud above. Lightning is hard to see in the daytime. Flying through showers you can't see the cloud they came out of in a light aircraft without wx radar however is always a bad idea. You never know what goes on in there, or what it looks like above. Some of the most bumpy rides I've been in was around weather that was barely in the 20's.....

(23 years in big airplanes)

I'm not exactly sure what you're saying here, with respect to the 20's or weather radar. I watched this cloud coming for some time. It was in a scattered layer of fair weather cumulus at about 3000 feet. I don't think there was more than a couple of thousand feet of vertical development. The cloud was basically a flat pyramid shape, less than half as tall as wide. It was just a light summer shower. The rain was so light I could see the entire cloud base with no trouble, and it may not have even been reaching the ground. I have used airborne weather radar a number of times and I'm pretty sure this thing would have barely shown up. The kind of thing that shows light green on doppler if at all. I'm sure someone in a big airplane(above the clouds rather than below) would have a better view. My personal rule of thumb is not to fly under any rain producing cloud that I cannot see all the way through clearly. I suppose if we want to continue a discussion of airborne waether we should switch to nother forum.

The bizzare thing was this sudden dissipation event where in one to two minutes a cloud this size disappeared completely with no rain and a sudden downburst. I was about two miles from the shore line and there was no disturbance on the surface of the water until the initial gust hit.

[SailFastTri]

So is there any universal way to anticipate a microburst? I'm reading these accounts and find them quite disturbing. The bad weather I've seen always had accompanying visual and audible indicators (or sudden drop in temperature) that would give you a clue, at least minutes ahead.

[s/v Jedi]

When you anchor in Des Haies in Guadeloupe or Bahia Gayraca in Colombia, you get 50-60 knots down-bursts from the surrounding mountains all the time (willy waws). There are the miniature version of these microbursts. The only warning I see is the water flying up from the sea when they are approaching, something you probably don't notice off shore.

cheers,
Nick.