Since the issue of microbursts has come up, the following article that I wrote and that was published in Sail magazine some years ago, might be useful to some members of the group. I'm a scientist and it is based on a talk that I gave at a Naval Architects meeting.
What happens in a microburst and how to handle it.
Allan H. Frey
During sailing season on the
Chesapeake Bay, it is not unusual for the weather to be unstable with resulting thunderstorms, microbursts and I’ve even seen funnel clouds.
Encountering a microburst is an
emergency situation; the first Pride of
Baltimore was
lost to one. However, if you understand the nature of the phenomenon, can recognize the early indicators and take appropriate action in a very short time – often that means only seconds – you can come through without damage. Let’s look at what is involved.
A microburst is a dense, rapidly descending column of cold air. When the column nears the surface of the sea it is deflected and flows outward radially like the spokes of a
wheel.
Research indicates that the column of air forms when raindrops evaporate before they reach the surface,
cooling the air in a localized area. The cold air falls rapidly to the ocean surface and its kinetic energy is dissipated as the air spreads out laterally at the surface.
Although microbursts do tend to occur in the unsettled weather that accompanies a depression, they are not necessarily associated with rain. They measure approximately ½ to 2 miles in diameter at the water surface and usually last from about 5 to 15 minutes (Fig.1). The lateral wind velocity of the column of air at the surface tends to be about 25 knots, though velocities as high as 80 knots have been documented. Microbursts can, and often do, occur in thunderstorms and this may well be part of the reason thunderstorms are often so violent. The good news about this type of microburst is that most sailors can recognize an approaching thunderstorm and know how to prepare for it.
The real problems come if a microburst is encountered essentially without warning when the weather seems to be merely unsettled; overcast skies, for example, with moderate wind or possibly occasional light rain. Meeting a microburst under these seemingly benign conditions puts a boat at risk, since the crew neither expects nor understands what is happening and therefore reacts slowly and possibly incorrectly.
Research to date hasn’t defined obvious indicators of an approaching microburst. But even in the absence of obvious signs such as those associated with a thunderstorm, there are a few subtle clues. In addition to the dirty-looking day, you will notice an approaching patch of flattened water that is similar to the flattened water resulting from a thunderstorm’s heavy rainfall flattening the waves. Another indicator is a rapid increase in wind speed, similar to what you would experience in a strong gust, which lasts 30 seconds or more and is considerably colder than the ambient air temperature.
What you do in this
emergency situation depends on your point of sail, which part of the microburst is passing over you and which direction it’s approaching from. To illustrate, let’s assume you are on a broad reach on a port tack, and the microburst approaches you from the port beam. Assume that the microburst is 1 mile in diameter, the system is moving at 20 knots, and the center is going to pass over the boat. You would feel a cold wind increasing rapidly from the port side; it would build to maximum strength in approximately 70 seconds and then fall off abruptly. About 40 seconds later a high-velocity wind would strike from the opposite, or starboard, side. The velocity would then taper down over the next 70 seconds, and the wind would return to what it was before the microburst appeared. Whether the wind backs or veers is a primary clue to the location of the microburst’s center and therefore to the wind conditions you can expect.
If you took no action, the boat would most likely be knocked down on its starboard side in a matter of seconds. If the boat didn’t flood, it would come back up about 90 seconds later as the center of the microburst passed over it. Then the boat would be laid flat on its port side - if the
sails and boom were still intact.
In this situation you probably wouldn’t have time to drop the sails; you must instantly release the
sheets and allow them to run free. When the microburst’s center passes overhead and before the wind starts to fill in from the other side, you must immediately prepare for a gybe. Bring the
sheets in so the main and
jib are approximately amidships, and then let the sheets run immediately when the wind starts coming from starboard. In about 3 minutes the extreme winds should have passed.
How you should react to the microburst depends, as I have said, on where the microburst is coming from relative to your boat. You can familiarize yourself with the appropriate actions in each case in a few minutes by working with simple models on the kitchen table. Draw an 8-inch diameter circle on a piece of paper to represent the microburst’s location at sea level. Then draw radii from the center of the circle to the periphery with arrows pointing out from the center to represent the microburst’s wind direction at various places. Next, cut out a 1-inch long piece of paper shaped like a boat and use a toothpick as the boom. Now place the model boat on top of the model microburst and slide the circle (the microburst) along under the boat. You can see what will happen, moment by moment, in an encounter with a microburst and how it affects the boat depending on the point of sail.
To simulate the situation described earlier, for example, place the “microburst” to the left of the model boat. Slide it under the boat, its center passing underneath the boat (the microburst would actually pass overhead), and then move it to the right of the boat (Fig. 2).
Or, as another example, put the model boat on a broad reach on port tack and assume the microburst will approach off the port bow parallel to the boat’s course. Then only the left side of the microburst will pass over the boat, and the wind will continually back (Fig. 3).
An encounter with a microburst is a rare event, but it does happen - probably more commonly than an encounter with a waterspout or a severe thunderstorm. If you’re caught out in unsettled weather, you should stay alert to the clues. Then, if you do happen to encounter a microburst, you’ll be able to recognize the fact quickly and to act immediately. If you do, the chances are excellent that you will come through with no damage at all. And you’ll have a darned good sea story to tell.
Fig. 1 Cross-sectional form of a microburst: a) in the atmosphere, b) at the surface of the ocean.
Fig. 2 Moving microburst viewed from above. It is approaching a boat on the port beam (panel 1) passing over (panel 2), and receding on the starboard beam (panel 3). Note the change in wind vector with respect to the boat. The boom will first be forced to starboard. If it is still intact when the center of the microburst is over the boat, it may tend to center because of the apparent wind from the boat’s residual forward motion. As the microburst recedes to starboard, the boom will be forced to port. The large arrows show the microburst’s direction of motion with respect to the boat.
Fig. 3 Moving microburst viewed from above approaching a boat on the port bow (panel 1), passing on the port beam (panel 2), and receding on the port quarter (panel 3). Note the change in wind vector with respect to the boat. The boom will first be forced to the starboard quarter, then abeam on the starboard, and then to the starboard bow. The shrouds,
mast and/or gooseneck may break. The large arrows show the direction of motion of the microburst with respect to the boat.
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