Thoughts on gusts?

N

nickfabbri

Well-Known Member
Joined
1 Sep 2006
Messages
290
Visit site
There seems to be a split in opinion in the yacht club bar. I am of the opinion that if the met office says 18 kts gusting 40kts then I won't set out on a trip as the wind strength is too high. Most of our passage plans involve both downwind and windward sailing and our current plan involves rounding North Foreland .

My older sailing friends scoff and tell me that you shouldnt bother looking at gust strengths too much unless they are stupidly high, just look at the average strength and if its below a 6 then you'll be fine.

Now I admit to alwats erring to the sides of extreme caution but would like to throw this open to the forum.

Many thanks.
 
We are to debating shall we sail on Sunday 0200 hours Dover to Brighton or stay another week in Dover marina....
SW 19 mph(Gusts 37 mph) - Weather On Line
 
Whether you decide to go or not has to depend on your comfort level. If the wind is particularly gusty, as it has been of late, sailing in confined areas is far more tiring as you are constantly trimming and spilling wind during the gusts. If you are fully crewed and on a shortish trip, then fine - you have a chance to rest as you rotate duties. Short-handed crews cannot cope as well as you are likely to be helming for long periods without the luxury of a mainsheet trimmer.

If you still decide to go, then in cruising mode you are best reefed to somewhere between the maximum and minimum windspeeds - undercanvassed in the lulls, but much more manageable in the gusts.

Rob.

P.S. my immediate plan for the weekend doesn't extend beyond motoring up and down the Hamble!
 
Gusts and even absolute wind speed are not the issue for us...it's sea state. And that's where the problem lies. Slight / Moderate / Rough etc are big ship terms in open water. Some years ago I reported in these forums a "rough" passage from Tenby to Milford in a F4...but wind against tide and a cross swell! Sea state forecast for the day...you guessed it...slight. So in the end it's down to first principles: Where's the wind coming from, how strong, how long a fetch, what's the tidal flow and swell doing, any overfalls or headlands, am I going upwind or downwind or on a rolly beam reach, how long is the discomfort likely to last? I ask myself all of those things before we decide to set-off or not. Sometimes we go anyway, others, well there's always the shops or the marina bar...:D
 
I reef to the gusts, so end up under canvassed most of the time, but with such a low freeboard as I have, its bad enough getting soaked with the spray, but getting covered in bruises from being thrown arround the cockpit is never fun. Im setting off on Saturday for my longest single handed trip yet (Trebeurden via Lymington, Portland Exmouth and Salcombe, oh, and back again) so heres hoping the wind calms down.
 
Had a 52 knot sustained gust / squall on Tuesday out of the blue (well out of the grey actually). Bit difficult to ignore the gusts in my experience as if felt pretty real and potentially serious depending on sea room at the time.
 
I am sure that the gusts they refer to factor the breeze to add to the predicted wind strength.
For example if you sail in the channel near the coast when the forecast wind force is low (1-2) the SSW breeze kicks in after 11am and can go up to an F4-F5 by the afternoon. So in case of a forecast for a SW F5 wind (wind generated by large pressure systems not local breeze), the breeze that adds to it can take the force to F8. However since breezes are local fenomena they cannot be predicted with certainty (although local knowledge helps) and the forecaster notes those as "gusts".

These gusts can be brief in squally conditions (clouds mitigate the condition to form a breeze by shading the land mass) when a weak cold front is passing through, but with clear sky (when the breeze conditions are at their best) they can last for hours, at least until the sun sets.

You may have noticed as well that spring and summer forecasts factor gusts more often, which is when the difference in temperatures between land mass and sea are more significant and favorable to generate local breezes.

To clarify:

Wind is the movement of air masses generated by large scale pressure systems.
Breeze is the movement of an air mass generated by a local fenomenon, like the difference in temperature between sea and land masses.
 
Last edited:
Racer tend to be fully crewed so carry sail for prevailing conditions and spill wind in the gusts. Cruisers reef for the gusts.
Just remember, any fool can be uncomfortable.

I sail to enjoy it, not to prove anything. And even then I'm in a constant state of what I call 'healthy paranoia'

If you're not happy, don't do it!
 
Last edited:
Getting back to the forecast gusts and where these figures come from, it does seem unlikely that the mean wind speed for this upcoming weekend in the SW is around 13-15 knots, yet gusts of close to 30! This can happen with an unstable air mass I guess, but surely the HIGH would make this unlikely. So where do these "gusts" come from? Are they even real or the fantasy of a computer model? Any meteorologists care to comment...
 
Some thoughts on gusts

I am probably teaching several grannies to suck eggs, I know, but wind is never steady; just how variable it is depends on a number of factors. The main one is the instability of the air but land will cause gusts simply by causing small scale turbulence and there can be some unusual effects particularly to the lee of high ground.

Generally, the most severe gusts occur when there are large vertical movements of air and air is rapidly descending to the surface. Partly, this air will have some speed because of the higher speeds where it originated but more speed because of conservation of energy. This is the same as when a falling stone loses height (potential energy) but gains speed (potential energy). This is known as conservation of energy. Those with long memories and who did a little physics at school may remember the equation –
mgh + 1/2mVxV. = constant. (sorry, I do noy know how to show V squard)
A stone of mass m at height h has potential energy = mgh. If the stone falls through that height, it loses its potential energy and gains kinetic energy = 1/2mVxV ie it loses height but gains speed.

When the air is unstable, mixing occurs through a great depth. Sometimes, rising air will create descending ait to compensate. If the mixing is through the lowest 4 km or so, as can happen, then gusts in clear air will be very strong.

If, as is very likely with such unstable air, there are heavy showers and thunderstorms, then the falling rain will drag air down with it. The cold strong blasts that are precursors of showers are due to that effect. As the large drops fall quickly, the air dragged down will also be moving quickly and will spread around the shower cloud.

Squall lines are like mini- but very active cold fronts. They are areas where there is very strong convergence of air and so, very strong up-currents leading to strong down currents. There will also be the effect of falling rain or hail.

How do Numerical weather models predict these effects? Basically, they probably use algorithms that calculate the instability of the air and use some form of statistical relationship to give a guess at the gust speeds. This is a routine output from the GFS and can be seen on zyGrib, for example. XCWeather and others will be extracting the same information. One of the more useful parameters on zyGrib is CAPE. This is an index of convection. I do not understand the numbers but, looking at the patterns, the bright red areas are obviously where there is strong convection, therefore thunderstorms are likely and strong gusts.

If you listen to French forecasts on VHF or get the tests from http://weather.mailasail.com/Franks-Weather/Northwest-European-Marine-Weather-Forecast-Texts, for example, you will always see the words – “Attention: en situation normale, les rafales peuvent être
supérieures de 40 % au vent moyen.” I need not translate! True enough, but not very helpful to add to each and every forecast. In fact, in extreme cases, gusts may be double the mean speed.

Where does all this leave us? Cold, unstable showery air will be gusty; thundery weather will probably be gusty, even if only from downdraughts; cold fronts or troughs often shown just as an anonymous line on synoptic charts may well have gusts. Warm sectors and air around highs are both fairly likely to have much steadier winds.
 
Thank you Frank...excellent. That now completes my egg-sucking knowledge :D

Of course going back to my point of why these large gusts in settled conditions? Well it may be the LOW over France squeezing up against the HIGH I guess. But at least I know what the models are trying to do now.
 
Bluedragon

There is rarely a quick or simple answer in meteorology-. One possible answer to your query might be a down-draught from a suoer-cell. On one occasion some years ago, we were sailing quietly along approaching les Sable d'Olonne. It was rather hazy and apparently settled. A swim off the boat before entering a hot marina was what we most wanted.

Suddenly with literally no warning we had a strong blast of hot dry air. All thoughts of a nice swim to cool down rapidly disappeared. When I asked former colleagues in Météo France, they said that it was a vigorous, isolated thunderstorm. It was so hazy the we saw nothing of the storm and heard no thunder. I only had their word for it.
 
Some areas are much more prone to gusts and/or violent changes in wind strength than others. The one which has the bad reputation down our way is Cap Bear. On more than several occasions I have gone from light winds to a steady F7 in a matter of minutes and also the other way around. Weather forecasts for the area don't always get it right (but do quite often).

On one occasion we have had a hard beat up round Cap Bear in a gale to hear another yacht coming down from France call up Cap Bear signal station and ask for the forecast for the area. They read the meteo France forecast out - slight seas and very light winds. At the end they did add "oh by the way, it is blowing a gale outside here at the moment". Sure enough once around the headland the wind disappeared, and we started the engine.

Also sometimes around this coast line you end up with areas where the wind blows downwards instead of horizontally. Most disconcerting - the sails are pulling well, you have the gunwhales under water and are hardly moving.

Cap Bear has the reputation for capsizing catamarans. My neighbour in the marina won't go near Cap Bear with his Cat, although he thinks nothing about heading straight to the Balearic Islands from our home port.
 
Another disconcerting experience is to go straight through the centre of a tight deep low. The last time this happened to me in winds hitting 45 knots, and at night, with no moon, the flag wound itself round the flagpole 11 times just within a period of barely one hour.:eek:
 
My view would be:

Does sthe boat have the sail plan, built and equipment to be out in the max wind speeds forecast plus some more. If not then stay on the mooring and entertain your friends instead.

If you have a reason and need to make good progress at a pace, then be prepared for attentive sailing and put everything way and hope you have a a young enough body to cope with being jerked around somewhat. On a monohull set your sail plan as if racing, then enjoy.

If you want to take it steady and be out there going somewhere, then set a conservative sail plan and be prepared for rather wallowing slow journey. Which is just fine if you have prepared your sea legs. On a Cat or Tri the motion might be better, and the pace still reasonable.
 
Channel Sailor said:
My view would be:

Does sthe boat have the sail plan, built and equipment to be out in the max wind speeds forecast plus some more. If not then stay on the mooring and entertain your friends instead.
Click to expand...

Down here you could be in for a nasty surprise. Max wind speed forecast F2. Reality F7. :eek:

If you have a reason and need to make good progress at a pace, then be prepared for attentive sailing and put everything way and hope you have a a young enough body to cope with being jerked around somewhat. On a monohull set your sail plan as if racing, then enjoy.

If you want to take it steady and be out there going somewhere, then set a conservative sail plan and be prepared for rather wallowing slow journey. Which is just fine if you have prepared your sea legs. On a Cat or Tri the motion might be better, and the pace still reasonable.
Click to expand...

So what do you do on a Cat down here? Reef down for a gale when the sun is shining and you have F2/F3 and the forecast is for a max of F3?

Or do as my neighbour does with his Cat and completely avoid certain areas altogether?
 
michaelchapman said:
Some areas are much more prone to gusts and/or violent changes in wind strength than others. The one which has the bad reputation down our way is Cap Bear. On more than several occasions I have gone from light winds to a steady F7 in a matter of minutes and also the other way around. Weather forecasts for the area don't always get it right (but do quite often).

................
Click to expand...

Like many such headlands, Cao Béar is notorious. Cap Sicié is another that often gets mentioned by CROSSMED forecasts in the same way that some headlands around our coasts get mentioned by name or, more usually, simply just as headlands..

An experienced, prudent sailor will know what happens in his or her local area but then be able to translate that knowledge to other areas. It is this kind of thing that I mean when I talk about the need for sailors to use any forecast with a mixture of experience, nous and commonsense. The last being the most important.
 
You must log in or register to reply here.

Share this page

Top