savageseadog
Well-Known Member
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Racing: How does air/wind move round showers please?
- Thread starter savageseadog
- Start date
ProDave
Well-Known Member
I assume you are talking about rain showers.
All I can say is a rain shower will either bring more wind, or will bring dead calm. I've experienced both.
All I can say is a rain shower will either bring more wind, or will bring dead calm. I've experienced both.
TSB240
Well-Known Member
I was always told if you see a big black cloud go to the right of it as much as possible and as it passes through tack to get a lift on both tacks.
Cant tell if it works, cant tell you what the theory is, but I expect some expert will come along and tell you it is:
Due to the rotation of the earth
Due to the rotation of air columns under clouds.
Due to the Jet Steam moving or not.
Due to MMGW.
Cant tell if it works, cant tell you what the theory is, but I expect some expert will come along and tell you it is:
Due to the rotation of the earth
Due to the rotation of air columns under clouds.
Due to the Jet Steam moving or not.
Due to MMGW.
savageseadog
Well-Known Member
I was always told if you see a big black cloud go to the right of it as much as possible and as it passes through tack to get a lift on both tacks.
Cant tell if it works, cant tell you what the theory is, but I expect some expert will come along and tell you it is:
Due to the rotation of the earth
Due to the rotation of air columns under clouds.
Due to the Jet Steam moving or not.
Due to MMGW.
I assume then the shower will be a small low pressure area and will therefore rotate anticlockwise in the Northern hemisphere thus giving a lift on the side that would be on the yacht's port side.
AntarcticPilot
Well-Known Member
Surely it will depend on the nature of the rain shower. Some will be like mini storm cells, with low pressure in the middle, so there will be the usual circulation of air round a low pressure. Others will be associated with frontal systems. I doubt there is a general rule. We need SimonJK for a more rigorous explanation! But my guess is that there isn't a general rule.
savageseadog
Well-Known Member
That's why I asked the question. At the extreme end there are thunderstorms, where air might possibly rush towards it from all directions. There's heavy showers where cold air rushes down from high altitude and propagates in all directions (I think) and lighter showers?
MissFitz
Well-Known Member
Don't want to make a twit of myself by paraphrasing, possibly incorrectly, other people's wisdom but would thoroughly recommend the below - contains detailed answer to your question & every other question you might have about wind for racing (lake, inshore, offshore, whatever). Exceedingly grateful to the forumites who recommended it to me!
http://www.amazon.co.uk/Wind-Strate...5124/ref=sr_1_1?ie=UTF8&qid=1343400382&sr=8-1
(Ok, I'll give it a quick go then - wind goes into cumulus clouds that aren't raining, comes out of those that are - so advice is, if you see a cloud that is raining or about to rain, sail towards it, you get a lift on the way & a lift again when you get closer.)
http://www.amazon.co.uk/Wind-Strate...5124/ref=sr_1_1?ie=UTF8&qid=1343400382&sr=8-1
(Ok, I'll give it a quick go then - wind goes into cumulus clouds that aren't raining, comes out of those that are - so advice is, if you see a cloud that is raining or about to rain, sail towards it, you get a lift on the way & a lift again when you get closer.)
Last edited:
grumpy_o_g
Well-Known Member
Disclaimer: All the below is just opinion and observations as it's well over 20 years since I taught this stuff.....
Thermals at least do rotate so I guess rain showers do as well as they're the same thing. It shouldn't just be rain-showers you should be thinking of but any thermal really. Whether it makes a difference if you fly against the rotation of the thermal is the glider pilots equivalent of "should you let your prop freewheel" and causes just as much argument whilst making just as little difference in real life. It's pretty much agreed that you can't predict which way they'll rotate though - and the rotation is fairly slow too unless you've got a very strong trigger point such as dust devil - in which case the rotation is fast but only localised - in general the bigger the thermal the slower the rotation.
You might want to take a trip to your local gliding club on a day when there's a few thermals around and get them to take you up and then get one of their instructors or serious cross-country pilots to give their opinion. I'd agree with Antarctic Pilot for what it's worth - I don't think there's a rule you could rely on. What I would say is that the base of a thermal is a long way upwind of the cloud it's created and you will need to allow for windshear as well. If there's a lot of thermals around then you may struggle to understand what effect a particular cloud is having at any particular point at ground level. A reasonable thermal will rise vertically at maybe 4 knots so it will take 10 minutes or so to get to a 4,000' cloudbase and it's quite common to get underneath a cloud and find the thermal has finished down where you a re whilst others are still climbing away smuggling above you. In general when you travel between thermals in a glider you will go through area of air that is neither climbing or sinking (just travelling horizontally as per the wind) - though any pilot would swear blind it sinks more than it rises. As you approach the thermal the air starts sinking at increasing rate and then, fairly quickly changes to rising air. The strongest lift is under the cloud in the centre of the thermal and the change from still to sink to lift seems to be quicker at the upwind edge of thermal. Unless something else is acting on it the air will simply flow horizontally at a lower level into the thermal, rise, then spill out the top and fall down the outside in what is often called the "doughnut model". This link is a well-used text and is written totally for glider pilots but there's a few good diagrams in there to explain the predicted airflow in thermal http://www.aviationweather.ws/095_Thermal_Soaring.php
The lift is stronger the nearer cloudbase you are - over buildings, hilly ground, etc., there's a boundary layer of a few hundred feet or more where the flow is very broken. Wind sheer is typically 30-45 degrees over 1,000 feet vertically (at over 1,000' it's getting roughly parallel to isobars on a chart). Over the sea, with a smoothish surface and a fairly uniform temperature, that boundary layer is a lot thinner and the really big boys (100' + loa) with tall rigs will often have the the top of the rig in a windflow with a substantially different direction to the foot, which must make life interesting.
Once you start talking CuNims the top cover tends to kill any activity around them, hence the calm before it arrives. When it does hit, it hits big time but they can be far worse on the coast where there's often local geography to channel the effect and multiply it.
It's all very simplistic though - in theory the same behaviour is shown by a cumulus or even blue thermal and whether or not it's managed to generate a cloud that produces rain that reaches ground level is irrelevant I'd have thought. Remember the rain is probably not falling straight down the thermal of course. If you're in the rain the base of the thermal may well be a distance away.
Thermals at least do rotate so I guess rain showers do as well as they're the same thing. It shouldn't just be rain-showers you should be thinking of but any thermal really. Whether it makes a difference if you fly against the rotation of the thermal is the glider pilots equivalent of "should you let your prop freewheel" and causes just as much argument whilst making just as little difference in real life. It's pretty much agreed that you can't predict which way they'll rotate though - and the rotation is fairly slow too unless you've got a very strong trigger point such as dust devil - in which case the rotation is fast but only localised - in general the bigger the thermal the slower the rotation.
You might want to take a trip to your local gliding club on a day when there's a few thermals around and get them to take you up and then get one of their instructors or serious cross-country pilots to give their opinion. I'd agree with Antarctic Pilot for what it's worth - I don't think there's a rule you could rely on. What I would say is that the base of a thermal is a long way upwind of the cloud it's created and you will need to allow for windshear as well. If there's a lot of thermals around then you may struggle to understand what effect a particular cloud is having at any particular point at ground level. A reasonable thermal will rise vertically at maybe 4 knots so it will take 10 minutes or so to get to a 4,000' cloudbase and it's quite common to get underneath a cloud and find the thermal has finished down where you a re whilst others are still climbing away smuggling above you. In general when you travel between thermals in a glider you will go through area of air that is neither climbing or sinking (just travelling horizontally as per the wind) - though any pilot would swear blind it sinks more than it rises. As you approach the thermal the air starts sinking at increasing rate and then, fairly quickly changes to rising air. The strongest lift is under the cloud in the centre of the thermal and the change from still to sink to lift seems to be quicker at the upwind edge of thermal. Unless something else is acting on it the air will simply flow horizontally at a lower level into the thermal, rise, then spill out the top and fall down the outside in what is often called the "doughnut model". This link is a well-used text and is written totally for glider pilots but there's a few good diagrams in there to explain the predicted airflow in thermal http://www.aviationweather.ws/095_Thermal_Soaring.php
The lift is stronger the nearer cloudbase you are - over buildings, hilly ground, etc., there's a boundary layer of a few hundred feet or more where the flow is very broken. Wind sheer is typically 30-45 degrees over 1,000 feet vertically (at over 1,000' it's getting roughly parallel to isobars on a chart). Over the sea, with a smoothish surface and a fairly uniform temperature, that boundary layer is a lot thinner and the really big boys (100' + loa) with tall rigs will often have the the top of the rig in a windflow with a substantially different direction to the foot, which must make life interesting.
Once you start talking CuNims the top cover tends to kill any activity around them, hence the calm before it arrives. When it does hit, it hits big time but they can be far worse on the coast where there's often local geography to channel the effect and multiply it.
It's all very simplistic though - in theory the same behaviour is shown by a cumulus or even blue thermal and whether or not it's managed to generate a cloud that produces rain that reaches ground level is irrelevant I'd have thought. Remember the rain is probably not falling straight down the thermal of course. If you're in the rain the base of the thermal may well be a distance away.
Jamesuk
Well-Known Member
Awesome reply Grumpy!! But can we have it as a you tube video please ;-)
I try and understand the weather, i am still waiting for the Eureka moment tho.i am getting there with localised low pressure systems and such but with my head in mastervolt manuals and hydraulic servicing intervals i get little time these days to do what i enjoy, racing yachts/ dinghies.
I try and understand the weather, i am still waiting for the Eureka moment tho.i am getting there with localised low pressure systems and such but with my head in mastervolt manuals and hydraulic servicing intervals i get little time these days to do what i enjoy, racing yachts/ dinghies.
Searush
Well-Known Member
You shouldn't be wasting time in the shower when you should be hanging off the side of the boat in your wraparound sunglasses. 
And breaking wind in an enclosed cubicle also seems a bad idea.
And breaking wind in an enclosed cubicle also seems a bad idea.
simonjk
Well-Known Member
Okay, here goes...
A typical shower cloud forms by air rising, cooling and condensing (a process we see as the final cumulus or cumulonimbus cloud).
Now, air rising wthing these clouds 'removes' air from the surface, and so pressure here falls. As a result, the air away from the cloud is now higher. As air flows from high to low pressure, air flows into the 'hole' left by the shower cloud, which in turn also rises and hence the cycle continues.
In a large shower cloud, or supercell, it is possible to identify rotation within the cloud in exactly the same way as in a standard area of low pressure. Given enough observation one can also identify warm and cold front!
So, winds in a large shower cloud attempt to rotate in an anticlockwise direction in the northern hemisphere. HOWEVER life is not that simple is it? Because the scale of the 'low' is far smaller than in a standard surface depression, factors such as coastlines, large thermals, temperature gradients can have a significant impact on the dynamics of the cloud and hence the wind within it.
As others on the thread have noted and observed, there are gusts in and around the showers as air is both sucked in and 'blown out' of the cloud.
Reading the observations made by others has been really interesting, and I'd be delighted to see more of what you have experienced in and around showers.
Phew, sorry for the long reply.
Simon
A typical shower cloud forms by air rising, cooling and condensing (a process we see as the final cumulus or cumulonimbus cloud).
Now, air rising wthing these clouds 'removes' air from the surface, and so pressure here falls. As a result, the air away from the cloud is now higher. As air flows from high to low pressure, air flows into the 'hole' left by the shower cloud, which in turn also rises and hence the cycle continues.
In a large shower cloud, or supercell, it is possible to identify rotation within the cloud in exactly the same way as in a standard area of low pressure. Given enough observation one can also identify warm and cold front!
So, winds in a large shower cloud attempt to rotate in an anticlockwise direction in the northern hemisphere. HOWEVER life is not that simple is it? Because the scale of the 'low' is far smaller than in a standard surface depression, factors such as coastlines, large thermals, temperature gradients can have a significant impact on the dynamics of the cloud and hence the wind within it.
As others on the thread have noted and observed, there are gusts in and around the showers as air is both sucked in and 'blown out' of the cloud.
Reading the observations made by others has been really interesting, and I'd be delighted to see more of what you have experienced in and around showers.
Phew, sorry for the long reply.
Simon
savageseadog
Well-Known Member
You shouldn't be wasting time in the shower when you should be hanging off the side of the boat in your wraparound sunglasses.
And breaking wind in an enclosed cubicle also seems a bad idea.
Always shower with a friend, preferably with no wind.
jimbaerselman
Well-Known Member
Great post Grumpy. Glider pilots stay airborne with this sort of knowledge.
Luckily, sailors don't sink if they don't have a feel for this detail!