Images in the Sky
Ever since humans started sailing they had to forecast changes in the wind in order to help them navigate and be prepared for lack of wind or a big storm and try to avoid it. They used many methods but one remains critical and that is the effect of clouds or in other words, what does the images in the sky tells us.
In modern racing we are often demanded to become short term weather forecasters to help us predict changes within the next 10-20 minutes that will help us to get an advantage over our competitors and sail the course quicker than them.
There are several types of clouds: high clouds usually gives us an indication of days to come, low clouds (clouds generated on the boundary layer) will affect us over hours or minutes.
Since low clouds will effect over a short term we are mainly interested in those during a racing day as they will help us predict changes that will come into effect during our race and may alter our plan completely.
Low clouds comes in different forms and sizes and can be looked at as two main types: A sucking cloud – This is a cloud that is condensing and sucking air into it, and a blowing cloud – a cloud that is dumping wind away from it.
How can we tell the difference between the two, It is actually much easier than you would think. Usually when a cloud is sucking air it means it is condensing as we speak, that means the cloud will be getting bigger, taller, and turn from white to gray, As long as it keeps collecting energy into it, it will be sucking wind towards, we will shortly discuss the effects on the wind we feel.
Blowing clouds are usually rain clouds, when a cloud is raining it is losing energy hence spreading it around it, when clouds are gray/black color we can assume they will be dumping wind, when a cloud is getting smaller it is losing energy hence dumping wind away.
Clouds may also have a cooling effect on the surface and influence thermal flow (usually in sea breeze)
We know this is a generalized description but as dinghy sailors this differentiation will give us the right information to make good decisions.
Let’s discuss how the different cloud types will affect us over the race course
The easiest one is clouds created over land by a sea breeze, these clouds will be condensing pretty quick and eventually prevent the surface from heating up, once the cloud covers a big enough patch of land it will block the hot air from rising by cooling the surface down and cause the temperature difference between the sea and the land to become smaller which in turn will cause the wind to drop in velocity. Depending on it’s position relative to us it will also cause the wind to shift
Dumping clouds are next in line. As mentioned above if a cloud is raining or very dark it will be dumping wind, the wind will be dumped at 90 degrees to the surface of the cloud
Depending on the direction the cloud is moving and it’s relative position to us we can predict how it will affect the wind over a short period of time. For example if the cloud is moving from our left to our right we can predict the wind to first shift left and increase as the cloud is getting closer and drop and shift right after the cloud has passed us to the right
Sucking clouds are less obvious to understand as we may read their color wrong, once we acknowledge a cloud is condensing and sucking energy into it, it will be sucking air into it exactly the opposite a dumping cloud will dump wind, that means it will be sucking wind all around it. You may be surprised by how quickly a cloud can suck wind into it and increase in size.
Similar to the dumping clouds depending on its relative position to us it will influence the wind we may encounter over the race course. For example if a cloud is being formed behind us and to the right, as long as it is forming the wind might have a tendency to increase and shift left, beware since it will all be reversed as soon as the cloud starts dumping wind
Gradients: A gradient is a line of clouds formed over an isobar, It will usually move towards us in some form, depending on its distance, rate of advancement, relative bearing to us and the relative angle of the gradient to us we can predict how the wind will behave, A gradient will be dumping wind at 90 degrees to it. For example if we see a gradient on the distance to our right at a 45 degree angle to us getting closer we can predict the wind to be shifting up to 45 degrees as soon as it is close enough to influence.
The ability to keep track of these changes and assess the distance a cloud will start influencing us takes training, it takes failing and it takes continuously observing our environment.
During our training at Toplevel Sailing we ensure to “hammer down” those observation skills to our sailors and coaches, coupled with the understanding of how the different types of clouds will affect the behaviour of the wind.
My apologies for the drawings, they are better than the ones I draw on the board and not as half as bad as Pete’s drawings!
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