We have all encountered this question in the past “What is wind?”.
As coaches we have also encountered many different answers to this, but usually the initial response is far from the very simple answer that wind is air movement from high pressure to low pressure.
That would lead to a further question; how is that pressure difference created? Again, the answer should be very simple when it comes to sailing, the pressure difference is created by difference in temperature between different surfaces. As Einstein once said “everything is relative”, so is the case regarding wind.
A warmer surface (warmer relative to neighboring surfaces) will cause the air above it to become lighter and raise up, which will cause the pressure on its surface to drop. That pressure will be filled up by air from any surrounding colder surface, when a surface is colder it will cause the air column above it to become heavier and fall down hence increasing the pressure, the air from the colder surface will flow towards the warmer surface and generate wind (in our language that would mean in many cases either a gust, a shift or both).
The bigger the temperature difference the stronger the wind would be.
How does it apply to our short course racing?
We can think of wind being created over a big scale, usually a weather system, typically this will measure between 1000 and 3,000 km across and be very predictable in its passage. This is what most forecasting is based on. Then we have the medium scale, which is maybe 100km to 300 km across, and will factor in major mountain ranges, typical sea temperature/land temperature differences, and even the thermal effects of big cities. This is the local forecast base for most models. Then we have our racing area forecast. This will be what we can see. Usually between 5 and 20 Km around us. To make this forecast we must develop both observation and meteo knowledge.
We must acknowledge that even though we have a prevailing wind (or dominant wind) we have various winds that are generated on a smaller scale, those winds will create gusts and shifts. When analyzing your racing area, you must pay attention to those factors that will allow you to estimate how is the wind going to behave next.
For example, a current will cause the water temperature to drop (the stronger the current the better the mix) that drop-in temperature will be generating more gusts from the side of the current. Mountains will have cold air falling off them, as will clouds. Clouds may also convect – the one thing for sure is that clouds WILL affect the wind!
Other factors to consider:
Topography of the surroundings, as well as surface color may also generate temperature differences
The angle of the sun (the wind in the morning will behave differently to the after noon)
Clouds shading will cool a surface and affect the wind. Sun heating will cause the air to rise and the wind to flow towards that area.
Gusts are also caused by air coming down from aloft. The air at the level of the lowest clouds usually travels at about three times the speed of the surface wind that we feel. Over land the direction of this wind in the Northern hemisphere is approximately thirty degrees to the right of the surface breeze. Over the sea it is approximately 10 degrees to the right of the surface breeze. Where the surface breeze meets the cloud base layer is called the boundary layer. When two winds of different speed are trying to flow along the same path, turbulence is created. This produces gusts. If the air spins straight down from the turbulence, then we get intense “dumping” gusts with big pressure differences. If the turbulence causes the air to spin off from the boundary layer at a less acute angle, we get gusts which are not as strong, but last a lot longer and travel slower across the water. These gusts are the biggest gain in sailing, usually lift on starboard in the northern hemisphere, and port in the southern hemisphere, and increase our chances of gain if we observe where the areas of greatest activity are on our course – almost invariably they will keep being the areas of greatest activity!
We will see different heights of the boundary layer (where the cloud sits) on any given day. Typically, the height will be between 200 and 500 meters, although anything is possible. The lower the boundary layer the more chance we will have gusts, this will also determine how fast a gust will move, lower layer means colder heavier layer (will commonly be accompanied and enhanced by clouds), so we can expect stronger and faster gusts, higher layer means warmer layer and we can expect lighter gusts and slower moving gusts.
Taking all those factors into consideration should help you as a sailor to determine what is the correct course of action at any point of time
At Toplevel Sailing we encourage development of meteorological knowledge and systematically develop a keen eye for observations among our sailors that will allow them to predict with a higher probability how the wind is going to change and plan the right course of action to take maximum advantage of it.