Article:

To shift or not to shift………
First of all, a little bit of thought about definitions and understanding meaning of strategic elements. 

"All persisting shifts are oscillations" and "Oscillations are persisting shifts"! It's all a matter of perspective or more precise; a scale problem.

In both cases we are talking about a wind shift in a defined period of time, but as the time of the shift is relative to the time of the race or, even better, the time of a leg ( for instance one upwind) This doesn't helps us any futher in dealing with it on the water so we need another criteria to deal with the problem.

In my basics about strategy I use the following index for the several systems and their scale of influence. (figure 1)
With that it automatically comes down to the total time of a regatta, race or leg, whether we can say that something is oscillating or not. For instance the movement of a continental system like a low pressure area could for the guys on a Volvo Ocean Race leg very well be a oscillating shift be over several days. But for the little club racer on the Lake in the Queens country be a very persistent shift to the left during the racing Sunday, but never realizing the righter back on Monday, when he is back in the office. Of course this principle also goes on an smaller scale let's say one 20 min upwind. 


So I tend to use this layer model for myself to get a grip on what's going on when I'm preparing a race. 
-Most of us don't need to worry too much about the intercontinetal systems but they sure can be interersting sometime when you're planning a Trans Atlantic crossing or so!! 
-But knowing what the nearest low or high pressure areas are going to do over the next 6-12 hrs is defintely worthwhile knowing. 

So wake up early let's say 6 am. (I know it's early but there won't be any thermic system working yet). And have good long look at the clouds. What shape they have. How high they are How thick they are. In what direction they move. How fast they go. What visibility is over the water etc.. 
Then check the weather forecast and try to get youre hands on the latest analsis chart in surface and at best a vertical wind and temp reading from a near local station or airport. This is your reference and if you can also get yourself into noticing the direction in which the smoke from the factory you just saw the other day from your boat while sailing back in. you'll actually will have more chance of observing the chance that are about to come.
Combine that with for instance a weather forcast telling you in which direction this low is actually moving away from you (or whatever way it's moving). It will be actually be very easy to find out in which direction the gradient wind (the wind blowing from one pressure area to the other) will turn and if it's going to increase, decrease or hardly chance at all over the day.

Local Effects

Once activity in this continental layer is known, we know what is going to happen if there would be no lokal effects. Which I must confess is not ever the case. 
Since most short course racing is always taking place within the vincinity of land there will always be temperature differences and so there will always be lokal effects. 
And since these effects are depending on temperature difference, their time of occurrence, duration and scale are limited compared to the "Big" continental systems. Therefore they could be defined as an oscillating shift because the direction changed because of this effect and turned back once the sun has left us.

Some of the Lokal effects have relatively long live span and some don't. 
If we look at the lokal effect convection. It can happen that the temperature difference and humidity between surface, bottom air layer and upper air layer are so organised, that with two hours of blue skies and sunshine at 11:00 hrs in the morning, can result in a very shifty (shifts of about 5 - 30 secs) morning breeze and a slow development of the thermal afterwarts. If that same two hours of blue sky and sunshine would appear let's say at 8:00 hrs in the morning one will never have this shifty period of breeze but still have the slow development of the thermal afterwarts.

So what I hope to make clear is that before we can talk about persistent or oscillating we need to know what systems are responsible for the airflow in the first place before we can actually say something about the time frame in which wind shifts might actually occur.

Next time I'll be more in the boat with you and give you another model of dealing with this sh*&^&ty wind..

Good winds!

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