The weather station reports "gusts" and regular wind speeds, but I think that just means "peak" and "average" for the one minute interval of each measurement. There's a reason to believe that these don't correspond neatly to what we think of as gust and lulls, as will be clear from looking at the data. However, the one minute interval is fast enough to notice some effects. Here goes.
- LimitedShifts.gif (17.39 KiB) Viewed 2040 times
The first graph is a scatter plot of wind speeds against direction on a day dominated by gradient wind (wind driven by large-scale pressure differences, as opposed to thermally induced ones, such a sea breeze). As you can se, as soon as the wind picks up, the direction aligns with a preferred value, in this case 210 degrees. During lulls, the wind is backed on average, but really mostly the direction isn't as well determined. The lulls and gusts don't divide according to the peak (Gust) and average (Wind) data sets. Both sets show the same relation to direction, except that peak values by their nature are a bit higher.
When sailing in this wind, it would seem useful to largely sail to the gust and not try to follow the temporary wind direction in the lulls. Particularly as the lulls will not propel you as forcefully as the gusts.
- GustWindShift.gif (16.51 KiB) Viewed 2040 times
The second graph shows quite different conditions from a different day. Here there is a strong correlation between wind speed and direction (again, the distinction between peak (G) and average (W) don't matter as much as the correlation for each. I've added an eyeballed fit for each of them and they are pretty similar. In sailing terms, with the wind from starboard, you get a lift of nearly 45° in the gusts, but the wind direction in the lulls is also predictable. That means you really want to play the windshifts.
If you are overpowered in the gusts and respond by letting out the main, instead of luffing up, you will give away a lot of upwind potential. The cause may have been a competition between gradient and thermal winds, with the gusts bringing the gradient wind to the surface. But whatever the cause, that day made playing the windshifts more rewarding than usual.
Note that this is different from the lift you get due to the gust shifting the apparent wind aft (because the contribution from the headwind is less, with the wind speeding up relative to the boat speed).