Wind I: Spirit of the Wind

I love what Here Dragons Abound did with wind. I want to do that, perhaps adding it to my pressure model.

Mainly, this will help with my move away from hand-drawing the rain. If I know where the wind is blowing, I can model the rain, yielding better results.

Once again, the primary source of my frustration is the sector rotation. It's hard to think in polar coordinates sometimes.

I applied a constant wind at an angle of 135$^\circ$, just for testing. The velocity is 25 knots, which is pretty high. I found a source showing a sea breeze strength of a max of 5 knots. No matter.

There are a few more rules for spreading the wind across land:

• Every time a wind is propagated, it loses 10% of its strength
• If it goes downhill, it gains some of that back depending on the slope
• If it goes uphill, it's deflected up to 90 degrees. I use a logistic function to make a nice smooth transition, using $K=90$, $P_0 = 1$, and $r={\ln(89)\over 2400}$. The rate is one of the settings I'm playing with to get a good result. The 2400 is the change in altitude where the deflection angle is 45$^\circ$. So if I want winds to make it higher up a slope, I make this number bigger.
• I talked about vector projection/rejection here, and I found that the best model for the wind is to use this to project the wind along the direction of the target hex. This "splits" the wind between the three hexes it's pointed toward the most.
• Everything gets added up using vector notation. At first I thought this propagated stuff forward too much but it seems ok after a few bugs were worked out. And I'm not using real data so that can affect things.

The wind barb notation shows the direction and strength of the local wind:

It's off to a good start. I'm going to think a bit more about the pressure systems so that maybe I can figure out how to integrate those into the model. There's also much tweaking to be done to get results I'm happy with.

Once that's done, I can move on to pushing moisture around.