June 11, 2018

Rain, Rain, Go Away

I totally redid the pressure maps (several times) and figured out a way to discretize the values (so, getting the pressure at each hex) in a much more efficient manner (essentially, I used Pixelize instead of Hexify in Gimp and I discard any pairs that don't overlap bounding boxes when doing the comparison). From there, I redid the wind maps and finally was able to move on to the precipitation. A little behind schedule but that's ok.

I want to say a bit about orographic precipitation. As mountains rise, water bearing air is forced up the pressure column, and also undergoes cooling. This means that all the water in the air will drop out (so, rain). I've modeled this as an increase in the monthly precipitation based on the elevations (where $\Delta P$ is the change in rainfall and $z$ is the altitude):
\[\Delta P = \textrm{max}\left(0, 0.005 \cdot z - 7.6\right)\]
For the life of me I can't remember where I got these numbers so I may just play around with them until I get something I like. Most of the scientific literature on the subject deals with absolute pressures and precipitation rates, not necessarily easy to translate into additional millimeters per foot of altitude.

Rain also does not fall in months with an average temperature less than freezing (technically, it could fall as snow, but for the purposes of climate determination, only liquid is considered). I still have to redo the temperature maps, so that cutoff, like everything else here, is subject to change.

Winter season rain


  1. I saw this before on the other pressure map. Why is there a straight lined band around each hexisphere?

    Also, I've just started going through your work from the beginning, I am enjoying it thoroughly. I can't wait to get to the present.

    1. The coastline value for rain is a function of the latitude. Because at this point I hadn't quite figure out how best to propagate the rain, it just goes straight across the map, creating a band.

  2. oh I get it. thanks. Its interesting to see the process as its evolving.