...
So, from the data we know that:
- There are set "target" maximum (daytime) and minimum (nighttime) temperatures.
- Heating is either sinusoidal or logarithmic, but cooling is linear.
- Daylight hours are a bit whack; if the hot edge of the map gets 16 hours of daylight, even if it's only at the summer solstice, that would mean the equator is actually still quite a ways off, and probably quite !!FUN!! (what's called a scorching desert in DF would probably feel refreshing after a walk at the equator).
I realized some erroneous assumptions/conclusions in my previous post (the short list of conclusions is above, but there's some other stuff too). First of all, cooling in itself isn't necessarily linear, but the effect of lower insolation late in the day (as the sun is setting) and the cooling has a total effect that looks linear. Or my new guess is that maybe the sun sets at around 13 hours of daylight (the cooling isn't completely linear before that, showing that there could still be some small heating), after which cooling is linear as speculated earlier. So the daylight hours are also a bit shorter at the South map edge, but still about an hour longer than expected for the equator, which should be a few minutes over 12h year-round (google e.g. "length of day Quito" or Singapore or something, check January&June). Also found
this nice visualizer. 13 hours at summer solstice corresponds to about 17 degrees north, which would mean that the equator would still be somewhat habitable: at worst, only about as bad as the existing scorching climates.
As a side note, It's kinda annoying that the plateaus are so big, with enough accuracy (1 degree F or U should be enough, I think), there should be a curve visible over the whole day, or at most a couple of hours of even temperatures. In real life, you need averages e.g. off the same day every year for several years to get rid of weather effects, of course. Also, lack of a clear peak/trough means it's harder to pinpoint what kind of delay specific heat capacities introduce to the system.
Readings from a desert/rocky badland vs. ocean on the same latitude would be interesting, since it might help isolate the effects of latitude vs. surface material. The desert reading definitely suggests my "target temperature" claim from the quote above, since barring statistical flukes due to weather, you wouldn't get plateaus that flar in real life, pretty much ever, at least not at tropical latitudes. So yea, the temperature model is "broken", or to put it another way, pretty simple, and doesn't fully take into account how high temperatures can reach during the day and so forth. My guess is that the max. temperature plateau might actually a safety feature for gameplay, to prevent dwarves from melting in scorching climates. The low temperature plateau is probably a "minimum ambient temperature" defined by latitude and/or biome, that cooling occurs to.
Regarding the temperature plots from Phoenix, yes, temperature does flatten out a bit especially on warm days, but I'm not totally convinced by the examples yet, either. Like I've said, one, or a few, days do have slight weather fluctuations even in a desert with little to no "weather" as usually defined in temperate zones (rain, clouds, etc.). Also, the y-scale is really flat in those plots, leading to an illusion that the temperature is more even that it is. Possibly.
Oh, and we have different energy units because the calorie was probably invented first; it's pretty easy to define experimentally, you just need a heat source, a thermometer, and a scale (to measure how much water there is), and a closed water container to prevent evaporation. However, it's not a neat, SI, base-10 unit, unlike the Joule. Why are there (in the imperial system, of which "you Americans" are the last big die-hard users) so many arbitrary units of distance/time? At least the calorie is somewhat scientifically defined.
...and finally, regarding the 2nd sand desert plot, yea, I'd say the "hump" is a feature of the heating model. Either the insolation function only approximates a sine curve with linear segments, or the temperature curve is formed directly, and the slope of temperature increase is just set to drop by some % 3 hours after sunrise.