Cap windchill heat from vapor pressure to 50C

The model we use to approximate windchill seems to have degenerate
behavior at high "dry-bulb" temperatures; this manifests most obviously
by generating apparent temperatures in the millions of celsius for input
temperatures of ~1000C. Since most of this runaway extra temperature is
from the vapor pressure term, and since it seems unreasonable that a
given 1000C fire would actually have vapor pressure of 10MPa in its
vicinity, we cap that vapor pressure-induced extra temperature to only
100C.

src/weather.cpp

@@ -715,8 +715,14 @@ int get_local_windchill( double temperature, double humidity, double windpower )
         tmpwind = tmpwind * 0.44704;
         tmptemp = temp_to_celsius( tmptemp );
 
-        windchill = 0.33 * ( humidity / 100.00 * 6.105 * exp( 17.27 * tmptemp /
-                             ( 237.70 + tmptemp ) ) ) - 0.70 * tmpwind - 4.00;
+        // Cap the vapor pressure term to 50C of extra heat, as this term
+        // otherwise grows logistically to an asymptotic value of about 2e7
+        // for large values of temperature. This is presumably due to the
+        // model being designed for reasonable ambient temperature values,
+        // rather than extremely high ones.
+        windchill = 0.33 * std::min<float>( 150.00, humidity / 100.00 * 6.105 *
+                    exp( 17.27 * tmptemp / ( 237.70 + tmptemp ) ) ) - 0.70 *
+                    tmpwind - 4.00;
         // Convert to Fahrenheit, but omit the '+ 32' because we are only dealing with a piece of the felt air temperature equation.
         windchill = windchill * 9 / 5;
     }