Add PRESSURE_MSL magic constant

lib/xmet/skew_t.py

@@ -22,7 +22,7 @@ TEMP_CENTER     =   0 # degrees C
 TEMP_STEP       =   5
 TEMP_STEP_COUNT = math.ceil(TEMP_RANGE / TEMP_STEP)
 
-SKEW = 0.7
+SKEW = 0.6
 
 def clamp(value, lowest, highest):
     if value < lowest:

lib/xmet/thermo.py

@@ -8,6 +8,7 @@ LAPSE_RATE_DRY   = 9.8 / 1000 # degrees C per km
 LAPSE_RATE_MOIST = 4.0 / 1000
 
 PRESSURE_MAX  = 1050 # millibar
+PRESSURE_MSL  = 1013.25
 PRESSURE_MIN  =  100
 PRESSURE_STEP =   50
 
@@ -68,12 +69,12 @@ def lcl(temp: float, dewpoint: float) -> float:
     """
     return (temp - dewpoint) / 0.008
 
-def pressure_height(pressure: float) -> float:
+def pressure_height(pressure: float, surface: float=PRESSURE_MSL) -> float:
     """
     Return the approximate altitude, in meters, for a given pressure in
     millibar.
     """
-    return (1 - (pressure / 1013.25) ** 0.190284) * 145366.45 * 0.3048
+    return (1 - (pressure / surface) ** 0.190284) * 145366.45 * 0.3048
 
 def lapse(temp: float, delta: float, rate=LAPSE_RATE_DRY) -> float:
     """