Hi Thomas,
Sorry to rehash an old thread, but I was looking though your wxformulas.py 
for more info on the ET calc.  As you mentioned, ET used to be calculated 
as a rate but now is just a measured quantity, like rain.  However, the 
calcs and the examples in wxformulas.py still reference a rate.

For example, line 687:
Returns: Evapotranspiration in mm/hr
Example (Example 19 in the reference document):
>>> sr_mean_wpm2 = 680.56 # == 2.45 MJ/m^2/hr
>>> timestamp = 1475337600 # 1-Oct-2016 at 16:00UTC
>>> print "ET0 = %.2f mm/hr" % evapotranspiration_Metric(Tmin_C=38, 
Tmax_C=38, rh_min=52, rh_max=52,
... sr_mean_wpm2=sr_mean_wpm2, ws_mps=3.3, wind_height_m=2,
... latitude_deg=16.217, longitude_deg=-16.25, altitude_m=8, 
timestamp=timestamp)
ET0 = 0.63 mm/hr
Another example, this time for night
>>> sr_mean_wpm2 = 0.0 # night time
>>> timestamp = 1475294400 # 1-Oct-2016 at 04:00UTC (0300 local)
>>> print "ET0 = %.2f mm/hr" % evapotranspiration_Metric(Tmin_C=28, 
Tmax_C=28, rh_min=90, rh_max=90,
... sr_mean_wpm2=sr_mean_wpm2, ws_mps=3.3, wind_height_m=2,
... latitude_deg=16.217, longitude_deg=-16.25, altitude_m=8, 
timestamp=timestamp)
ET0 = 0.03 mm/hr 

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