Weather values are instantaneous:
#CFHT/GEMINI Weather Tower Data
2008/12/01 19:26:41 #Date/time of acquisition
WS=    8.42   #Wind speed (knots)
WD=  307.66   #Wind direction (degrees) (0-360)
T =    1.02   #Temperature (degrees celsius)
RH=   44.71   #Relative humidity
BP=  615.56   #Barometric pressure (millibars)