GRW/SKYRAD/M1 - Short direct normal questionable

Starting on 05/08/2010 values of short_direct_normal from the NIP instrument were reading 
low (up to 50W/m2).  This was most apparent during sunny/partly sunny periods.  The 
values spontaneously returned to normal on 5/14 with no Ops intervention.  Data system checks 
after that date revealed no apparent problems with instruments or datalogger. The problem 
has not recurred.

Only the short_direct_normal values during this time period are suspect.  If direct normal 
values are needed they can be re-constructed using the following relationship for each 
minute of data:
down_short_hemisp = short_direct_normal * cos(zenith angle) + down_short_diffuse_hemisp

• Instantaneous direct normal shortwave irradiance, pyrheliometer thermopile
  voltage(inst_direct_normal)

• Shortwave Direct Normal Irradiance, Pyrgeometer, Standard Deviation(short_direct_normal_std)
• Shortwave Direct Normal Irradiance, Pyrheliometer, Maxima(short_direct_normal_max)
• Shortwave direct normal irradiance, pyrheliometer, minima(short_direct_normal_min)
• Shortwave direct normal irradiance, pyrheliometer(short_direct_normal)