TWP/SKYRAD/C1 - Shaded vs. unshaded radiometer comparison

The daily average difference between the shaded and unshaded PIRs
reversed on 19980428.  Previous to then, the unshaded PIR was typically
about 2 Wm-2 less than the shaded PIR.  Since the 28th, the unshaded
PIR has been averaging about 4-5 Wm-2 greater than the shaded PIR.
This latter difference worsened for the period 5/25 to 5/31, when the
daily average difference increased to between 8-10 Wm-2.    The
disagreement is typically greater at night, reaching differences of
greater than 12 Wm-2 during the period 5/25-5/31.  The cause for the
switch in sign of the difference can most likely be attributed to
instrument replacement, however the (in general) larger absolute
disagreement after Apr 28th indicates a possible calibration problem.
The cause of the large discrepancies from 5/25 to 5/31 is unknown.

I noticed that the daylight differences increased slightly, with an
attendant slight decrease in the night differences. This daylight
difference is on the order of an additional 1 Wm-2.

Note: the unshaded PIR is incorrectly named "shaded2" during this
time period.  This is covered in a separate DQR.

• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer1, Minima(down_long_hemisp_shaded1_min)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer2, Minima(down_long_hemisp_shaded2_min)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer1, Maxima(down_long_hemisp_shaded1_max)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer1, Standard
  Deviation(down_long_hemisp_shaded1_std)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer2, Maxima(down_long_hemisp_shaded2_max)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer2, Standard
  Deviation(down_long_hemisp_shaded2_std)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer1(down_long_hemisp_shaded1)
• Downwelling Longwave Hemispheric Irradiance, Shaded Pyrgeometer2(down_long_hemisp_shaded2)

• Instantaneous Downwelling Pyrgeometer Case Thermistor Resistance, Shaded
  Pyrgeometer2(inst_down_long_shaded2_case_resist)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Resistance, Shaded
  Pyrgeometer1(inst_down_long_shaded1_dome_resist)
• Instantaneous Downwelling Pyrgeometer Case Thermistor Resistance, Shaded
  Pyrgeometer1(inst_down_long_shaded1_case_resist)
• Instantaneous Downwelling Pyrgeometer Dome Thermistor Resistance, Shaded
  Pyrgeometer2(inst_down_long_shaded2_dome_resist)
• Instantaneous Downwelling Pyrgeometer Thermopile, Shaded Pyrgeometer2(inst_down_long_hemisp_shaded2_tp)
• Instantaneous Downwelling Pyrgeometer Thermopile, Shaded Pyrgeometer1(inst_down_long_hemisp_shaded1_tp)