The VAQR may be unavailable during the following situations:
- At night or within a few hours of sunrise and sunset. During those periods the VAQR is not a reliable indicator of pollution impacts on visibility. Around sunrise or sunset the low angle of the sun in the sky causes visibility to vary dramatically depending on which direction one looks. During the night visibility depends on illumination much more than on pollution levels.
- When humidity is high (>75% relative humidity). Many of the periods with high humidity are affected by clouds or rain. Even without clouds or rain, high humidity generally causes poorer visibility as the humidity causes existing haze particles to grow. For this reason no VAQR is calculated during high humidity periods.
- When the air pollutant measurements used to calculate the VAQR are unavailable. At times the measurements may not be available due to planned instrument maintenance, unplanned power outages or data transmission errors.
