In addition to the date and time you should also make note of the Volume Coverage Pattern (the VCP number) in the upper left side of the image. The National Weather Service WSR-88D employs 9 different Volume Coverage Patterns. Without going into the technical aspects of these patterns, you should note one of the patterns you are likely to see during non-precipitation scans; VCP 32. This is commonly referred to as "Clear Air Mode" and in this mode, the radar is utlra-sensitive. Images in VCP 32 are frequently filled with ground clutter, anomalous propagation or false echoes. If you notice a very busy radar image, especially at night and early morning, when no precipitation is falling, you should check the VCP number. Chances are, the forecast office is monitoring the radar in VCP 32 - Clear Air Mode.
Briefly, the Volume Coverage can be changed to fit the type of weather that a forecaster is monitoring. The change can include a slower or faster scan rate, a larger sample size, or more slices of the atmosphere, and a slower or faster pulse rate frequency. VCP 21, 12, 11 and 212 are all common VCPs during stormy weather. If you want to know more about the Volume Coverage Patterns, how they are used and what the numbers mean, please check the tutorial offered on line from the National Climatic Data Center.