Comet Panstarrs & M31
The night of April 6-7 brought a cloudy, humid (dripping wet) evening, making astrophotography a miserable, futile effort. But before sunrise, Comet Panstarrs was due to rise into view.
At 4AM, things looked promising, but in 15 minutes, a blanket of fog rolled over the observatory and things seemed hopeless. Fortunately the fog cleared out as quickly as it rolled in... at least enough spot the comet (and M31, the Andromeda galaxy) between thin clouds.
Moonrise occurred a few minutes later, making it clear just how hazy the sky was.