I noticed an interesting cloud formation: A horizontal line that was cloudy on the bottom and blue sky above. What was it?
—Evelyn Schmidt, Naperville
Your description is vague, but appears to be a cirrus cloud streak. The day in question was mainly sunny. “Horizontal” implies an elongated cloud, as opposed to a vertically-stacked cloud form like cumulus. “Cloudy at the bottom” suggests that virga were falling from the cloud. Virga are produced when cloud particles descend from the cloud, but then evaporate and disappear as they descend. This is common with cirrus clouds, which are high-level clouds that are generally 20,000 feet or higher above the surface, and consequently are composed of ice crystals. Virga trail off beneath the cirrus and dissipate far above the ground.