I live on the seventh floor of high rise building in the Loop. One day last summer I saw rain fall from a clear sky. Can you tell me how this was possible?
Tom Albers, Chicago
Though the sky was likely cloud-free overhead, the rain you observed precipitated out of a cloud. Rain always originates from clouds, but raindrops take time to reach the ground, and during that time the clouds from which the rain originated may have moved out of your view or had dissipated after being scavenged up in the precipitation growth process. The fall speed of raindrops varies, with the largest drops falling fastest. Largest raindrops descend at a top speed of about 20 mph, while the smallest raindrops falling at about three mph. Droplets smaller than raindrops, referred to as drizzle, can fall at speeds of less than one mile per hour.