Dear Tom,
I saw drizzle with no clouds in the sky. Explanation?
—Bill Wofther

Dear Bill,
We can offer two explanations for rain without clouds. Very small raindrops, implied by your mention of drizzle, fall at speeds of two feet per second or less. Descending at that rate, a raindrop would require 21 minutes to reach the ground if it originated from a cloud whose base was at 2,500 feet — ample time for the “mother cloud” to move out of sight.

Another possibility: Consider the very rare situation in which still air is supersaturated with moisture (its relative humidity is above 100 percent) and clouds have not formed because condensation nuclei are practically nonexistent. Water vapor will condense onto the few nuclei that are present and those droplets will rapidly grow into raindrops.