Dear Tom,
At 9:40 a.m. Sept. 2, it began to drizzle and there wasn’t a cloud in sight. How can this happen?
—John Peterson, Grayslake
Dear John,
We can offer two possible explanations for rain without clouds. Very small raindrops, implied by your mention of drizzle, fall at 2 feet per second, or less. Descending at 2 feet per second, a tiny raindrop would take about 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 quickly condense onto the few nuclei that are present and the droplets will rapidly grow into raindrops.
