Cloud Droplet Fall Time Completion

Droplets and ice crystals behave somewhat like dust in the air made it visible in a shaft of sunlight. To the casual observer, dust seems to act in a totally random fashion, moving about chaotically without fixed direction. But in fact, dust particles are much larger than water droplets, and they finally fall. The average size of a cloud droplet is only $0.0004$ inch in diameter. It is so small that it would take sixteen hours to fall half a mile in perfectly still air, and it does not fall out of moving air at all. Only when the droplet grows to a diameter of $0.008$ inch or larger can it fall from the cloud. The average raindrop contains a million times as much water as a tiny cloud droplet. The growth of a cloud droplet to a size larger enough to fall out is the same cause of rain and other forms of precipitation. This important growth process is called “coalescence.”

Complete the following question.

A cloud droplet is so small that it would take ..... hours to fall half a mile in perfectly still air, and it does not fall out of moving air at all.

Answer: [______]