Rain drops have to nucleate on solid particles floating in the atmosphere. The sizes of these particles are very small, on the order of hundreds of nanometers. These particles are normal (at a natural level) and include:
dust
soot from forest fires (or smokestacks)
salt from the ocean
volcanic ash
sulfates from plankton
Average raindrops are 2 millimeters in diameter. Cloud droplets are smaller than that. They are large enough to interfere with light, so we can’t see through clouds. 

Rain drops have to nucleate on solid particles floating in the atmosphere. The sizes of these particles are very small, on the order of hundreds of nanometers. These particles are normal (at a natural level) and include:

  • dust
  • soot from forest fires (or smokestacks)
  • salt from the ocean
  • volcanic ash
  • sulfates from plankton

Average raindrops are 2 millimeters in diameter. Cloud droplets are smaller than that. They are large enough to interfere with light, so we can’t see through clouds.