Wednesday, July 22, 2009

Apple #398: Raindrops

It's raining. So I want to know, what's in a raindrop?

Looking for the answer to that question, I found out some other stuff about raindrops, too.

(Raindrop flood photo by Lyn Evans)

  • When raindrops first form and drop out of clouds, they are very large -- sometimes as big as baseballs. They are also nicely spherical due to the surface tension inherent in water.
  • But as raindrops fall, air resistance drags on the drops and distorts them. We typically represent that distortion as a teardrop because that's the shape they make when they're hanging off something here on the ground. But in the air, raindrops actually take on other shapes that are more like pancakes, parachutes, or broken grocery bags.

What happens to raindrops as they fall and the shapes they make. These photos appeared in a study published this week.
(Photo by Emmanuel Villermaux, sourced from Science News)

  • Each raindrop, still falling, reaches a point where the air resistance is so great that the drop flattens out and then bursts. What hits the earth, then, are the fragments of larger raindrops that have exploded on the way down.
  • Very small droplets, those with a radius of 0.014 cm, remain spherical the whole way down.
  • There's another fact about raindrops that may also be related to the way they break up on the way down. You would think that larger raindrops fall faster than smaller raindrops; i.e., the force of gravity acting on an object with more mass would give it greater acceleration than a smaller object. However, two researchers from Michigan Tech University discovered that some smaller raindrops actually fall faster than larger drops.

Some raindrops fall faster than you'd expect.
(Photo from My Two Cents' Worth)

  • Using a spectrophotometer, they analyzed 64,000 raindrops during rainfalls. They discovered clusters of smaller drops that were not only falling faster than larger drops, but they were in fact traveling faster than mathematical formulas had predicted was even possible for their size.
  • They think that when the larger rain drops break up, the resulting smaller fragments continue to travel at the rate they were falling when they used to be bigger. Because not all small raindrops are the result of a ruptured larger raindrop but were small to begin with, this would explain why not all small raindrops travel faster than the big ones.
  • But why the broken-up raindrops should maintain the speed they had before they broke up, I don't know and as far as I can tell, neither do the researchers.
  • They do think, though, that weather forecasters have been overestimating rainfall for years. Meteorological software that predicts rainfall has been designed based on the assumption that larger raindrops always fall faster than the smaller ones. That has led meteorologists to estimate higher numbers of large raindrops, and thus a greater amount of overall rain.

  • You probably remember from your elementary school science class that raindrops first form around a particle of dust.
  • Once they've formed, raindrops grow larger as the wind turbulence in the cloud where they were born whips them around and bumps them into each other.
  • But what else is in a raindrop besides that first bit of dust?

(Raindrop photo from EduPic)

  • What's in a raindrop varies depending on where the rain is falling. If it's falling over oceans, the raindrop will have more salt in it. If it's falling over cities with lots of pollution, it might have higher levels of acids. But here are some of the things that have been found and are probably present in most raindrops:
  1. Water
  2. Salt
  3. Dust
  4. Smoke particles
  5. Pollen
  6. SOx (sulfuric acids, primary component of acid rain)
  7. NO2 (nitrogen dioxide, a toxic air pollutant)
  8. NH3 (ammonia)
  9. Hydrogen
  10. Calcium
  11. Magnesium
  12. Potassium
  13. Sodium
  14. Nitrates
  15. Chlorine
  16. Fluorine
  17. Lead
  18. Iron
  19. Bromine
  20. Manganese
  21. Vanadium (metal that comes from minerals)
  22. Aluminum
  • Not entirely a happy list, but that's a lot of stuff in one raindrop.
  • I thought I remembered learning at some point that little living organisms might be present in raindrops, but now I can't find anything that says that.
  • Raindrops do help disperse seeds, though. A falling raindrop may strike a plant and as the drop splashes, so do the seeds. Planting and watering all in one shot!

The splashing shape and the shape the raindrops make when they explode in the air look similar.
(Rail Top Rain Drop by Robert Case)

Victoria Gill, "Why raindrops come in many sizes," BBC News, July 20, 2009
"Solved: The amazing pictures that explain why no two raindrops are alike," Daily Mail, July 21, 2009
NOAA University Corporation for Academic Research, What is a rainbow?
"Maybe It's Raining Less Than We Thought: Physicists Make a Splash With Raindrops Discovery," ScienceDaily, June 11, 2009
Jeanna Bryner, Clue Found in Mystery of How Raindrops Form, LiveScience, December 8, 2006
How Raindrops Form, Physical Review Focus, March 26, 2001
University of Hawaii, Ask an Earth Scientist, What is a chemical salt recipe for "typical" rainwater?
USGS, The Water Cycle: Precipitation
Willem A. H. Asman et al., "Meteorological interpretation of the chemical composition of rain-water at one measuring site," Water, Air, & Soil Pollution, August 1981
Ellen Root, et al., Rainwater Chemistry Across the United States, Carleton College, November 22, 2004
Jane Norris-Hill and Jean Emberlin, "The incidence of increased pollen concentrations during rainfall in the air of London," Aerobiologica, April 1993


  1. You have gotten the following song in my head, Apple Lady:

    And now I know that smaller raindrops fall faster than larger!

  2. Only SOME small raindrops fall faster than the large ones. Remember some of the small raindrops are not broken-up but started small and stayed small.

    Did you know that song, and the movie for which it was written (Butch Cassidy & the Sundance Kid) is 40 years old this year?

  3. This explains the mud sheen on my car when there's not enough rain to clean it but just enough to get it very dirty. :o)


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