MadSci Network: Physics

Re: why do bubbles form when rain drops land in water

Date: Wed Mar 28 21:31:58 2001
Posted By: Denni Windrim, Director of Education, Sylvan Learning Centre
Area of science: Physics
ID: 980718056.Ph

The most likely reason is a combination of two things: that air is trapped 
under the raindrop, and that there are some impurities in the water 
(certainly the case in a public swimming pool). Contrary to popular belief, 
raindrops are not teardrop-shaped. Because they are moving through the air 
fairly quickly, and, being water, are easily deformed, they tend to take on 
a shape more like a jelly-filled donut or a pizza pop as they fall. The 
leading (bottom) edge of the raindrop is fairly flat on its bottom center. 
The swimming pool water itself is not smooth, and has little rises and 
hollows in it because it is being constantly agitated. So it is not hard for 
a small bubble of air to be trapped underneath, which then rises to the 
surface. In absolutely pure water this bubble would wink out of existence 
very quickly, but if there are impurities in the water, ie, shampoo 
residues, the bubble will be able to remain intact for a while. What you may 
also be seeing is water droplets floating on the surface of the water. Water 
with a thin layer of oil (which swimmers contribute freely to the water) on 
top will momentarily prevent the tiny droplets which form when a raindrop 
splashes into the water from rejoining the water in the pool, and the 
droplets may skitter around on the surface of the water for up to several 

You can try both of these at home, by using an eyedropper to drip water into 
a bowl of water which has a very small drop of detergent or cooking oil 
added. Have fun!

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