MadSci Network: Earth Sciences

Re: What shape, size, etc. is a raindrop?

Date: Wed Jul 8 19:53:21 1998
Posted By: Nezette Rydell, forecaster,National Weather Service
Area of science: Earth Sciences
ID: 899867448.Es

In real life, raindrops aren't shaped like teardrops.  The shape of a falling 
raindrop is determined by the interaction of hydrostatic, aerodynamic, and
surface tension forces.  That shape is primarily dependent on the size of
the drop.  Very small drops, less than 2 mm in diameter, will remain round. 
The attraction forces between the water molecules (surface tension)  are
stronger than the other two forces.  Larger drops flatten on the bottom as
they fall, become rounded on the top, and are wider than they are high. 
The pressure of the air is greatest on the bottom of the drop, flattening it, 
and least on the sides, allowing it to bulge.  As long as the surface tension 
balances the  hydrostatic (related to the weight of the liquid above the bottom 
surface of the drop) and the aerodynamic (the force of the air against the 
bottom of the drop) forces, the drop will remain intact.  If the drop becomes so 
large that surface tension cannot overcome the other two forces, the drop will 
break into two raindrops.

Duncan Blanchard wrote a nice little book some years back called From
Raindrops to Volcanoes.  It is no longer in print but is available in most
libraries.  In it, he describes the forces acting on the shape of raindrops,
and several very simple experiments that demonstrate the effects of these
forces.  It's worth looking for.

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