London: The mystery why raindrops come in different sizes has puzzled scientists for years. Now, a team in France claims to have finally cracked it -- it's all due to air resistance.
For their study, researchers at Aix-Marseille University filmed a single falling drop of water -- about six millimetres in diameter -- with a high-speed camera. In fact, they recorded how air resistance caused it to deform and eventually break up.
Air resistance causes a raindrop to deform as it falls, and eventually to fragment. The large, round drop fell, gradually flattened out and, as it got wider, "captured" the air in front of it to form the shape of an upturned bag.
This bag finally "inflated" and burst apart into many smaller droplets - all within six hundredths of a second. This happened because drops were too large and heavy to remain intact, according to the researchers.
Each large, heavy drop accelerates as it falls and "has to displace the air molecules" on its way down, explained the study's lead author Emmanuel Villermaux. "This produces the air resistance or drag."
At a certain speed, the number of air molecules -- and therefore the intensity of this drag -- is greater than the surface tension holding the round drop together, so the drop starts to deform.
"When it bursts, the fragments match exactly what we find in raindrops. This is a precise, quantitative explanation for their distribution and size," Villermaux told 'BBC News'.
The study has been published in the 'Nature Physics' journal.