Raindrop size distribution
Why do raindrops in a storm follow a characteristic size distribution -- many small drops, few large ones -- as described empirically since the 1940s?
History and references
Settled in 2009 by Emmanuel Villermaux, Benjamin Bossa (Aix-Marseille University).
Meteorologists Marshall and Palmer empirically fit an exponential law to raindrop size distributions in 1948, but for six decades the underlying physical origin of that law was not derived from first principles. In 2009, Emmanuel Villermaux and Benjamin Bossa used high-speed imaging of single water drops breaking apart mid-fall to show that the fragmentation cascade of one large, unstable drop alone reproduces the full observed distribution and its parameters, correcting the prevailing assumption that collective drop-drop interactions in a cloud were the main driver.
A statistical law about entire rainstorms was traced back to the fate of a single falling drop.
Explanations that circulated
- That the observed size distribution mainly reflects drop-drop interactions in the cloud (coalescence and collisional breakup among many falling drops); Villermaux & Bossa showed with high-speed video that the distribution is instead set almost entirely by the fragmentation dynamics of single, isolated large drops (>=6 mm) breaking apart as they fall, with drop-drop interactions playing a negligible role.
References
- Single-drop fragmentation determines size distribution of raindrops, E. Villermaux, B. Bossa, Nature Physics, 2009
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