Abstract
One of the chief problems in cloud physics1 is to explain how, in the absence of ice, small cloud droplets can coalesce to form raindrops. Cloud droplets are produced in concentrations of ∼108 m−3 by condensation on sub-micrometre atmospheric nuclei, but, because they are so small, collisions between droplets are rare events. In mature rainclouds there are2 usually about 1,000 drops per m3; here, however, I present differential radar reflectivity observations which indicate that, when ice is not present, first echoes of convective showers consist of large (>4 mm) raindrops present in much lower concentrations ( 30μm radius) which are present in similarly low background concentrations3. In some clouds this initial raindrop spectrum persists, whereas in other cases it changes rapidly and irreversibly to the raindrop distribution normally observed in mature clouds with many more smaller raindrops. The latter may occur when the break-up of the large raindrops becomes more common and many small satellite drops are produced.
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