The life, death and afterlife of a raindrop on Titan

A model is presented which describes the descent rate and evaporation rate of methane raindrops on Titan. The model, using conventional aerodynamics, with raindrop distortion parameterized by the Weber number, gives excellent agreement with terrestrial raindrop data. Terminal descent velocities for drops of various sizes at different altitudes are presented, and it is found that the largest raindrops may be larger than those on Earth (9.5 mm diameter vs 6.5 mm diameter) yet fall much more slowly (1.6 m S -‘vs9.2ms-‘. Under standard conditions on Titan, raindrops evaporate before they reach the ground : profiles showing the shrinking of drops due to evaporation during their descent are shown for various values of relative humidity. A 500 m increase in elevation can lead to a tenfold increase in rain mass flux, leading to increased “washing” of highland terrain. It is pointed out that evaporating raindrops will leave behind their condensation nuclei : fall times for these are presented. and it is noted that they may significantly affect visibility in the troposphere. The effects of additional factors on raindrop behaviour, such as the nonideal solubility of nitrogen in methane, are briefly considered.