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 aero dynamics, with raindrop distortion parameterized by the Weber number, gives excellent agreement with terrestrial raindrop data. Terminal des cent velocities for drops of various sizes at different altitudes are presented, and it is found that the largest raindrops may be larger th an those on Earth (9.5 mm diameter vs 6.5 mm diameter) yet fall much m ore slowly (1.6 m s(-1) vs 9.2 m s(-1). Under standard conditions on T itan, raindrops evaporate before they reach the ground : profiles show ing the shrinking of drops due to evaporation during their descent are shown for various values of relative humidity. A 500 m increase in el evation can lead to a tenfold increase in rain mass flux, leading to i ncreased ''washing'' of highland terrain. It is pointed out that evapo rating raindrops will leave behind their condensation nuclei: fall tim es for these are presented, and it is noted that they may significantl y affect visibility in the troposphere. The effects of additional fact ors on raindrop behaviour, such as the nonideal solubility of nitrogen in methane, are briefly considered.