Do raindrops behave differently in the Indian monsoon condition? Kirti Chandra Sahu Department of Chemical Engineering, Indian Institute of Technology Hyderabad, Sangareddy 502285, Telangana, India The Indian monsoon is a worldwide unique and unpredictable monsoon system since it is influenced by a variety of factors such as the intertropical convergence zone, El Niño, changes in conditions of pressure over the southern oceans, etc. While India sometimes gets good precipitation, other times drought-like circumstances are created. The rainfall modelling is much more challenging in the Indian subcontinent owing to large temporal and spatial gradients in temperature, humidity, aerosol presence and air current. The climate change is, of course, a global concern, and the understanding of rainfall in India would provide useful inputs to global climate models. While most of the rainfall models are empirical in nature, there is an increase in the demand for detailed microphysics to be included to improve predictions. The emergence of powerful supercomputers and sophisticated experimental techniques makes it now more feasible than ever 1-2 . The shape and size distribution of raindrops 3 is one of the many complexities that need to be understood in order to improve the rainfall modelling 4 . The measurement of rainfall by remote sensing and polarimetric radars assumes a function for eccentricity of raindrops that increases with the drop size. Moreover, the Marshall-Palmer formula, which essentially says that the distribution of raindrop size is a monotonically decreasing function of size, is often used in weather radars. These assumptions, however, are not true for large droplets that are unable to preserve spherical shapes. While the typical size of raindrops is about 1 mm to 4 mm in most of the world, the raindrops in the Indian monsoon condition appear to be much bigger. It is interesting to note that very large size of raindrops (up to 1 cm in diameter) have also been observed in some other countries (e.g. in Brazil and Hawai) 5 . The main difference between the small and large droplets is associated with their shapes. Small droplets of size less than a few hundred microns are spherical due to dominating surface tension. Large droplets not only show a wide range of shapes but also display many fascinating but less understood behaviours, such as their interaction and complex flow field in the wake region due to comparable surface tension, inertia and viscous forces. Such droplets do not attend the terminal velocity and continue to exhibit the shape oscillations until they hit the ground 6-8 . This increases the chance of the droplets to coalesce due to oblate-prolate shape oscillations (as shown in Fig. 1a). After reaching a critical size, a raindrop also undergoes fragmentaion 4,9 . A typical fragmentation process observed in raindrop (known as “beg breakup” mode) is shown in Fig. 1b. (a) (b) Figure 1: A sketch showing (a) coalescence due to shape oscillations and (b) fragmentation in falling raindrops. Nonspherical droplets undergo coalescence as they hit each other due to oblate-prolate shape oscillations, but small droplets will not coalesce because of large surface tension. Another puzzling question is can a raindrop be hollow (droplet with air as its inner core) 10 ; in a complex monsoon scenario, like the one observed in India, this possibility cannot be discarded. The first mention of this idea that raindrop can be hollow goes back to the late 1800s by E. J. Lowe. One can get hollow raindrop in two situations: (i) when the bottom part of an 'upward-turned bag' like shape (Fig. 1b) is closed, and (ii) from a crystal of frozen fluid inside a cloud droplet that melts under the heat of condensation to release inert gas that serves as the core of a hollow droplet. A normal liquid droplet