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After a hectic day at lab, my eyes were waiting for the clock to strike 5! Football has become a routine in my graduate life; my only stress buster. Cycling to the ground dressed up in a complete game attire, I looked up at the sky. Thank God, the sky is clear. Cloudy skies and heavy rainfall over the past week have interrupted our games almost everyday. No rain forecast for today! Yay! Finally, game day! I reached the ground to witness the usual quarrels over dangerous tackles, shouts for the ball and  the usual team words "Man On!", " behind you", "Pass"!

Suddenly, a dark hue escalated and the sky went dark. Wind started blowing faster. Bruised clouds banked up the horizon. But how come such a dark cloud form all of a sudden? We all ran as the thick cloud cover started raining cats and dogs. Everyone looked disappointed. No one saw this coming. A few started blaming weather predictions and some asked me why can’t the scientists predict rainfall accurately. Being a doctoral student in aerospace engineering, specializing in atmospheric science, I was responsible to answer, but I couldn't. It took quite some time for the rain to stop. But the thoughts on "why can't we predict the sudden onset of rain?" had just started.

We have already landed a space probe on a moving comet. Scientists have captured a picture of a black hole. Higgs boson aka God particle has been detected using Large Hadron collider. Flourishing innovations in medicine, communication, transportation are also seen in this 21st century. Still, why can't we predict the sudden onset of rain? "Innovations come from real needs", someone murmured.

Are these predictions vital? I googled it. After reading a lot about it over dinner, I started wondering why the grey cloud didn’t rain as expected though there were enough indications? Why unthreatening clouds can result in a deluge? Unpredicted sudden downpour in Kerala, Mumbai, North-eastern regions, Chennai, etc., has taken away many lives, displaced over millions, destroyed homes and agricultural yields and has caused huge loss in revenue.

“Yes, this is an interesting topic for a wider range of audience, especially during these extreme climatic changes because of global warming”.

Finally I have connected the whole problem to "global warming"! Of course, all are familiar with that term. Things learnt at a young age are imprinted in our minds. I started recollecting the concept of rain formation learnt in high school.

I tested my little brother’s knowledge on rain formation. He replied, "Ocean water evaporates and rises into the atmosphere and cools down to form tiny droplets through condensation and it starts raining". He said it all in a breath. I am pretty sure he had given some science test recently. "Rohit bhaiyya, how tiny are these droplets?", he asked me curiously. "10 to 20 micron in size" was my answer. He looked uncomfortable because of the term "micron"; I continued "smaller than the diameter of a human hair. At this size, it's hard to detect droplets by the naked eye". I could see how excited he was. He continued "But, when it rains, we can see the droplets, right?". “He is smart”, I thought to myself and went on to answer him elaborately. “Yes, droplet size increases to millimetres (which we can easily detect) by the time it reaches the ground. Droplet growth reaches a maximum of 30 microns by condensation. However, a minimum of 60 micron size is needed for the droplet to leave the cloud and start the rain. This time taken for 30 micron cloud droplets to increase its size up to a minimum of 60 micron rain droplet decides whether it's a sudden onset of rain or not. This size further increases once the droplet leaves the cloud followed by collisions and merging with other droplets.” 

My current research motivated by all these questions is to find factors that  accelerate the growth of the rain droplets. During the football game, I still remember the wind blowing unusually fast. "Did the wind cause the increase in droplet size?”. My advisor replied, “Yes, wind can induce turbulence in clouds and can enhance coalescence”. Turbulence: it is the violent irregular movement of air/water as seen in a hurricane or a river current during flood. Turbulence is like many whirlpools put together; it stirs and mixes the flow. When a droplet goes through the whirlpool, it experiences rotations and eventually gets thrown out of the whirlpool with a higher speed. This droplet collides with some other droplet and merges to form larger droplets. “But, how is the wind velocity related to the time taken for the onset of rain? Larger the wind speed, the shorter the time to start rain? Or is it the other way round?” “Rohit, you have to quantify the turbulent intensity for different velocities and find out how much the growth rate is varying?” By turbulent intensity, he meant how strong could the whirlpool be. It is like the swinging stone experiment, where you tie a string to a stone and rotate it as fast as you can. The stone tries to fly off and the speed of the stone depends on how fast one rotates the string. So stronger the whirlpool, the higher the speed with which the droplets are thrown out and hence they collide with nearby droplets in a shorter time. But, can I increase wind speed indefinitely and get rain in a few seconds? I wish we could do that to help farmers during drought. But unfortunately, it isn’t possible. We have seen cloudy sky turn into a clear sky with high speed wind. So there should be an optimum wind speed that can start rain in a shorter time. Once we solve this question or at least show that such an optimum speed exists, it will be a promising initial step towards a better rain forecast. It took me almost 6 months for fabricating an experimental setup.

Working in a centre of national importance, I am used to explaining my research to visitors. It is hard to explain, especially when the group is mixed varying from school kids to journalists to professors. Here is how I do it. “Dear kids/Sir/Madam, this setup is to study the effect of wind velocity (air turbulence) on rain formation (droplet size growth rate). We produce whirlpools (turbulent vortices) by passing air through rotating rods (active grids). Water droplets are injected using an atomizer, just like a garden atomizer (pressure swirl atomizer) and we measure droplet size increase over a fixed time using this equipment.” By the time I reach this line, I would have already pointed to a hi-tech equipment called Phase Doppler Interferometry. To kids, I refer to it as a black box which measures diameter as small as a micron. Kids are happy if I just switch-on the spray and shine laser on it. However, the other set of people are keen on the final results. I continue “We have estimated the time taken for the conversion of cloud droplets to rain droplets at different wind velocities. We have observed an optimum wind velocity at which the rain formation time is minimal. Above or below this optimum, it takes longer.” 

A famous journalist from The Hindu stopped me and asked, "so you can predict rain from your experiments, right?" I replied, “Indeed! Now I am sitting on a huge bulk of data obtained from my experiments. We have shown that the intuition of the existence of an optimum wind velocity for shorter rain formation time is true. We have also shown that the wind velocity and initial cloud droplet size account for a better prediction of the sudden onset of rain. However, we need to verify these results using real cloud data. If it stands, then including our new results to the existing prediction technique could improve the rain prediction”. She scribbled something in her notes.

"What could be the reason for that increased time for rain initiation beyond the optimum value?” questioned a Professor from IISc Bangalore. "Clustering", I replied confidently as my paper on that work just got published. "We have seen a group of droplets come closer and move together as a packet without colliding, which reduces the collisions".

Today my advisor called me to his room and showed me an article about my experiment in The Hindu, titled “Delving deep with predicting rain”. I felt really happy and more responsible. I rushed to my desk and wrote an email to Indian Meteorological Department(IMD) seeking permission to access real cloud data so that we can verify our results and incorporate it to rain prediction.

*Declaration:

This article is an original work of the author and is based on his own research work done at IIT Madras. It has been exclusively written and submitted for AWSAR award nomination only.