![]() These droplets are around one tenth of a millimetre across. Since they are small, they have a lot of surface area compared to their mass. This means they fall slowly, and surface tension squeezes them into near perfect spheres. Think of a droplet like a well inflated football. The strong skin on the surface squeezes it into a near perfect sphere. What about a much bigger droplet, around two millimetres across? This is a true rain drop. Its surface area is 400 times that of the drizzle, but its mass is 8000 times bigger. So the mass has become more important, and the surface tension finds it less able to squeeze into a perfect sphere. It also falls faster, and so it has wind rushing past it pushing on the bottom - this is known as drag force. The Real Shape of Raindrops via NatureVolve ApIt’s the same thing that makes a skydiver’s face look funny when they jump out of a plane. This increase in drag force pushing on the underside of the drop, combined with the reduction in the surface tension’s ability to squeeze the drop into shape, means that the drop ends up with a flat bottom and broad domed top. The exact opposite of what most people expect. Maybe rather than a football, you could imagine this like a half inflated giant beach ball. It has a surface area 2500 times our first drizzle droplet, but a mass 125 000 times bigger. Now, the droplet falls much faster and gets so flat that is ends up stretched thin and domed like a parachute, where the wind pushes it up. At this point the surface tension tends to fail completely and the droplet is ripped apart into dozens of smaller droplets. ![]() The biggest ever measured raindrops are around this size. I like to think of this like throwing a bouncy castle out of a plane. Distorted by the wind, it starts flapping around and is eventually ripped to shreds. There are two main reasons this is important. With a maximum size set for the raindrops, we can use this information in the simulations that give us our weather forecasts and help us predict how much it rains. This is also useful when we measure the weather with radar that sends a beam of radio waves into a cloud and measures how the waves bounce back. The best radar can send these radio waves oriented vertically and horizontally, known as polarisation. If we get the same signal from vertical and horizontal waves, we know we are measuring spherical drops, which must be small drizzle. ![]() If we get a different signal from each orientation, we must be measuring flattened drops, which are much bigger and indicate heavier rain.Raindrops can adopt remarkably similar shapes, sizes and movements as they fall to the ground, but so far, our understanding of their behaviour has remained far from complete. Through her research, Dr Merhala Thurai at Colorado State University uses innovative techniques to learn more about what happens as they fall, particularly during stormy weather.
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