This depends on the forces acting downwards - gravity - and the aerodynamic drag which acts in the opposite direction. The researchers can also determine the terminal velocity of raindrops in any atmosphere. “As raindrops grow in mass, they evolve from spheres to oblate spheroids to shapes resembling the top of a hamburger bun.” “Falling raindrops adopt a range of shapes depending on their size - though never the teardrop shape inscribed in the public imagination,” says Loftus and Wordsworth. The shape of raindrops is relatively well understood. “From these properties, we demonstrate that, across a wide range of planetary conditions, only raindrops in a relatively narrow size range can reach the surface from clouds,” say the researchers. Loftus and Wordsworth say that just three properties play a role - the shape of the raindrop, its terminal velocity and its rate of evaporation as it falls. The size is uniform because the physics that governs the drops as they pass through the atmosphere is the same for them all, regardless of how they form. Most raindrops are about a millimeter in scale and almost none are bigger than 4mm across because any larger drops break apart. Nevertheless, the drops that form in this process are remarkably similar. Exactly how this happens over the vast range of spatial scales inside clouds is poorly understood. Condensation redistributes heat and humidity within the cloud, which in turn affects future drop formation. This turns out to be a complex non-linear process. Raindrops form and grow inside clouds when water vapor condenses around much smaller particles such as aerosols. Because of this, Loftus and Wordsworth say raindrops on other planets are likely to share remarkable similarities to raindrops here on Earth. This pair says just three factors determine the size of raindrops in any given atmosphere, regardless of the way the drops form. Now we get an answer, thanks to the work of Kaitlyn Loftus and Robin Wordsworth, both at Harvard University in Cambridge, Massachusetts. Although gas giants such as Jupiter and Saturn have no surface, raindrops are thought to play a crucial role in the dramatic storms visible on their surfaces because the drops transport heat through the atmosphere.Īnd that raises an interesting question: what of rain on even more distant how similar is it likely to be to rain on Earth? Mars also seems to bear the marks of rainfall, albeit from some billions of years ago. Titan, too, has rivers, lakes, valley networks and rain, even though the fluid involved is quite different - liquid methane instead of water. We know similar processes are also at work elsewhere. It shapes both our skies and our landscape, carving valleys while filling rivers and lakes. Rainfall is one of the defining characteristics of our weather, with amounts varying dramatically from one region to another.
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