Tag: rain

Reader Question: Non-Coalescing Droplets
Reader ancientavian asks:
I’ve often noticed that, when water splashes (especially as with raindrops or other forms of spray), often it appears that small droplets of water skitter off on top of the larger surface before rejoining the main body. Is this an actual phenomenon, or an optical illusion? What causes it?
That’s a great observation, and it’s a real-world example of some of the physics we’ve talked about before. When a drop hits a pool, it rebounds in a little pillar called a Worthington jet and often ejects a smaller droplet. This droplet, thanks to its lower inertia, can bounce off the surface. If we slow things way down and look closely at that drop, we’ll see that it can even sit briefly on the surface before all the air beneath it drains away and it coalesces with the pool below. But that kind of coalescence cascade typically happens in microseconds, far too fast for the human eye.
But it is possible outside the lab to find instances where this effect lasts long enough for the eye to catch. Take a look at this video. Here Destin of Smarter Every Day captures some great footage of water droplets skittering across a pool. They last long enough to be visible to the naked eye. What’s happening here is the same as the situation we described before, except that the water surface is essentially vibrating! The impacts of all the multitude of droplets create ripples that undulate the water’s surface continuously. As a result, air gets injected beneath the droplets and they skate along above the surface for longer than they would if the water were still. (Video credit: SuperSloMoVideos)
June 25, 2013
Airborne Aerosols
This numerical simulation from NASA Goddard shows the motion of particulates in Earth’s atmosphere between August 2006 and April 2007. These aerosols come from various sources including smoke, soot, dust, and sea salt. As these fine particles move through atmosphere, they can have significant effects on weather as well as climate. For example, the particles serve as nucleation sites for the condensation and formation of rain drops. (Video credit: NASA Goddard SFC)
December 19, 2012
Reader Question: Dry Rear Windshields in the Rain
Reader sheepnamedpig asks:
I was driving through the rain down the highway when I noticed something strange: though the rain was heavy enough to reduce visibility to a quarter mile, the rear windshield of my Corolla was bone dry except for the streams of water flowing off the roof. There was no wind so far as I could tell, but I had to slow down all the way to ~20-25 mph for rain to start falling on the rear windshield. Why is that?
That’s a wonderful observation! Like many sedans, your Corolla has a long, sloped rear window that acts much like a backward-facing step with respect to the airflow while the car is moving. Note the smoke lines in the photo above. At the front of the car, we see closely spaced intact lines near the hood and windshield, indicating relatively fast, smooth airflow over the front of the vehicle. At the back, though, there is a big gap over the rear windshield. This is because flow over the car has separated at the rear windshield and a pocket of recirculating air. This recirculation zone is, for the most part, isolated from the rest of the air moving over the car; that’s why the smoke lines continue relatively unaffected a little ways above the surface. This same pocket of recirculating air is protecting your rear windshield from rainfall. It’s an area of low-speed, high-pressure fluid, and the raindrops are preferentially carried by the high-speed, low-pressure air over the recirculation zone. This is one reason why many sedans don’t have rear windshield wipers. (Photo credit: F-BDA)
ETA: Reposted by request to make it rebloggable.
November 29, 2012
Donut-Shaped Bubbles
Here researchers simulate rain-like droplet impacts with large drops of water falling into a tank from several meters. The momentum of such an impact is significantly higher than many other droplet impact examples we’ve featured. In this case, the coronet, or crown-like splash, caused by the collision collapses quickly, closing the fluid canopy around a trapped bubble of air. The remains of the coronet fall inward, preventing the development of the usual Worthington jet associated with droplet impacts. Instead, the air bubble takes on an unstable donut-like shape. (Video credit: M. Buckley et al.)
November 5, 2012
How Mosquitoes Fly in the Rain
One might think that rainfall would keep the mosquitoes away, but it turns out that rain strikes don’t bother these little pests much. Because the insect is so small and light compared to a falling raindrop, the water bounces off instead of splashing. This results in a relatively small transfer of momentum, although the mosquito does get deflected quite strongly. Researchers estimate that the insects endure accelerations up to 300 times that of gravity, which is more than 10 times what a human can withstand. (Video credit: A. Dickerson et al; submitted by Phillipe M.)
June 12, 2012
Fragmenting Raindrops
This numerical simulation demonstrates the fragmentation of droplets of water falling through a quiescent medium–essentially how a raindrop behaves. As the initial droplet falls, drag forces deform the droplet, contorting it until surface tension causes it to break into smaller droplets, which can themselves be broken up by the same mechanisms.
January 6, 2012
Water Spray from a Tire
The spray thrown up by a rolling tire is simulated in the lab by running a single-grooved tire (top) against a smooth tire (bottom) that simulates the road. A supply of water flows from the left at the speed of the rolling tires (6 m/s). The resultant sheet of water is a familiar site to motorists everywhere. Holes in the the sheet of water collide to form the smallest droplets, whose diameters are comparable to the thickness of the sheet, of the order of 100 microns. Thicker parts of the sheet form ligaments and break down into large droplets through the Plateau-Rayleigh instability. (Photo credit: Dennis Plocher, Fred Browand and Charles Radovich) #
September 14, 2011
How Not to Get Wet in the Rain
[original media no longer available]
Ever wonder how to minimize how wet you get if you’re caught in the rain without an umbrella? This lecture discusses just that problem and how to calculate an answer. I actually solved a version of this problem when studying for my PhD quals, only I first had to determine the terminal velocity of a rain drop (~10 m/s assuming a 4mm spherical drop) and work from there. We also had to compare moving upright to running at an angle. It makes for an interesting little diversion. (via physicsphysics)
April 20, 2011