Even when the sky is mostly blue, there’s a lot going on at different altitudes. The winds do not move in a consistent direction or at the same speed, something which becomes apparent when watching clouds move relative to one another. When different layers of air move past one another, there is shear between them, not unlike the friction you feel when running your hand along a table. Under the right circumstances, this shear creates Kelvin-Helmholtz waves like the ones in this image over Helena Valley, Montana. Fast-moving winds (blowing right to left in the image) above a layer of clouds created these breaking wave-like curls. The same phenomenon creates many of the ocean’s waves from the shear caused by wind blowing across water. (Image credit: H. Martin, via EPOD)
Category: Phenomena
The Impressive Take-Off of Pigeons
One reason that peregrine falcons are such amazing fliers is that their prey, pigeons, are no slouches in flight, either. Able to take off vertically and accelerate to 100 kph in two seconds, pigeons are pint-sized powerhouses. With this high-speed video, BBC Earth highlights the mechanics of this vertical take-off. Pigeons begin by bending their legs and jumping high enough that their first downstroke can extend fully and still clear the ground. That gives them a headstart on generating the force they need to propel themselves upward.
Note the way the pigeon’s wings move, sweeping from directly behind the bird’s back to a full extension in front of it. With the bird moving vertically, this motion tells us that it’s thrust – not aerodynamic lift – from the wingstroke that’s powering this take-off. In that sense, the pigeon is something like a Harrier jet, using the thrust of air downward to take off vertically. (Image and video credit: BBC Earth)
Ferrofluid in a Cell
Ferrofluids are a colloid consisting of magnetically sensitive nanoparticles suspended in a carrier liquid, like oil. They’re often associated with a distinctive spiky appearance when exposed to a magnet, but this isn’t their only magnetic response. Above we see a ferrofluid confined to a Hele-Shaw cell – essentially two glass plates with a small gap between them. In the upper image, the ferrofluid is exposed first to an axial magnetic field, which stretches it to form spidery arms. Then the magnetic field switches to a rotating configuration, which curls the arms around and causes the ferrofluid to slowly rotate.
In the lower image, you see the reverse. First, the ferrofluid feels a rotating magnetic field. When this is changed to an axial field, the ferrofluid bursts into a cell-like center with straight arms. As the magnitude of the axial field increases further, the arms begin to curl. For more fantastical ferrofluid formations, check out these previous posts featuring artists Linden Gledhill and Fabian Oefner. (Image credit: M. Zahn and C. Lorenz, source; via Ashlyn N.)
How Waves Travel
When playing in the surf, it’s easy to imagine that the incoming waves are a wall of water crashing into the shore. And, in a way they are, but probably less so than you imagine. Waves travel through a medium, whether it’s solid or fluid, but for the most part, they’re not translating the medium itself. You can see that in the animation above by watching the dye beneath the surface. The passing waves don’t cause much mixing in the dye, and though their passage distorts the underlying water, we see that everything returns more or less to its starting position once the wave has passed. (Image credit: S. Morris, source)
Feathered Fighter Jets
Peregrine falcons are built for speed. They’ve been clocked at more than 380 kilometers per hour when diving. This video from Deep Look examines some of the features that make these birds of prey so fast, from the shape of their eyes to the tubercles in their nostrils that help them breathe during high-pressure dives.
Part of the falcon’s speed comes from its signature stoop, where it pulls in its wings to form a tight, streamlined shape. This reduces drag forces on the falcon, letting gravity pull it toward a high terminal velocity. But even with its wings extended, the falcon exudes speed and agility. Its wings form a sharp leading edge to cut through the air, with stiff, overlapping feathers that slice the flow. Compare this to the feathers of an owl, which specializes in silence rather than speed for catching its prey. (Video and image credit: Deep Look)
Grayscale Aurora
This swirling grayscale image shows a spring aurora over the Hudson Bay, as seen by the Suomi NPP satellite. As energetic particles from the sun zip past Earth, they interact with our magnetosphere, which tends to channel particles toward the poles. At these higher latitudes, some of the particles get trapped along Earth’s magnetic field lines and crash into the upper atmosphere where they excite oxygen and nitrogen molecules. It’s this molecular bombardment that creates the distinctive colors of the aurora. (Image credit: J. Stevens; via NASA Earth Observatory)
Seeing with Sound
Sound carries rich information about the environment through which it’s traveled. And while many sighted people never take the time to notice this, using sound to build a mental picture of the surrounding world is something many blind people do constantly, either by studying how sounds produced by others change (passive echolocation) or by using their own sounds to pinpoint what’s around them (active echolocation).
In the latest It’s Okay to Be Smart video, you have a chance to learn some of the basics of active echolocation and how you can train your brain to recognize and process this extra environmental information.
Personally, I am not very good at this. I can hear edges but it turns out I’m very bad at figuring out where they are. That said, having spent time recently in a few anechoic chambers – where sound reflection is almost completely damped out – I’ve come to realize that even as a sighted person, I rely on sound a lot more than I think I do! (Video and image credit: It’s Okay to Be Smart)
Lensing in a Straw
While doing the sort of experiment only a kid or a scientist would pursue – namely, staring down a straw – Dianna noticed that water in a straw creates a lens-like magnification effect as the straw moves or down. This happens thanks to the curvature of the air-water-straw interface. Because water has strong surface tension, it curves dramatically as it meets the wall of the straw, and moving the straw up or down will drag some of the fluid with it, enhancing the curvature. When light refracts across that interface, it gets bent the same way it would through a lens, thereby shrinking or magnifying the objects beneath. (Video credit: D. Cowern/Physics Girl)
Volcanic Plume
Astronauts aboard the International Space Station captured this dramatic image of Raikoke Volcano’s eruption in late June. This uninhabited Pacific Island is part of the Kuril Islands off mainland Russia. The hot plume of ash and volcanic gas rose until its density matched that of the surrounding air, at which point it could only expand horizontally. This is why the plume appears to have such a flat top. It’s similar to the cumulonimbus clouds we associate with severe thunderstorms. Scientists speculate that the white ring around the plume’s base might be water vapor condensed from ambient air pulled in to the plume’s base or a side-effect of magma flowing into the surrounding sea. (Image credit: NASA; via NASA Earth Observatory)
Pouring a Liquid Mirror
In this video, the Slow Mo Guys play with liquid gallium, giving us a chance to see how molten metals behave (outside of, say, the Terminator movies). Near its melting point, gallium is about six times denser than water, with a viscosity three times higher, and a surface tension about ten times greater. So how do those properties affect its behavior?
You may be surprised that when watching the gallium vibrate on a speaker or get poured into a pan, it doesn’t look all that different from water. Yes, it’s highly reflective, but, on the whole it doesn’t look radically different from a distance. We can use the Reynolds number to quantify what’s going on here. It’s a dimensionless number that compares the fluid’s inertial force to the viscous force. Imagine we have two versions of an experiment, one where we pour gallium at a given speed and one pouring at the same volume and speed but with water. If we compared the Reynolds numbers of the water and the gallium, they only differ by a factor of two. Overall, that’s not very much. That’s why the two pours look similar.
The story is different, though, if we look at individual drops of gallium and water, like when the first few drops of our pour hit the surface. Check out the gallium drops below. They’re conical on either end! This looks very different from what we expect with water droplets. You might think that’s because the metal is more viscous, but if we compare a water drop with a gallium drop of the same characteristic size and impact speed, we find a different story. For this, we’ll use the Ohnesorge number, which compares the viscous forces to a combination of inertia and surface tension. In this case, we find that the gallium drop’s Ohnesorge number is almost an order of magnitude smaller than the water droplet’s. That means that viscosity isn’t a major factor for our gallium drop. Both surface tension and inertia are more important.
But if the surface tension is so high, then why aren’t the droplets spherical? Mostly because they don’t have time to form spheres before they hit. Their shape suggests that they’ve only just broken into droplets, which makes sense if the pour is fast and the surface tension is strong. (Video and image credit: The Slow Mo Guys)
