Stripe-like wave clouds can often form downstream of mountains. This satellite image shows such clouds in the South Pacific where rocky mountains jut 600 meters (2,000 ft) above the sea. This disrupts air flowing east by forcing it to move up and over the island topography. The air does not simply settle back down on the other side, though. It must come back into equilibrium with its surroundings in terms of density and temperature. While doing so it will travel up and down along a wavy path. As it reaches the crest of the wave, humid air cooling condenses and forms a cloud. At troughs, the air warms and the condensation disappears. This creates the stripey cloud pattern in the mountain’s wake, which fades out as the atmospheric gravity waves die out. (Image credit: NASA/J. Schmaltz; via NASA Earth Observatory)
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Spinning Paint
Several years ago Fabian Oefner started spinning paint, and it’s been a perennial favorite online ever since. Here the Slow Mo Guys revisit their own paint-spinning antics by super-sizing their set-up. In some respects, it’s a little dissatisfying; as with their first time around, they don’t moderate the drill speed at all, so after the initial spin-up, the centrifugal acceleration is so strong that it just shreds the paint instead of showing off the interplay between the acceleration and surface tension’s efforts to keep the paint together.
In their largest experiment, though, the Slow Mo Guys get some interesting physics. Here there’s only a single slot for paint to exit, so the set-up doesn’t lose all its paint at once. The centrifugal acceleration flings the paint out in sheets that stretch into ligaments and then tear into droplets as they move further out. But there’s some more complicated phenomena, too. Notice the bubble-like shapes forming in the yellow paint on the lower right. These are known as bags, and they form because of the relative speed of the paint and the air it’s moving through. This is actually the same thing that happens to falling drops of rain! (Video and image credit: The Slow Mo Guys)
Nestling Droplets
Pay attention after a rainfall, and you may notice beads of water gathering in the corners of a spider’s web or along the leaves of a cypress tree (bottom right). Look closely and you’ll notice that the largest droplets don’t form along a straight fiber. Instead they nestle into the corners of a bent fiber (top image). Researchers recently characterized this corner mechanism and found that the angle at which the largest droplets form is about 36 degrees. This angle provides the optimal conditions for capillary action and surface tension to hold large drops in place. At smaller angles, a growing droplet’s weight pulls it down until the thin film holding the droplet near the top ruptures and the droplet falls. At larger angles, a heavy droplet will slowly detach from one side of its fiber and shift toward the other side until its weight is too great for the wetted length of fiber to hold. Then it detaches completely and falls. (Research and image credit: Z. Pan et al.; via T. Truscott)
What Makes Joints Pop?
Cracking one’s knuckles produces an unmistakable popping noise that satisfies some and disconcerts others. The question of what exactly causes the popping noise has persisted for more than fifty years. It’s generally agreed that separating the two sides of a joint causes low enough pressures to form a cavitation bubble in the sinovial fluid of the joint. But researchers have been divided on whether it’s the formation or the collapse of this bubble that’s responsible for the sound. Studying the phenomenon firsthand is difficult with today’s imaging technologies – none of them are fast enough to capture a behavior that takes only 300 milliseconds. As a result, scientists are turning to mathematical modeling and numerical simulation.
A recent study tackled the problem by modeling a joint that already contains a bubble and examining the bubble’s response to changes in pressure inside the joint. The pressure changes alter the bubble’s size and cause it to generate sound. When compared to experiments of people cracking their knuckles, the simulated sounds are remarkably similar in both amplitude and frequency. It’s not even necessary for the bubble to collapse completely to make the noise. Just a partial collapse is enough to sound just like that old, familiar pop. (Image credit: G. Kawchuk et al.; research credit: V. Chandran Suja and A. Barakat; via Gizmodo)
Jupiter’s Belts and Zones
Jupiter’s distinctive bands of colored clouds, known as belts and zones, have been an iconic part of the planet since they were first observed by Galileo. (The scientist, not the space mission!) They are considered part of Jupiter’s weather layer, the region of its atmosphere where storms reign. Thanks to gravitational measurements by the Juno spacecraft, we now know how deep these bands persist; they stretch about 3,000 kilometers into Jupiter! That means that Jupiter’s weather layer accounts for about one percent of the planet’s total mass. By comparison, Earth’s entire atmosphere makes up less than one millionth of its mass. What lies beneath Jupiter’s colorful clouds is also intriguing. The same gravitational measurements that indicate the weather layer’s depth also suggest that, beneath these storms, the rest of Jupiter rotates like a solid body. (Image credit: NASA, source; research credit: Y. Kaspi et al., submitted by Kam-Yung Soh)
Fissures in Africa
Pictures of an enormous fissure in Kenya’s East African Rift Valley have gone viral in recent weeks along with breathless reports about how part of the African continent is splitting away. And while Africa is splitting – very, very slowly – this crack, impressive as it is, may not have anything to do with it. Geologists familiar with the area are confident that the fissure is the result of recent torrential rains and flooding – not fresh seismic activity. For one, there have been no earthquakes in this area stretching back for several years. One theory is that the crack had actually been present for quite some time but was filled with softer volcanic ash that’s been swept away by the rains. Geologists will need to study it more closely to be certain.
One thing geologists agree on, though, is that the tectonic plates that make up Africa are slowly pulling apart, or rifting. (That’s why the area is known as a rift valley in the first place.) This happens as mantle convection causes two land masses to move away from one another. That’s happening right now along a fault running through Ethiopia, Kenya, and Tanzania, and it’s happened before. A similar rift caused the South American and African continents to separate. This doesn’t mean that the countries in East Africa are in danger of being parted by ocean any time soon, though. Geologists predict it will take on the order of 50 million years for the break to happen. (Image credit: Getty Images; Reuters/T. Mukoya; DailyNation)
Dune Networks
In sandy deserts, winds can build a vast network of dunes whose shapes depend on the winds that built them. This photograph, taken by an astronaut aboard the International Space Station, shows part of a Saharan dune field known as the Grand Erg Oriental. Of the five basic types of sand dunes, this field features all but one. The predominant winds of the region build most of the dunes into long, straight chains separated by interdune flats some 150 meters lower in elevation. Within the chains, there are linear dunes, created by winds blowing nearly parallel to the dune’s long axis. In places where winds tend to change directions, several linear dunes may merge to form star dunes, like the one just below and right of center in the image. Transverse dunes form perpendicular to the predominant wind direction. The one shown in the upper left of this image may have formed when multiple crescant-shaped barchan dunes merged. (Image credit: NASA, via NASA Earth Observatory)
Cloud Chambers
Cloud chambers were one of the first methods used to study radioactive decay and cosmic particles. Such chambers are filled with a cool, supersaturated cloud of alcohol vapor. When high-energy particles pass through, they collide with atoms in the chamber, ionizing them. Those ions then serve as nucleation sites for the alcohol vapor, creating a condensation streak that marks the particle’s passage. In some respects, they’re similar to the contrails that form behind airplanes. What you’re seeing is not the particle itself but evidence that it went by. YouTuber Nick Moore built his own cloud chamber. Learn more about it and see lots more great footage of it in action in the full video below. (Image and video credit: N. Moore)
Water on Mars
Recurring slope lineae (RSL) are seasonal features on Mars that leave behind gullies similar to those left by running water on Earth. Their discovery a few years ago has prompted many experiments at Martian conditions to determine how these features form. At Martian surface pressures and temperatures, it’s not unusual for water to boil. And that boiling, as some experiments have shown, introduces opportunities for new transport mechanisms.
Researchers found that water in “warm” (T = 288 K) sand boils vigorously, ejecting sand particles and creating larger pellets of saturated sand. Water continues boiling out of the pellets once they form, creating a layer of vapor that helps levitate them as they flow downslope. The effect is similar to the Leidenfrost effect with drops of water sliding on a hot skillet; there’s little friction between the pellet and the surface, allowing it to travel farther.
The mechanism is quite efficient in experiments under Earth gravity and would be even more so under Mars’ lower gravity. It also requires less water than alternative explanations. The pellets that form are too small to be seen by the satellites we have imaging Mars, but the tracks they leave behind are similar to the RSL seen above. (Image credit: NASA; research credit: J. Raack et al., 1, 2; via R. Anderson; submitted by jpshoer)
Space Shuttle Sonic Booms
The Space Shuttle had a famous double sonic boom when passing overhead during re-entry. This schlieren flow visualization of a model shuttle at Mach 3 reveals the source of the sound: the fore and aft shock waves on the vehicle. The nose of the shuttle generates the strongest shock wave since it is the first part of the vehicle the flow interacts with. This initial shock wave turns the flow outward and around the shuttle. The second boom comes from the back of the shuttle and serves to turn the flow back in to fill the wake behind the shuttle. (The actual shock wave would look a little different than this one because there’s no sting holding the shuttle like there is with the model.) The other major shock wave comes from the shuttle’s wings, but, at least for this Mach number, the wing shock wave merges with the bow shock, making the two indistinguishable. (Image credit: G. Settles, source)
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