Although they may look sinister, roll clouds like this one are no tornado. These unusual clouds form near advancing cold fronts when downdrafts cause warm, moist air to rise, cool below the dew point, and condense into a cloud. Air in the cloud can circulate around its long horizontal axis, but the clouds won’t transform into a tornado. Roll clouds are also known as Morning Glory clouds because they often form early in the day along the Queensland coast, where springtime breezes off the water promote their growth. The clouds do form elsewhere, though; this example is from Wisconsin in 2007. (Image credit: M. Hanrahan; via APOD)
Tag: science
Diving From Above
Blue-footed boobies, like many other seabirds, climb to a particular altitude before folding their wings and diving head-first into the water. This acrobatic feat balances the bird’s force of impact and the depth it can reach to ensnare fish swimming there. It’s an incredible process to watch, a fascinating one to study, and, here, a beautiful glimpse of the natural world from a perspective we don’t typically see. (Image credit: H. Spiers, Bird POTY; via Colossal)
Butterfly Scales
Catch a butterfly, and you’ll notice a dust-like residue left behind on your fingers. These are tiny scales from the butterfly’s wing. Under a microscope, those scales overlap like shingles all over the wing. Their downstream edges tilt upward, leaving narrow gaps between one scale and the next. Experiments show that, although butterflies can fly without their scales, these tiny features make a big difference in their efficiency.
At the microscale, a butterfly’s scales overlap like roof shingles but are tilted upward, leaving cavities in the downstream direction. When air flows over the scales, tiny vortices form in the gaps between. These laminar vortices act like roller bearings, helping the flow overhead move along with less friction and, thus, less drag. Compared to a smooth surface, the scales reduce skin friction on the wing by 26-45%. (Image credit: butterfly – E. Minuskin, scales – N. Slegers et al., experiment – S. Gautam; research credit: N. Slegers et al. and S. Gautam; via Physics Today)
This lab-scale experiment shows how air moves over butterfly scales. As flow moves from left to right, small persistent vortices form in the gaps between scales. These act like roller bearings that reduce the skin friction from air moving past. 
Fishing With Mucus
The scaled wormsnail isn’t much for travel. It lives its whole life cemented to a rock in the tidal lands. And when you can’t go out for food, you have to wait for the food to come to you. During high tides, the snail lets out tendrils of mucus that capture bits of kelp, plankton, and whatever else the water brings. The snails haul their catch directly into their mouths, relying on the mucus’s impressive viscoelasticity to withstand the journey. (Video and image credit: Deep Look)
Drag Is Greatest Before Submersion
A new study shows that partially submerged objects can experience more drag than fully submerged ones. This unexpected result comes from the excess fluid that piles up ahead of the object, as seen in the image above, where flow is moving from left to right. The experiments used centimeter-sized spheres and showed that the maximum drag on a nearly-submerged sphere could be 300-400% greater than the drag on a fully submerged sphere.
Even more surprisingly, they found that water-repellent hydrophobic coatings — which are often suggested for drag reduction — actually increased the drag even further on partially submerged spheres. That’s because the water-repelling coating caused an even larger build-up of fluid ahead of the sphere, increasing the pressure on the front side of the sphere and creating even more drag. Spheres with a hydrophilic coating had less water build-up and thus lower drag.
The study suggests that — at the centimeter-scale — drag physics at the air-water interface may be more complicated than we assume. (Image and research credit: R. Hunt et al.; via Physics World; submitted by Kam-Yung Soh)
Hitting Molten Steel
Watching droplets burst is often fascinating, but it’s rare that we get to watch droplets of molten metal. In this Slow Mo Guys video, though, they’re shattering globs of molten steel and filming the results in slow motion. It’s the kind of starburst that breaks compression algorithms but remains beautiful regardless. (Video and image credit: The Slow Mo Guys)
“Black Ice”
Ice, black ink, and flowers combine in filmmaker Christopher Dormoy’s “Black Ice.” Filmed during the COVID-19 lockdowns, the video is an exploration of the creativity one can achieve when constrained. I especially enjoy seeing the tiny bubbles trapped in the ice escape as ink billows past, and the views of ice tunnels invaded by ink are incredibly cool. For a behind-the-scenes look at how Dormoy achieved many of the shots, see this video. (Video and image credit: C. Dormoy)
Changing Climes on Mars
China’s Zhurong rover explored Utopia Planitia on Mars from May 2021 to December 2022. During that expedition, the rover uncovered evidence of a major shift in climate that took place some 400,000 years ago. Originally, the area was covered in crescent-shaped barchan dunes formed by winds from the northeast. But after Mars exited its last ice age — courtesy of a shift in its rotational axis — the winds shifted around 70 degrees, coming from the northwest. Those shifted winds eroded the barchan dunes and caused new transverse ridges to form atop them. (Image credit: NASA/JPL-Caltech/UArizona; research credit: J. Liu et al.; via Gizmodo)
Surviving the Dry Season
The Zambezi River winds through eastern Africa, providing much-needed water to plants and animals there. But during the dry season, when rain and river water are scarce, most trees go bare. The apple ring acacia is the exception. These towering trees rely on their taproot, which delves 30 meters or more into the ground, to deliver an ongoing supply of water. Flush with water, the trees remain green, providing vital food and shade to animals during the harshest season of the year. (Image and video credit: BBC Earth)
Aquatic Escape Artists
Springtails are tiny hexapods found living on the air-water interface. Like other creatures living at the interface, they sometimes need to make a quick escape. For the springtail, that means a high-flying leap, driven by their fork-shaped furcula. The springtail soars into the air, where it contorts its body and uses aerodynamic forces — along with a droplet it carries on its belly — to orient itself. For landing, it uses that droplet as a sticky anchor that helps it adhere to water (or ground) instead of bouncing. Nailing that landing sets it up to make another daring escape as quickly as needed. (Video and image credit: Deep Look; research credit: V. Ortega-Jimenez et al.)
