Although we are most familiar with the white, branching lightning caused by electrical discharge between clouds and the ground, there are many types of lightning. This fortuitous image captures two: tentacled red sprites and ring-like ELVES. Sprites extend upward from the top of a thunderstorm, in a large but weak flash that lasts only seconds. ELVES appear as a rapidly-expanding disc, thought to be caused by an energetic electromagnetic pulse moving into the ionosphere. They were first discovered in footage from a 1992 Space Shuttle mission. (Image credit: V. Binotto; via APOD)
Category: Phenomena
“Broken Water, Like Broken Glass”
How can you break water? By accelerating it so quickly that the pressure drop forms cavitation bubbles. Here, a steel piston rests against a transparent plate, all underwater. When a hammer strike accelerates the piston away at around 1000g, the severe pressure drop tears the water into bubbles (bottom, left). As the bubbles expand, the nearby piston squishes them into pancakes (bottom, center). As they continue growing, the bubbles press into one another, squeezing thin ridges of water between them. The result (center) resembles broken glass. (Image credit: J. da Silva et al.)
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Milano Cortina 2026: Speedskating Team Pursuit
Track cycling and speedskating often mirror one another, with similar events in each sport. In the team pursuit, for example, cyclists and skaters compete as a team to post the fastest time for a given distance. In cycling events, riders spend the race tucked into a line, with the lead rider providing a draft for their teammates. But that’s a tiring position for a cyclist, so every few laps the lead rider will pull off, move up the track, and drop behind their teammates for a rest. Speedskaters used to use the same technique. But no longer.
After working with aerodynamic simulation specialists, U.S. Speedskating pioneered a new race technique, in which skaters never change positions. Instead, each racer specializes in one position and skates while pushing the skater ahead of them. The technique requires a lot of practice, finesse, and trust; skaters in the later positions cannot see, skating as close as they can to the skater in front of them.
But, performance-wise, the new technique works. It’s taken U.S. women’s team pursuit from eighth in the world to number one. Other teams have adopted the technique, too, so this is likely what team pursuit will look like in the years to come. (Image credits: various, see image captions; via NPR)
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A Bubbly Heart
Next time you fill your water bottle, watch closely and see if you can spot a bubble heart like these. When a jet falls into a pool, it pulls air in with it. The low pressure of the jet pulls bubbles inward, even as shear pulls the bubbles downward with the sinking liquid. If the bubbles are large and there’s enough momentum in the jet, the lower portion of the bubble will get pulled into a conical shape, while the upper portion remains a hemisphere. That forms one lobe of the heart. The other half requires a second bubble. But with a little patience and luck, you can form a complete heart. Happy Valentine’s Day! (Image credit: S. Tuley et al.)
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Milano Cortina 2026: How Ski Skins Work
The 2026 Olympics include the debut of ski mountaineering (a.k.a. skimo), a sprint race heading both up and down the mountain on skis. During the uphill segment of the race, competitors use skins on their skis to help them climb; these skins then get ripped off (see below) before skiing back down.
As their name suggests, the first climbing skins used on skis were made from seal skin. By angling the seal fur, skiers could glide in the forward direction and resist sliding backwards. Modern skins may have animal or synthetic fibers, but they use the same physical mechanism. The angled hairs let skis slide forward easily, then grip and resist sliding backward. (Image credits: touring – H. Morkel, skins – Josefka, video – NBC Bay Area)
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Milano Cortina 2026: Cortina Sliding Center
This year’s sliding events–bobsleigh, luge, and skeleton–will take place at the brand-new Cortina Sliding Center. Built on the site of a historic sliding track, this new venue came together in only the last couple of years. It features a state-of-the-art refrigeration system that pumps a mixture of water and ethylene glycol beneath the track surface to keep the ice properly chilled. Each section of the track is continuously monitored to optimize the flow rate, temperature, and pressure of the refrigerant to keep the track at maximum performance while minimizing environmental impact.
According to the designers, it’s the first competition track to use a glycol-based refrigeration system, which should be more sustainable than the ammonia-based systems used elsewhere. For a sense of what a run is like, check out this skeleton driver POV run from the facility’s shakedown competition last year. (Image credit: LMSteel; video credit: tuff sledding)
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Milano Cortina 2026: Curling Stones
Ailsa Craig sits about 10 miles off the Scottish coast, a granite dome left behind by a volcanic event millions of years ago. This small, now-uninhabited crag is the birthplace for every Olympic curling stone. It’s where Kays of Scotland, which has made curling stones for the Olympics since the sport appeared in the first Winter Games in 1924, gets their granite.
Curling stones have to withstand both cold and collisions, something Ailsa’s microgranite excels at. Its elasticity keeps it from cracking, and Ailsa’s unique blue hone granite resists water absorption, so that freeze-thaw cycles don’t erode the surface. That waterproofing makes for the perfect running surface. It’s no wonder that the majority of curling stones in the world originate in Ailsa. (Image credit: A. Grant/AP; via AP)
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Instabilities in a Particle Flow
![Composite image of bed layers for 4 different particle density ratios. Text reads, "The wave amplitude and growth rate increase with particle density ratio but only if [the density of large particles is greater than the smaller particle density]."](https://fyfluiddynamics.com/wp-content/uploads/KHbed3.png)
Even though particles are not (strictly speaking) a fluid, they often behave like one. Here, researchers investigate what happens when two layers of particles–with different size and density–slide down an incline together. The video is tilted so that the flow instead appears from left to right.
When the larger, denser particles sit atop a layer of smaller, lighter particles, shear between the two layers causes a Kelvin-Helmholtz instability that runs in the direction of the flow. This creates a wavy interface that lets some small particles work upward while large particles shift downward.
At the same time, a slice across the flow shows that plumes of small particles are pushing up toward the surface, driven by a Rayleigh-Taylor instability. The researchers also look at what happens when the particles are fluidized by injecting a gas able to lift the particles. (Video and image credit: M. Ibrahim et al.; via GFM)
A Supernova in Motion
In 1604, astronomers first caught sight of Kepler’s Supernova Remnant, a massive explosion some 17,000 light-years away. Twenty-five years of observations from the Chandra X-ray Observatory went into making this timelapse, which shows the supernova remnant‘s material pushing into the surrounding gas and dust.
In its fastest regions, the supernova remnant is moving around 2% of the speed of light–some 22 million kilometers per hour. Slower parts of the remnant are moving at just 0.5% of light-speed. (Image credit: NASA/CXC/SAO/Pan-STARRS; via Gizmodo)
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Jupiter in a Lab
The vivid bands of a gas giant like Jupiter come from the planet’s combination of rotation and convection. It’s possible to create the same effect in a lab by rapidly spinning a tank of water around a central ice core. That’s the physical set-up behind this research poster–note the illustration in the lower right corner. The central snapshots show how temperature gradients on the water surface change the faster the tank rotates. At higher rotational speeds, the parabolic water surface gets ever steeper and Jupiter-like temperature bands form. (Image credit: C. David et al.)
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