Clouds spiral behind the islands of Tenerife and Gran Canaria in this nighttime satellite imagery. Although it’s not entirely unusual to see these von Karman vortex street clouds in the wakes of islands, this is the first time I’ve seen them at night. They form when winds off the ocean are forced up and around rocky islands. Like air moving past a cylinder, the flow forms a swirling vortex off one side of the island, which separates and moves downstream while another forms on the island’s opposite side. When the resulting flow mixes with a cloud layer, we can see the pattern from space. (Image credit: J. Stevens; via NASA Earth Observatory)
Category: Phenomena
Supercooling Thermodynamics
In the latest Gastrofiscia episode, Tippe Top Physics takes on thermodynamics and the complicated truth behind certain phase changes. Although we’re accustomed to thinking of water freezing at 0 degrees Celsius and boiling at 100 degrees Celsius, reality is more complex, and temperature is only one of the factors that goes into a change of phase. Pressure and purity also play an important role.
This is why it’s possible, for instance, to supercool purified water to below 0 degrees Celsius without freezing it. Liquid water needs a nucleus to serve as a seed for its freezing. Without dust or other impurities, it takes a lot of energy for water to spontaneously generate its own nucleus. Check out the full video to see how and why that’s so. (Image and video credit: Tippe Top Physics)
Bay of Fundy Tides
Canada’s Bay of Fundy has some of the wildest tidal flows in the world. Every six hours, the flow direction through the strait shifts and tidal currents rise to several meters per second. This creates distinct jets a couple kilometers long that pour from one side of the strait to the other.
What you see here is a numerical simulation of the flow using a technique called Large Eddy Simulation (or LES, for short). It’s one method used by fluid dynamicists to model turbulent flows without taking on the complexity of the full Navier-Stokes equations. At large lengthscales, like those of the jets and eddies we see above, LES uses the exact physics. But when it comes to the smaller scales – like the flow nearest the shores or the bottom of the strait – the simulation will approximate the physics in order to make calculations quicker and easier. Models like these make large-scale problems – including modeling our daily weather patterns – possible. (Image credit: A. Creech, source)
Waves on a Supercell
This Colorado supercell thunderstorm features an unusual twist. Notice the sawtooth-like protrusions along the outer cloud wall. These are Kelvin-Helmholtz waves, like these fair-weather clouds we’ve seen before, but instead of occurring vertically, they project horizontally! That implies that the invisible layer of air just outside the cloud wall is moving faster than the wall itself. That creates shear along the outer edge of the cloud wall and causes these waves to form. This is the first time I’ve ever seen this sort of thing. What an awesome photo! (Image credit: M. Charnick; submitted by jpshoer)
Getting Cold
Just as some chemical reactions produce heat, many chemical combinations absorb heat. In “Getting Cold,” the Beauty of Science team demonstrates this by showing endothermic processes in both visible and infrared light. Combinations that appear humdrum from our normal perspective suddenly become vibrant and interesting when we see the temperature variations accompanying them.
Evaporation is a good example. As humans, we sweat so that when our sweat evaporates off our skin, it takes heat away with it. Water (the main ingredient in sweat) isn’t the fastest evaporating liquid, however. Here it’s shown alongside ethyl acetate, a common ingredient in nail polish remover. And anyone who’s used nail polish remover is familiar with the chill it leaves behind as it evaporates. Just look how much colder and darker it is when evaporating! (Video and image credit: Beauty of Science)
Superheating
Being hot isn’t always enough to make water boil. To form vapor bubbles, water and other liquids need imperfections that serve as seeds. In the absence of these, the liquid can become superheated, reaching temperatures higher than its boiling point without forming bubbles. Superheated water can be quite dangerous because it appears to be cooler, but once it’s disturbed – thereby breaking its surface tension – vapor bubbles form rapidly and explosively. You can see in the animation above just how quickly and unsteadily a sudden vapor bubble expands as it rises to the surface. (Image credit: C. Kalelkar and K. Raj, source)
Waterspouts
Despite their ominous appearance, these waterspouts – like most of their kind – are fair-weather phenomena unrelated to tornadoes. They can form when cold, dry air moves over warm waters. As warm, moist air rises from the water’s surface, air is drawn in from the surroundings to replace it. Any vorticity in that air comes with it, growing stronger as it gets pulls in, thanks to conservation of angular momentum. That action creates the waterspout, which becomes visible when the warm, humid air cools enough to condense and form a cloud wall. (Image credit: R. Giudici; via EPOD)
Seeing Sound
It’s not always easy to imagine how waves travel, but with this demonstration, you can see sound waves and how they reflect and defract. The set-up uses schlieren optics that show light and dark bands where strong changes in density take place. This, combined with a stroboscopic light, makes it possible to see the wave fronts from the acoustic transducer on the left side of the screen. Once the wave is apparent, introducing a reflective object lets us see exactly how sound waves bounce, reflect, and interfere. (Image and video credit: Harvard Natural Sciences Lecture Demonstrations)
How to Build a Lava Moat
If you’re looking for a new and impractical way to protect your home, here’s a great option: a lava moat. Nothing says “Don’t try to knock on my door” like a glowing inferno of molten rock. And Minute Physics – along with xkcd – has put together a short, handy guide to some of the challenges you’ll face in building and maintaining this fearsome fortification. If running your own commercial-scale power plant seems overly daunting but you still want to see what lava’s all about, I have good news; here’s a selection of some of my favorite looks at lava here at FYFD:
– Upstate NY’s homemade lava
– What happens when you step on lava
– A veritable river of lava in action
– What happens when water meets lavaNow, if you’ll excuse me, I’m off to Hawai’i for the next two weeks. There will be lava. (Video credit: Minute Physics)
Why Do Backwards Wings Exist?
Over the years, there have been many odd airplane designs, but one you probably haven’t seen much is the forward-swept wing. While most early aircraft featured straight wings, rear-swept wings are fairly common today, especially among commercial airliners. A rear-swept wing has its forward-most point at the root of the ring, where it attaches to the fuselage. The sweep breaks up the incoming flow into a chordwise component that flows from the leading edge to the trailing edge of the wing and a spanwise component that flows along the wing. Compared to straight wings, a swept wing provides better stability and control when flying at transonic speeds where shock waves can form on the wing (even though the plane itself is not supersonic).
The trouble with rear-swept wings is that when they stall, they do so from the wingtips inward. Since the ailerons that control the plane’s orientation are out near the wingtips, that’s a problem. Forward-swept wings were supposed to solve this issue because they would stall from the root outward. But they came with a whole new set of problems, which included the need for robust onboard computers controlling them constantly to keep them in stable flight. In the end, the disadvantages outweighed any gains and so, for the most part, the forward-swept wing design has seen little flight time. (Image and video credit: Real Engineering)
