FYFD

Radiation is invisible, but it's not too difficult to build an apparatus that lets you see it. This video shows the ghostly aftermath of passing radiation in a cloud chamber, one of the first set-ups used to study radiation. (Image and video credit: L. Gledhill)

Joe Hanson over at "It's Okay to Be Smart" has a great video on the random walk photons have to make to escape the core of the sun and other stars, but it’s missing a key ingredient: convection!  (Video and image credit: It's Okay to Be Smart)

At high, dry altitudes, fields of snow transform into rows of narrow, blade-like formations as tall as 2 meters. Known as penitentes -- due to their similarity to kneeling worshipers -- these surreal snow sculptures form primarily due to solar reflection. Surrounded by dry air and intense sunlight, the snow tends to sublimate directly into water vapor rather than melt into water. This turns an initially flat snowfield into one randomly dotted with little depressions. The curved surface of those depressions helps reflect incoming sunlight, causing the indentations to grow deeper and deeper over time. Although the high Andes are best known for their penitentes, they form elsewhere as well. Recent work has even identified them on Pluto! (Image credit: G. Hüdepohl; research credit: M. Betterton)

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)

We’re used to radiation being invisible. With a Geiger counter, it gets turned into audible clicks. What you see above, though, is radiation’s effects made visible in a cloud chamber. In the center hangs a chunk of radioactive uranium, spitting out alpha and beta particles. The chamber also has a reservoir of alcohol and a floor cooled to -40 degrees Celsius. This generates a supersaturated cloud of alcohol vapor. When the uranium spits out a particle, it zips through the vapor, colliding with atoms and ionizing them. Those now-charged ions serve as nuclei for the vapor, which condenses into droplets that reveal the path of the particle. The characteristics of the trails are distinct to the type of decay particle that created them. In fact, both the positron and muon were first discovered in cloud chambers! (Image credit: Cloudylabs, source)

Thermal imaging of emperor penguins in Antarctica shows that, in still conditions, large portions of their bodies remain colder than ambient temperatures. In the image above, the heads, beaks, eyes, and flippers of this pair of penguin are the warmest while much of their feathered surface remains several degrees colder than the temperature around them. Not only does this indicate that the penguins' skin and feathers are extremely effective insulators--the core temperature of each penguin is roughly the same as a human's--but it means that the penguins are losing heat via radiative cooling toward the sky, the same way your car does when frost forms. The measurements in the study are for penguins at least one body length away from any other penguins; of course penguins typically huddle together to generate additional warmth. The mathematics of this behavior are under active research. (Photo credit: D. McCafferty et al.; via Wired)

Reader Mike L asks:

Great question! Frost forms on surfaces when their temperature drops below the freezing point of water and the dew point of the surrounding air. The water vapor in the air gets deposited as a solid directly; this is called deposition. This means that the surface--in this case your car--has to be colder than the nearby air. Neither conduction nor convection of heat between your car and the surrounding air can cause this drop; heat transfer between your car and the surrounding air would tend to make them the same temperature, not make the car colder than the air. The third--and typically least effective--type of heat transfer, radiation, is the answer because it allows heat transfer between two objects that are not in direct contact like the air and car are.

Frost typically forms on still, clear nights with little clouds or wind. A car sitting beneath a clear night sky will radiate heat out into space. Since space is much, much colder than the air, this radiation cooling to space allows the car's surface temperature to drop below that of the surrounding air, which is not a good radiator by comparison. On a night with little wind (and thus little convection), this radiation cooling can be quite effective. Frost will tend not to form on one's car under a carport because the car is sheltered from the night sky, blocking such radiative cooling. Having a tree or house blocking the car from the night sky is also effective at preventing frost formation. (Photo credit: N. Sharp; with thanks to Keri B and Jerry N for the meteorological assistance)