Fairbanks, Alaska
If the temperature is -40 °C or colder, toss a pan of boiling water into the air (taking great care, of course, not to scald yourself or others). It won’t splash to the ground. Instead, with an odd ripping sound, the water rapidly freezes into a cloud of tiny ice crystals, plus a few larger chunks of snow or ice. “It’s an amazing thing to throw this water in the air that never comes to Earth,” says Paul Ward, a biology teacher who spent two winters working with the British Antarctic Survey and has photographed the phenomenon.
The trick works best at -40 °C or below because all moisture in the air freezes spontaneously at these temperatures. Any warmer and water likes to be nudged into freezing by tiny particles of other substances, which it collects around in a process called nucleation. If such particles are missing, water can stay in a supercooled liquid state at temperatures well below its usual freezing point. But below the -40 °C mark, no nucleation is needed. Physicists refer to this critical temperature as the “point of homogenous nucleation”.
The rip you hear is probably the sound of ice crystals bursting, according to David Newman, a physicist at the University of Alaska Fairbanks. In such cold air, water droplets are bound to freeze from the outside in. Ice is less dense than water, so when the middle freezes, it expands and pops its already-frozen outer shell.
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Look for thin, glowing arcs and rings around and near the Sun or moon. You get these haloes when ice crystals in the atmosphere act as prisms, refracting the light. “The shapes of the crystals are very simple when it’s cold and they’re growing slowly,” says atmospheric scientist Stephen Warren of the University of Washington, Seattle, who studies Antarctica’s climate. “They’re long in the hexagonal direction, like pencils, or long in the other direction, and you get a hexagonal plate. Because they’re so simple, they have nice flat faces and nice sharp corners. They can make beautiful haloes.” Some rare haloes may also be formed by crystals that are shaped like pyramids. Haloes are easiest to see when you block the centre of the light, which you can do with your fist. Even a nearby street lamp can work as your light source.
Ice crystals can form in air at any temperature below freezing, but really spectacular displays tend to appear between -28 and -34 °C. Below -34 °C the crystals tend to be too small for great haloes. Above -28 °C too much moisture may hang around as water vapour instead of forming crystals. Of course, the temperature in the sky may be much colder than that at ground level. On Easter Sunday in 1974, a rare display of multiple haloes was seen from England and the Netherlands.
If the temperature is -17 °C or colder, partially fill a balloon with water, tie the top and place it outside. At these temperatures, the top and sides will freeze much faster than the inside. Time it right and you can peel off the balloon and dump the slush from the middle. Turn over the icy shell, place a candle inside, and you’ve got a lantern to light up your front walk for carollers on Christmas Eve.
Blow soap bubbles outside at temperatures below about -20 °C, and they’ll freeze into icy spheres only a few molecules thick before they hit the ground. That’s because soap bubbles, despite their name, are mostly made of water plus a bit of soap to relax the surface tension. When they strike the ground, the frozen bubbles either shatter or roll around. “They could last for hours,” says Newman.
On a cold, windless day, look to the horizon for the phenomena called “looming” or “towering”, in which distant mountains appear to either float or rise taller than they really are. On a winter’s day in Fairbanks, for example, 6194-metre Denali (also known as Mount McKinley), can seem much larger than usual, dominating part of the horizon 250 kilometres to the south-west. This happens because cold, calm conditions often lead to temperature inversions, in which cold air pools close to the ground under a layer of warmer air. “Inversions are more likely to happen the colder it gets,” says atmospheric scientist Warren. “At the South Pole they happen all the time.” Desert mirages are a related phenomenon: they form when hot air pools close to the ground under a layer of cooler air.
Cold air is denser air, and when light rays cross a density boundary, they refract, or bend, toward the denser layer, which makes distant objects look unnaturally tall when that layer is at ground level. The same sort of bending can create the mesas, walls and spires known as Fata Morgana, which appear when an inversion layer channels several images of a distant landmark to your eyes. These copies stack up to turn even normally insignificant bumps in the distance into dramatic skyscrapers.