Why Is Water Still Liquid at Minus Twenty? Nothing to Freeze Around

Freezing begins when enough molecules happen to arrange themselves into a tiny ice-like cluster that is large enough to keep growing rather than falling apart, and forming that first cluster from pure water alone is improbable, so it requires substantial cooling before it happens spontaneously. A surface or particle with a structure resembling ice makes it far easier, since molecules can arrange against that template rather than assembling from nothing. Ordinary water freezes at zero degrees because dust, container walls and impurities provide such templates everywhere. A cloud droplet is tiny, clean and surrounded by air, so it frequently contains no suitable template at all and stays liquid down to around minus thirty eight degrees, below which it freezes without help.

The particles capable of starting ice are surprisingly specific and surprisingly rare:

• •Mineral dust, particularly clay minerals blown from deserts
• •Volcanic ash
• •Certain bacteria carrying proteins that mimic an ice surface almost exactly
• •Pollen and fungal spores
• •Existing ice crystals, which start freezing on contact
• •Soot and some industrial particles, though these are less effective than once assumed

Most rainfall outside the tropics depends on this situation existing, through a mechanism worked out in the 1930s. Where ice crystals and supercooled droplets coexist in the same cloud, water vapour is at a pressure that is above saturation with respect to ice and below it with respect to liquid, so the ice crystals grow while the droplets evaporate to feed them. Crystals therefore grow rapidly at the expense of the surrounding liquid, becoming heavy enough to fall within minutes, collecting more on the way down and melting into raindrops before reaching the ground in warm conditions. A cloud of droplets alone grows them far too slowly to produce rain, so the presence of a few ice crystals among many liquid droplets is what makes precipitation efficient.

One group of templates deserves separate attention because it is alive and because it complicates the picture considerably. Certain bacteria that live on plant surfaces carry a protein arranged to match the spacing of an ice crystal closely, and they are the most effective naturally occurring starters known, triggering freezing at temperatures only a few degrees below zero where mineral dust needs far more cooling. The bacteria benefit, since frost damage to the plant releases nutrients they feed on. Those same organisms are carried into the atmosphere on wind and are found in clouds, in snow and in rain, which supports the idea that biological particles contribute meaningfully to precipitation. The protein was commercialised decades ago and is what allows snow machines to work at marginal temperatures.

The same physics causes serious problems and gets exploited deliberately. Aircraft flying through such clouds are hit by droplets that freeze on contact with the airframe, building ice on wings and control surfaces, which changes the shape of the aerofoil, adds weight and can bring an aircraft down, and which is why heated leading edges and inflatable de-icing boots exist. Freezing rain occurs when droplets fall through a cold layer near the ground and freeze on contact with everything they touch, bringing down power lines and making roads impassable. Cloud seeding introduces silver iodide, whose crystal structure resembles ice closely, to start the process deliberately, and its effectiveness remains genuinely difficult to demonstrate despite decades of programmes.

Ice needs a template to start on, and a clean cloud droplet frequently has none, so it stays liquid down to around minus thirty eight degrees. Mineral dust, volcanic ash and certain bacteria act as templates and are comparatively rare. Where crystals and droplets coexist, the crystals grow at the droplets' expense, which is how most rain outside the tropics forms, and the same process ices up aircraft.