So far it’s been a mild and soggy start to winter, but there has been the odd snowflake for some. And even if you haven’t observed any snow on the ground, it’s likely that your rain started life as snow. Curiously, different types of snowflake lead to different kinds of rain.
Every snowflake is different, and their shape and size depends greatly on temperature. In the 1930s, Japanese physicist Ukichiro Nakaya discovered this when he grew snowflakes in his lab. At -2°C they form thin hexagons, at -5°C hollow slender pencil-shaped crystals, and at -15°C, you get the familiar six-armed hexagonal snowflakes. At -25°C multiple, hexagonal crystals sprout from the same frozen droplet.
Chris Westbrook, a meteorologist at the University of Reading, uses radar to watch how snowflakes change shape as they fall, and what kind of precipitation they are likely to produce when they hit the ground.
“If the snowflakes don’t grow very large then the raindrops won’t be very big either, and so the rain will be light,” he explains. By contrast, in regions with strong updrafts, the flakes tend to collect and freeze lots of liquid droplets to make big, heavy ice particles, which are more likely to produce a rainfall deluge on the ground. Feeding this kind of information into weather forecast models will ultimately improve rain and snowfall forecasts.
So next time you are feeling fed-up with December drizzle, perhaps you can draw comfort from thinking about the incredible journey each droplet has taken over the last hour or two, tumbling through the sky and turning from snowflake to rain.
