The other day, NASA’s Earth Observatory posted some interesting pictures about localized snow in the Netherlands. Several fields in Heensche Molen, a hamlet in the western Netherlands, glowed white as though a spotlight had been shone over them, leaving nearby areas untouched. According to their interpretation, the snowfall was caused by a drop in temperature that led to the condensation of the tiny droplets of water in the fog over these areas into ice crystals, which fell as snowflakes.
As Pola Lem explains on Earth Observatory, fog-induced snow is a rare and somewhat obscure phenomenon, that typically forms next to industrial sites.
“Big chimneys release water vapor and other gases and particulates, which can lead to the formation of fog. It also turns out that these emissions can create snow when the weather gets cold enough,” she wrote.
Such an incident occurred in December 2007, also in The Netherlands, when a cold fog started to develop into a thick stratus cloud. Dutch meteorologist, Wim van den Berg, who described the event in a 2007 paper, wrote:
Mixing within the fog caused the temperature to drop gradually to −4 or −5 °C throughout the cloud. The temperature decrease caused additional condensation into tiny water droplets but also ice crystals. In such a mixed cloud the Wegener-Bergeron process starts, and ice nuclei grow at the expense of water droplets with small snowflakes as a result. To start the process, condensation nuclei are necessary, and they are more widespread downwind of cities and industrial zones. As the snowflakes grow, they start to fall down and reach the surface close to their production area.
The 2007 event caused scattered spots of snow, about an inch or two in depth.
The same happened in January 2017, when these images were taken.
“After many days with fog and/or low clouds and subfreezing temperatures, on January 17 several places reported snow,” Wim van den Berg confirmed with NASA in an email. “It was very local, mostly west of industry,” as surface wind was light easterly, “but it did cause some unexpected slipperiness.”
