Reykjavík residents and visitors may have noticed a distinctive stripe on Esja’s slopes in the last few days.
As can be seen, a white band of snow stretches up Esja’s slope for about 300m. Above the 300m mark there is much less snow, and in many places no snow at all, leading to the interesting band of colour.
The Meteorological Office of Iceland claims on social media that they’ve received many questions about the phenomenon and have provided a brief public explanation.
Typically, we see the opposite on mountain slopes: white peaks, with bare sides. This is because the higher the elevation, the lower the average temperature. So precipitation falling at the peak is much more likely to be snow, while precipitation falling on the slopes may simply turn to rain.
The pattern visible on Esja for the last few days, according to the Meteorological Office, can be explained by a cycle of freezing and thawing.
Average temperatures have been very low in Iceland his winter, but data shows brief temperature spikes in low-lying areas. These warming periods, followed by continued cold averages, create a cycle of thawing and re-freezing that compacts the snow, making it denser and icier.
However, because the peaks have remained at freezing temperatures, the snow at higher elevations has remained powdery. Powdery snow is of course more susceptible to wind and is more likely to be blown away in storms. The Meteorological Office pointed out the night of January 8-9 as especially windy, with recorded wind speeds of 20 m/s (45 mph). Sure enough, the next day was when the distinctive snow pattern became visible.