A cloud of dust from Tunisia brought coloured snow to Italy and rain to other parts of Europe, on 19 February.
29 September 2022
18 February 2014
In this SEVIRI Dust combination (IR12.0–IR10.8, IR10.8–IR8.7, IR10.8) from 19 February 2014 at 00:00 UTC a cloud of dust is seen to blow from Tunisia to Italy.
The 10m wind speed was 5–13m/s. The dust is pink-magenta, having high contributions from both red and blue components.
The red hue comes from the transmissivity in 12.0µm being larger than in 10.8µm for thin (non-opaque) dust clouds. The blue contribution (temperature) is high for warm low clouds like dust.
Download RGB animation 17 Feb, 00:00 UTC–19 Feb, 12:00 UTC
In the daytime convection RGB image from 12:00 UTC on 19 February, the effect of the dust on the high level clouds can be seen. Dust aerosols have a large impact on cloud microphysics such as cloud phase and cloud particle size. In general, clouds that have ingested dust glaciate more quickly, forming a large number of very small ice particles. These have a higher reflectance than large ice particles in the 'microphysical' channels (near-IR 1.6 and IR 3.9 micrometer). Thus, they appear as bright orange in the day microphysics RGB or as bright yellow in the convection RGB.
The dust was carried from the Sahara to Europe on an intense meridional flow (an airmass of warm air) which also brought above-average temperatures to some countries in February.
When the mass of warm air started to withdraw under the thrust of fresh Atlantic currents, dust arrived from the Sahara. This was suspended in the atmosphere providing the condensation nuclei for condensation of water vapour, leading the clouds to quickly glaciate, forming many small ice particles.
This photograph taken at the Italian ski resort Riserva Bianca by Andi Jacomet, shows the resulting coloured snow which fell overnight of 19/20 February.
Air quality measurements taken at the Sonnblick observatory by ZAMG and at the Jungfraujoch in the Alps, taken by the Swiss Federal Office for the Environment, found high concentrations of PM10 (particulates up to 10 micrometers in size) during the dust event.