Le 1er avril est la date souvent utilisée par les hydrologues pour caractériser le stock de neige disponible avant la saison de fonte. A partir des produits neige Theia, j'ai calculé la durée d'enneigement par pixel de 20 m dans les Pyrénées depuis le début de l'année hydrologique (le 1er septembre 2018) jusqu'au 1er avril 2019.
Durée de l'enneigement entre le 01 Septembre 2018 et le 01 Avril 2019
The snow cover is not always white. Sometimes it is orange, sometimes it is black. The images below were captured by Sentinel-2 in Prokopyevsk, Russia.
According to the Siberian Times the deposition of black dust on snow in February 2019 was due to the failure of a filtering system in a coal processing plant. Looking at the picture below, I have the feeling that this kind of event was not exceptional in Prokopyevsk this winter...
Source: Siberian Times 15 Feb 2019. Pictures from Kemerovo region by Orlovprklife, Willravilov, Typical Kemerovo
Here in southwest France ski lovers did not really enjoy the beginning of the snow season... But how does it compare to the previous years? Using Sentinel-2 and Landsat-8 data, we computed the snow cover duration since September 01 until January 20 for the past three snow seasons in the Alps and Pyrenees.
Snow cover duration (in days) from 01 September of year N-1 to 20 January of year N
We  have processed 6205 Sentinel-2 images and 593 Landsat-8 images to compute the annual snow cover duration in the Alps and the Pyrenees at 20 m resolution for hydrological years 2016-2017 and 2017-2018. The snow cover duration (or snow persistence) is defined as the total number of days with snow on the ground over a hydrological year (from 01 September to 31 August). We also added the ski runs from the great OpenSnowMap project.
We are thinking to distribute a new product that would provide the snow cover duration in the Alps and the Pyrenees over a hydrological year (snow persistence map). Here is a preview of a "beta" version that is only 100 m resolution for now.
Snow cover duration from 01 Sep 2017 to 31 Aug 2018. Pixels with less than 60 days were masked out.
A few words on the method (see also here): this map is generated after a linear interpolation at the daily time step of every available Theia snow product. Here we have selected 26 Sentinel-2 tiles, which represents 3189 snow maps for the study period, that is 96 billion pixels to process.
With the accumulation of Sentinel-2 data it becomes possible to look at interannual variability. Here is for example a visual comparison of the snow cover duration calculated from January to May for three years in the Pyrenees (31TCH):
Snow Duration from Jan 01 to May 31 in the Pyrenees (tile 31TCH)
Le conseil départemental des Alpes de Haute-Provence a mis en ligne le tracé des 6 500 km de sentiers aménagés et balisés sur l’ensemble de son territoire. Ces sentiers sont visibles sur le joli site www.rando-alpes-haute-provence.fr qui fonctionne avec le logiciel Geotrek.
When I present the potential of Sentinel-2 for snow science, I often tell that the spatial resolution of Sentinel-2 is sufficient to detect snow at the scale of the ski runs. Because a picture is worth a thousand words, here is the Sentinel-2 view of the only two ski resorts in southern Africa on July 11.