How it works ?
These snow cover maps come from satellite observations (every 2 to 5 days). They are therefore not in real time and in the event of the presence of clouds, previous observations are used.
The data is partly generated from the Let-it-Snow algorithm (Gascoin & all. 2018)
The update of the data present on the site depends on 3 factors:
- Satellite acquisition: this is done every 2 to 5 days depending on the orbit of the satellite..
- Presence of clouds: in the event of the presence of clouds the ground is not visible, then, the latest information available is used. In case of prolonged cloud cover it is not uncommon to see delays up to a month.
- Updating data online. Facing the cost induced by the hosting method, the update can be spaced out. It is normally 2 days but can go up to 5 days.
How to use the map?
You have to understand the data differently depending on the time of year:
- If we are in a melting period (usually from March to September), a pixel without snow has a high probability of always being free of snow. A snow-covered pixel, on the other hand, may have seen its snow melt in the meantime. The date of the last snow observation gives you an indication of the reliability (a snow-covered pixel at low altitude that has not been updated for a month has a better chance of being cleared of snow than a pixel at high altitude).
- If you are in the winter period, a pixel free of snow on the map may very well have been covered by a snow event since then, treat the information with caution depending on the weather conditions.
- To have the date of the last observation of the snowy pixel simply click on it.
These limitations will soon see improvements by adding information on the map / on the pixels, actually:
- For reasons of cost and precision, altitudes below 800 m are never covered.
- The refresh rate as mentioned above.
- No information on the depth of snow, a snowy pixel can have a centimeter as well as a meter of snow.
- In the forests there are many false negatives (no snow detected even though there is some), refer intelligently to the pixels outside the neighboring forest (knowing that there is often less snow in the forest than in the open).
The GPS tracks are indicative and allow you to give some marks on the maps. If you want to add the trace of a great hike or a great trail, do not hesitate to contact me.