Help - problems with reading the map?


Is this map OK? It shows that there is snow where I live, but I see through the window that there is now snow! To answer those questions we have to know a little bit more about how the map is developed.

The map is produced within the The Interactive Multisensor Snow and Ice Mapping System (IMS) project, based on data collected with various satellite sensors. The data are interpreted manually. The sensors observe Earth mostly daytime, when the Earth's surface is illuminated by the solar light. With no clouds it is quite easily to see the ground, and decide if it is covered with snow, or not. The problem starts when clouds appear, as they cover the planet's surface, making impossible to indicate where snow can be.

On IMS maps information on snow cover extent is continuous - there are no gaps in the data due to clouds. How is it possible? IMS' philosophy is as follows. Suppose that one day a pixel is marked as 'snow'. Observation on a second day confirms that status (pixel still classified as 'snow'). However on the third day there were clouds that prevented the observation of land surface. In such case, the IMS project analyst maintains the status of 'snow' as long as clouds occur. It can be a day, or two, or week until next cloud-free observation will be available. Then the analyst is able to reinvestigate the status and maintain 'snow', or change to 'no snow' if appropriate.

In Poland (especially winter-time), it happens that the whole country is covered by the clouds for few consecutive days. If such situation takes place IMS data cannot be updated on a regular basis. Thus, even if the map was generated yesterday, this does not mean that yesterday a satellite was able to observe the actual extent of snow cover in Poland. One can note a consequence of that fact when looking at the plots of snow cover extend - sometimes the line seems to be 'frozen', indicating exactly this same fraction of country covered by snow during consecutive days.

Sometimes the discrepancy between map and reality is driven by the nature of snow cover. Consider following situation. At some location there was a sleet during night time. There was not enough now to make a snowman, but enough to cover the surface with thin layer of wet snow. During the day the temperature rises, and the snow melts. In the afternoon there is not a trace of snow. This ephemeral snow cover could be missed by the satellites at all: satellite could fly over the area not in the morning but in the evening.

Another aspects is the misclassification of 'snow' as 'no snow' (and vice versa) by the analyst, who investigated the data. Since the decision is made by specially trained personnel, the risk of misclassification is significantly smaller, than for purely automatic algorithms, however - it still exists.

Now, taking into consideration all above-mentioned circumstances, you are ready to draw conclusions from the map.


© 2010-2015  | Earth Observation Group at CBK PAN
Data analysis and web site concept: Andrzej Kotarba
Web site maintenance: Andrzej Kotarba and Adam Włodarkiewicz
Contact info:

Centrum Badań Kosmicznych PAN

Web site was partialy deweloped under the framework of the following projects:
  GEO Network for Capacity Building - GEONetCab (, 2009-2012
  Earth Observation for Economic Empowerment - EOPOWER (,

Disclaimer: We put our best effort to make this web site as accurate as possible. Nevertheless the web site comes with no warranty to its users. By using this web site and/or data published on this web site you acknowledge, that you are doing this on YOUR OWN risk and responsibility. Acknowledgement: Source data used by this web site (The Interactive Multisensor Snow and Ice Mapping System, or IMS) originates from NOAA servers and are freely available for everyone at