Upon closely examining satellite data on changes in snow cover extent in individual countries, analyzed at the Space Research Centre of the Polish Academy of Sciences (CBK PAN), it is immediately noticeable that not all winters are alike. In recent years, there have been both "harsh" and "mild" winters.

We would call a winter "harsh" if snow cover persisted for a long time and covered the largest possible area of the country. An example of such a situation in Poland is the winter of 2005/06, when snow covered almost the entire country from mid-December until the end of April. This significantly impacted the average for the entire year—statistically, on each day between July 1, 2005, and June 30, 2006, snow cover extended over as much as one-third of Poland!

A mild winter is one where snow cover appears and quickly disappears, only occasionally covering more than 80-90 percent of the country. A good example of such a situation is the winter in Poland during the 2006/07 season, when snow appeared on a larger scale only at the turn of January and February.

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