The dynamics of air temperatures during cold half-year to a large extent affect all human activities. Most significant damages are caused by minimum air temperatures dropping to very low values. Due to the current ongoing climate change, air temperatures in winter in general are increasing, however, this does not necessarily mean the frequency of these very low temperatures decreases. It is not easy to express the course of air temperatures during cold periods using some simple characteristic. Meteorologists and climatologists use various characteristics, which can be referred to as frost indexes. These include the number of frost, ice or arctic days. Since these indexes not always perfectly reflect reaction of the nature to a particular temperature threshold, the analysis also included number of days with minimum air temperature below -3, -5, -7, -10, -15, -20 and -25°C. In the last 15 years (2001-2015), there has been a decrease in the frost indexes in comparison to the normal period 1961-1990, this trend however, is not always statistically significant. Significant changes were found in particular in case of the number of days with a higher temperature threshold (frost, ice days and minimum air temperature above -3 to -10°C). In contrast, days with extremely low temperatures are relatively rare (sometimes only observed once in several years), so the dominant factor here is rather variability. The paper also includes an analysis of possible future development based on the most recent climatological models.