On the basis of values of day and night mean maximal and minimal temperatures taken from the years 1991-2000 the thermal conditions in the north-eastern part of Poland were characterised with respect to inter day and night changes of T on a scale of 6th class variability. Within the limits of the analysed area, it was divided into regions with large changes of T from day to day which occur most often in the east. It was also established that the biggest average annual sum of days with considerable fluctuations of external temperatures refers to negative changes Tmin, the smallest, on the other hand, refers to Tmax in the same range.