The aim of this work is to analyze the waves of warmth and coldness in central part of Poland basing on a daily maximum and minimum temperature in Łódź between 1931 and 2006. Warm and cold periods were distinguished following daily maximum temperature course (warm and cold days) and daily minimum temperature course (warm and cold nights). A particular day was involved in warm (cold) period, if its daily temperature was higher (lower) than average +1.28 (-l.28) of standard deviation within this calendar day. The value of l.28 was chosen because it counterparts percentile of 90% (10%) if we assume that temperature distribution is normal. The trends of seasonal and annual changeability of the number of cold and warm days and nights as well as maximum length of warm and cold waves were estimated by Sen's method. The most frequently observed were the short waves, such as one- or two days long (Fig. 3 and 9). The average of day numbers, within at least three day-long waves and an their average lasting time of the longest waves had distinct annual cycle with its maximum in summer and minimum in winter. However, the absolute maxima of the waves lengths fell on winter half-year. The longest waves were 18 and 17 days. For comparison, the lengths of the warm and cold nights periods were significantly shorter and were about II and 14 days. The longest cold waves were slightly longer than equivalent waves of warmth. The following years: 1962, 1989, 1992, 1994, 2000, 2002 and 2006 are the years of particularly long warm periods or years of a great number of warm days, and are grouped in last 20 years of analyzed series with the exception of 1962, which represents series center. On the other hand, the years, during which cold waves were exceptionally long or those, which were characterized by a great number of cold days and nights (1954, 1956, 1963, 1979, 1996) were gathered during first 30s of the analyzed period. In spite of this, only some of studied series have statistically significant trend. Lasting time of the longest wave of warmth increased significantly in winter (0.28 of the day per ID years), while the number of waves of warmth grew in summer (0.33 of the day per ID years) and during the whole year (1.79 of the day per ID years). If it is about warm nights, the lasting time of the longest wave increased significantly in winter (0.17 of the day per 10 years) and in autumn (0.20 of the day per 10 years), while the number of the days increased significantly in summer (0.36 of the day per 10 years) and within the whole year (1.61 of the day per 10 years). The length of the longest wave of cold days significantly decreased in winter (0.18 of the day per ID years) and the number of cold days in a year decreased in 0.87 of the day per 10 years. In case of cold nights the lasting time of the longest wave shortened in winter (0.17 of the day per 20 years) and during the whole year (0.2 of the day per 10 years), while the number of cold nights in the year decreased in 1.43 days per 10 years and 0.43 of the day per ID years in spring. The influence of the atmospheric circulation either in macroscale or mesoscale on frequency and intensity of extreme thermal events in Poland is visible only in winter, however it explains no more than 25% of their changeability. Better results are obtained by composite method, which allows to single out such synoptic situations, which favor maintaining strong positive and negative thermal anomalies. Thermal conditions of central Poland primarily depend on baric centers, which centers locate in country's proximity (Fig. 14-17). The correlations between number of days during periods of warm days and nights, and maximum length of waves and northern hemisphere's temperature are positive and statistically significant for all seasons except summer (fable 6). Nonetheless, no more than 25% of the changes might be the direct results of big-scale temperature changes, the rest is influenced by regional or even local factors. The correlations between number of days during periods of cold days and nights and maximum length of waves and northern hemisphere's temperature are negative and statistically insignificant.