This paper reports on a study of the variability of thermal conditions over a multi-year period. The study approaches the topic as a question of thermal stability and looks at the number of strings of consecutive days with the air temperature remaining within certain ranges. Thermal and percentile/quantile classification systems were used to determine specific ranges, which were then applied to records from five weather stations representing Central Europe (Potsdam, Krakow, Prague, Debrecen, Vienna) during the period 1775-2012. The study found a decrease in the numbers of strings of days with a certain temperature during the last 100 years at most of the stations. Fewer strings of days in a year means that they grew longer due to less day-to-day temperature change occurring when compared to the late 18th century and the 19th century.