INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 21: 617–636 (2001) ANALYSIS OF MAXIMUM AND MINIMUM DAILY TEMPERATURES RECORDED AT FABRA OBSERVATORY (BARCELONA, NE SPAIN) IN THE PERIOD 1917–1998 C. SERRA a, *, A. BURGUEN  O b and X. LANA a a Departament de Fı ´sica i Enginyeria Nuclear, ETSEIB, Uniersitat Polite `cnica de Catalunya, A. Diagonal, 647, 08028 Barcelona, Spain b Departament d’Astronomia i Meteorologia, Facultat de Fı ´sica, Uniersitat de Barcelona, A. Diagonal, 647, 08028 Barcelona, Spain Receied 28 March 2000 Reised 1 Noember 2000 Accepted 2 Noember 2000 Published online 3 April 2001 ABSTRACT Daily maximum and minimum temperatures recorded without interruption at Fabra Observatory (Barcelona) from 1917 to 1998 are analysed studying their homogeneity, randomness, possible trends and their statistical significance, and time irregularities detected by means of concepts of entropy and spectral power analysis. The homogeneity of the series is tested on a monthly scale using the adaptive Kolmogorov – Zurbenko filter. With respect to the randomness of the time series, the von Neumann ratio test is applied to standardized values of extreme temperatures in four different time-scales (daily, monthly, seasonal and annual). The statistical significance of trends is quantified by applying the Spearman and Mann – Kendall tests to daily, monthly and seasonal data. The Mann – Kendall sequential test also leads to the detection of sharp changes in the time series when monthly data is analysed. The quantification of irregularities through entropy is investigated for standardized temperatures on daily, monthly and seasonal scales, based on the concept of mathematical information theory. Periodicities derived from spectral power analyses are checked with the hypothesis of white-noise and Markov red-noise stochastic processes. The most notable features, common to maximum and minimum temperatures, are the lack of randomness of the series for all the time-scales considered and the different trends obtained for the periods 1917–1980 and 1917–1998, which are confirmed by the Spearman and sequential Mann – Kendall tests. Nevertheless, the maximum and minimum temperature series show quite a different behaviour from the point of view of results concerning time irregularities in terms of entropy and periodicities. The main features of the results are discussed by comparing them with those obtained for other areas of the Mediterranean domain. Copyright © 2001 Royal Meteorological Society. KEY WORDS: daily extreme temperatures; time series analysis; homogeneity; randomness; temperature trends; entropy and power spectral analyses DOI: 10.1002/joc.633 1. INTRODUCTION Long-term climate variability is of great importance for the estimation of its impact on human activities and for predicting the future climate. This variability can be detected from a great variety of records such as tree ring growth, ice cores, isotope ratios and historically documented evidences, or quantified from meteorological instrumental records for different domains of the Earth. When thermometric time series are analysed, solar and volcanic activities have to be considered in order to interpret periodicities or abrupt changes in the time series. Other factors to be considered are, for instance, the influence of ocean dynamics, concentrations of greenhouse gases and sulphate aerosols in the atmosphere and the local urban heat island effect. * Correspondence to: Departament de Fı ´sica i Enginyeria Nuclear, ETSEIB, Universitat Polite `cnica de Catalunya, Av. Diagonal, 647, 08028, Barcelona, Spain. Copyright © 2001 Royal Meteorological Society