Theor. Appl. Climatol. (2008) 93: 75–89 DOI 10.1007/s00704-007-0325-1 Printed in The Netherlands 1 Laboratory of Climatology and Atmospheric Environment, University of Athens, Athens, Greece 2 Meteorological Institute, University of Freiburg, Freiburg, Germany Variability of tropical days over Greece within the second half of the twentieth century P. T. Nastos 1 , A. P. Matzarakis 2 With 10 Figures Received 23 August 2006; Accepted 18 April 2007; Published online 12 October 2007 # Springer-Verlag 2007 Summary Tropical days (TD) are defined as the days with a maximum air temperature greater than 30.0  C. It is clear that the study of TD includes also the absolute maximum temperatures, which are of great interest for the description of a region’s climate. These days are considered as very hot, and they particularly are of great importance not only for bioclima- tology and applied sciences, but also for the individuals who are sensitive in the heat-stress. The regime of the TD in Greece is the focus of this study. The aim is to demonstrate their changes from decade to decade, for the time period 1960–2000. For this study, the Annual Number of Tropical Days (ANTD) recorded by each of the 26 meteorological stations of National Meteorological Service, which are uni- formly distributed in the Hellenic peninsula, was calculated and analysed. In terms of quantifying the conditions in a human- biometeorological manner, the thermal index Physiological Equivalent Temperature (PET) and the consecutive days for Athens have been included in this study. The trends of the TD for each station were analysed through the Mann–Kendall technique, while the spatial dis- tribution per decade reveals the regions with change (in- crease or decrease) in the ANTD during the examined period. Two characteristic periods of change for the ANTD appear in the majority of the meteorological stations in Greece. The first period (1955–1976) is determined by a negative trend, which is statistically significant (c.l. 95%), for adequate stations. In the period between 1976 and 2000, the increase in the ANTD and the maximum temperature exceed the corresponding maximum that appeared in the beginning of the 1950s for several of the examined meteo- rological stations. The human-biometeorological analysis shows that the consecutive days of PET > 35  C have had a positive trend in the last two decades of the last century. 1. Introduction The rise in air temperature, as a result of the greenhouse effect, is the focus of current research from the local to global scale. This trend has been recorded in the 20th century and particular- ly in the last decade 1990–2000, which likewise was the warmest decade in the last century, while the year 1998 was the warmest year of the instru- mental observation period since 1861. The global average surface temperature (the average of near surface air temperature over land, and sea surface temperature) has increased since 1861. Over the 20th century the increase has been 0.6  0.2  C (IPCC 2001; Jones et al. 2001). A recent study undertaken by Jones and Moberg (2003) presents an extensive revision of the Climatic Research Unit (CRU) land station temperature database Correspondence: Dr. Panagiotis T. Nastos, Laboratory of Clima- tology and Atmospheric Environment, Department of Geography and Climatology, Faculty of Geology and Geoenvironment, University of Athens, Panepistimiopolis GR, 157 84 Athens, Greece, e-mail: nastos@geol.uoa.gr