The report presents the deformation of the field of air temperature caused by the town and expressed through the fact of appearance in this field of the so called urban heat island (a frequent, but not necessarily everyday phenomenon). The starting point of analysis was constituted by the data for the years 1961-1965 coming from the stationary observations conducted in 5 weather stations (posts) located within the town (the stations of Bielany, Astronomical Observatory, River Pumps Station and University of Warsaw) and a station located in the peripheries of the town (Okęcie Station, treated as a reference point). The results were presented concerning the appearance of the urban heat is land in the instants of observation (7:00 a.m., 1:00 p.m. and 9:00 p.m. local time), including determination of its annual rhythm and the changes of intensity during the day, as well as the frequencies of differences of air temperature between the urban stations and the one located in the peripheries (ΔT). The greatest differentiation of air temperature during the day occurs in the evenings and at night, while during the year - between April and October. The smallest differentiation occurs at about noon, and with respect to the year - in November and March. The values of air temperature differences between the town and the peripheries reach in terms of monthly averages up to 2°C, but the maximum differences (ΔT MAX) for particular days may even exceed 10°C, the maximum difference for the minimum temperature reaching 7.5°C and for the maximum temperature: 4.5°C. Relations between the intensity of the urban heat island and some weather elements like velocity and direction of wind as well as cloudiness were also analysed. Formation of the urban heat island- especially the one of high intensity- is induced by low cloudiness (between 0 and 2) and by lack or low velocities of wind (up to 2 m/s). It was stated that the limit velocity of wind at which the heat is land disappears is 9-10 m/s. Besides this, the dependence was identified of the urban heat island upon the synoptic situation and the kind of the air masses. The respective results proved to be essential for the explanation of the mechanisms advantageous for appearance and development of the heat island. Special emphasis was placed upon the analysis of daily/hourly changes of the urban heat island in connection with weather conditions. The instances of appearance of the phenomenon, of its maximum and its decline and demise were also determined. Likewise, the differentiation of the rates of warming and cooling of the air in the areas of downtown concentration of buildings (the University station) and outside of the town (the Okęcie station) was considered. In the final part of the report the results of studies for the five-year period 1961-1965 are compared to those for the period 1976-1980, with the purpose of uncovering the dynamics of changes of the heat is land in connection with the development of the town.