This study is dedicated to characterize the urban heat island (ICU) in the conurbated area of Florianópolis-SC based on synoptic conditions favorable to atmospheric stability. The relationship is established between these conditions and the variables used in the classification of days with stable and/or unstable weather types. To know the degree of daily atmospheric stability, the model called PGT (Pasquill-Gifford-Turner) was applied, in which the classification is made from the parameters of solar radiation, average wind and cloudiness in day and night periods. The monitoring was done from 15 measurement points distributed by the study area during the 1 year, from August 2018 to July 2019. 22 “days-type” were found according to the PGT classification. The results show that the days with the classes that indicate greater atmospheric stability are the ones with the highest intensity of ICU. The multiple linear correlation found from the parameters used and the magnitude of the heat island was r = 0.65, with cloudiness at night being the component with the greatest weight in the relationship.