Miles, V; Esau, I (2020). Surface urban heat islands in 57 cities across different climates in northern Fennoscandia. URBAN CLIMATE, 31, 100575.

The urban heat island (UHI) is one of the most evident local climate phenomena in urbanized areas. Although much is known about the UHI in low- and mid-latitude cities, knowledge about the UHI in high latitudes is still fragmentary. Understanding this urban climate phenomenon in the high latitudes is essential to support sustainability and resilience of northern settlements that experience accelerated Arctic warming. This study focuses on Fennoscandia, which is the most urbanized northern region. Here, small and medium sized cities are numerous. The urban population is expected to grow as well as the importance of the region for the global resource supply and geopolitical arena. This study includes all 57 cities located above 64 degrees N in this complex region. Their combined urban population is of 1,700,000. The study is based on statistical analysis of Land Surface Temperature (LST) derived from Moderate Resolution Imaging Spectroradiometer (MODIS) data in the period 2001-2017. The analysis of the LST data determines a surface UHI or SUHI. We find strong and persistent winter and summer SUHIs in most of the studied cities. The SUHI intensity varies from city to city. It is remarkably heterogeneous due to bioclimatic and socioeconomic differences between the cities. The SUHI is particularly intense in the coastal cities of the Atlantic region. Our analysis shows that the cities with warmer rural background and larger fraction of land covered by more productive vegetation (low albedo) around the cities have lower SUHI intensity. Conversely, the cities with colder background rural climate and less fraction of land with more productive vegetation (high albedo) have higher SUHI. Large thermal inertia of water bodies additionally complicates the determination and interpretation of the SUHI as both the relative strength and the direction of the urban effects (cooling or warming) depend on the fraction of water surfaces in the rural background. In this maritime climate zone, neither population nor the city area size reveal strong correlations with the SUHI intensity. The size of the population is found to be the strongest SUHI predictor in cities with more continental climate. The mean SUHI intensity is found in the range 0-5 degrees C. The intensity is larger for the largest cities of Murmansk and Oulu (3-5 degrees C).