Identification and Evaluation of Urban Heat Islands in Skopje Using Satellite Remote Sensing and Climate Scenarios
This study utilizes multi-decadal Landsat satellite data, land-surface indices, and regional climate projections to characterize the spatial persistence of surface urban heat islands in the Skopje Valley, revealing that recurrent thermal hotspots align with urbanized areas and that future warming trends necessitate climate-responsive urban planning.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Imagine the Earth's surface as a giant, complex kitchen. Some parts of the kitchen are covered in cool, damp sponges (like forests and grass), while others are paved with hot, black asphalt or covered in shiny metal roofs. When the sun turns on the stove, the sponges stay relatively cool because they can "sweat" water into the air, but the asphalt and metal get scorching hot and hold onto that heat. This difference creates a "thermal map" where some spots are always the hottest, regardless of the day. Scientists call this the Surface Urban Heat Island (SUHI). It's not just about the air feeling warm; it's about the ground itself radiating heat. This matters because when our cities get hotter, it affects our health, how much energy we need to run air conditioners, and how comfortable our summers feel. To understand this, researchers use special satellite eyes that can "see" heat from space, tracking how different surfaces warm up over decades.
This paper is like a detective story set in the Skopje Valley in North Macedonia, a city nestled in a bowl-shaped valley surrounded by mountains. The authors, Vlado Spiridonov, Floranda Muharemi, and Mladjen Ćurić, decided to investigate why some parts of this valley get so hot and whether these hot spots are just temporary or if they are permanent features of the landscape. They didn't just look at one summer; they dug through a massive archive of satellite photos spanning 30 years, from 1996 to 2025, to see the "thermal fingerprint" of the city.
The team used a clever trick to separate the "always hot" places from the "temporarily hot" ones. Imagine looking at a classroom and asking, "Who is the tallest?" If you ask every day for 30 years, the same few students will likely be at the top of the list, even if their heights change slightly. The researchers did this with temperature. They identified the top 10% of the hottest spots in the valley for every single summer over those 30 years. They found that the places that kept showing up as the hottest were almost always the low-lying valley floor, where the city, factories, and roads are packed together. In contrast, the cooler spots were always the green, mountainous areas surrounding the city. The long-term average summer temperature across the whole valley was 35.51 °C, but the hottest spots could reach an average of 43.37 °C or even higher in extreme cases.
They also took a closer look at the summer of 2025, which turned out to be a particularly scorching year. They compared the heat map of 2025 against the 30-year average to see how much hotter it was than usual. The result was a widespread "heat wave" across the entire valley, not just the city center. The average temperature in 2025 was 38.18 °C, which is a 2.78 °C jump above the long-term average. This means the whole valley, from the city center to the rural fields, was baking hotter than it had in decades. However, the paper is careful to point out that this doesn't mean the city got relatively hotter compared to the countryside; it just means everything got hotter than usual.
To understand why the ground gets so hot, the researchers looked at what the ground was made of. They used two special "spectral glasses" (mathematical formulas applied to satellite images) to measure greenery and buildings. One glass, called NDVI, measures how green and leafy an area is, while the other, NDBI, measures how much concrete and buildings are there. They found a perfect "opposites" relationship: where the greenery was high, the buildings were low, and vice versa. In 2025, the average greenness score was 0.551, while the building score was -0.127. The paper shows a very strong negative connection between the two (a correlation of -0.914), meaning that if you see a lot of concrete, you almost certainly won't see much grass, and that concrete is likely the reason the ground is burning hot.
The study also looked at the sky above the valley to see what kind of weather was cooking up this heat. They found that the hottest days happened when the atmosphere was stuck in a "heat trap." Imagine a giant, invisible lid (an anticyclone) sitting over the valley, pushing air down, stopping clouds from forming, and preventing wind from blowing the heat away. When this happens, the sun beats down on the ground for days in a row, and the valley, being surrounded by mountains, can't breathe. Sometimes, dust from the Sahara even drifted in, changing how the sunlight hit the ground, but the main culprit was this stagnant, sunny, and dry weather pattern.
Finally, the authors looked into a crystal ball made of computer simulations to see what the future holds. They used climate models to predict how the region will warm up by the year 2100. The simulations suggest that the whole region will get significantly warmer, especially in the summer. If the world continues to emit high levels of greenhouse gases (a scenario called RCP8.5), the hottest months of July and August could see maximum temperatures frequently reaching 40 °C or even 42–44 °C by the end of the century. The paper clarifies that these models predict the air temperature will rise, which means the ground will likely get even hotter, but they don't promise that the difference between the city and the countryside will necessarily get bigger. Instead, the entire "kitchen" will just get hotter, making the existing hot spots even more dangerous.
In short, this paper confirms that the Skopje Valley has a permanent "thermal map" where the city floor is always the hottest part, surrounded by cooler green mountains. While 2025 was an exceptionally hot year for everyone, the pattern of heat is stable and linked directly to where the concrete is and where the grass is. The study suggests that as the climate warms in the future, these existing hot spots will face even more extreme heat, making it crucial for city planners to know exactly where to plant trees and cool down the ground to protect the people living there.
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