Characterising Surface Urban Heat Island in Ibadan, Nigeria: Hot- Spot Clustering and Relationships With Vegetation and Land Cover

In cities across the world, the ground often feels hotter than the surrounding countryside. This happens because natural landscapes, with their soil and plants, interact with sunlight differently than the concrete, asphalt, and rooftops that cover a city. Plants cool the air by releasing moisture, while buildings and roads absorb the sun's energy and hold onto it, radiating heat back into the environment. When a city grows rapidly, replacing green spaces with hard surfaces, the local climate shifts, creating a pocket of warmth known as a surface urban heat island. This is not just about the air temperature people feel on a street corner, but the actual temperature of the ground itself, which can be measured from space. Understanding where these hot spots form and how they relate to the loss of vegetation is crucial for city planners trying to keep communities cool and comfortable as urban areas expand.

A team of researchers set out to map this phenomenon in Ibadan, one of the largest cities in West Africa. They wanted to see exactly how the city's heat is distributed, how it connects to the remaining greenery, and where the most intense pockets of warmth are located. Using satellite images taken during the dry season of 2025, they created a detailed thermal map of the city and its rural surroundings. The study focused on the surface temperature of the ground, a measurement that reveals how different materials heat up under the sun. By comparing the city center to the rural ring around it, the researchers found a clear and significant difference. The average temperature across the entire study area was 35.03 degrees Celsius, but the city center was much hotter, averaging 39.77 degrees Celsius, while the rural areas averaged 34.73 degrees Celsius. This created a heat island effect where the urban core was 5.04 degrees Celsius warmer than the countryside.

The researchers discovered that the heat was not spread evenly across the city. Instead, it formed a concentrated dome over the built-up center, with temperatures peaking near and above 42 degrees Celsius in the densest areas. As one moved away from the city center toward the vegetated edges, the ground temperature dropped steadily. The study also looked at how the amount of plant life, measured by a standard index of greenness, related to the heat. The connection was incredibly strong: as the amount of vegetation increased, the surface temperature dropped sharply. In fact, the presence of plants explained about 86 percent of the variation in temperature across the landscape. For every small increase in green cover, the ground temperature fell by nearly two degrees Celsius. This relationship was so consistent that the researchers could predict the temperature of a patch of land simply by knowing how much vegetation it held.

To understand exactly where the heat was most dangerous, the team used a statistical method to identify clusters of unusually high temperatures. They found that significant hot spots covered about a quarter of the study area, with the most intense heat concentrated in a core zone of roughly 345 square kilometers. This area, which corresponds to the city's dense built-up center, was a coherent block of extreme warmth rather than a scattered collection of hot patches. In contrast, the cooler areas were found in the vegetated surroundings, where significant cold clusters occupied the green periphery. The study also broke down the temperatures by land type. Built-up areas, such as neighborhoods with houses and roads, were the warmest, averaging 39.35 degrees Celsius. Bare land was slightly cooler at 38.12 degrees Celsius, while vegetation averaged 33.60 degrees Celsius, and water bodies were the coolest at 31.76 degrees Celsius. The difference between the warmest built-up areas and the coolest vegetation was nearly 6 degrees Celsius, a massive gap that highlights the thermal impact of replacing nature with concrete.

These findings offer a clear picture of how Ibadan's urban growth is reshaping its local climate. The study confirms that the city's heat is driven by the replacement of vegetation with impervious surfaces, creating a distinct thermal signature that is hottest in the center and cooler at the edges. The researchers emphasize that this heat is not just a general warming of the whole city, but a specific, intense concentration over the built-up core. The data suggests that planting trees and restoring green cover in the city center could provide immediate and significant cooling, potentially lowering surface temperatures by nearly two degrees for every small increase in greenness. The study also points out that bare, exposed land is almost as hot as the built-up areas, meaning that leaving soil uncovered before construction is just as damaging to the local climate as building on it.

The researchers conclude that managing these heat islands requires targeted action. They recommend focusing green infrastructure efforts on the specific districts identified as the hottest, rather than spreading resources evenly across the city. Protecting existing water bodies and peri-urban vegetation is also critical, as these areas act as natural cooling reservoirs. Furthermore, new developments should consider using materials that reflect sunlight or store less heat to reduce the overall thermal load. By using these maps of heat and vegetation, city planners can make informed decisions to protect residents from extreme heat, ensuring that the growth of Ibadan does not come at the cost of a livable climate. The study serves as a blueprint for how satellite data can be used to turn abstract temperature readings into concrete plans for a cooler, more sustainable urban future.