An urban heat island (UHI) is a phenomenon where a city is warmer than the surrounding rural areas. The temperature difference can be small or large, but it is often enough to affect human health, energy use, and urban ecosystems. More than 200 years ago, the British scientist Luke Howard noticed that London was consistently warmer than the nearby countryside. In 1818, he described this difference for the first time, laying the foundation for what we now call the urban heat island effect. Since then, urban heat islands have been documented in cities across the world and in many different climates.

Why do urban heat islands form?

Urban heat islands form mainly because natural surfaces are replaced with artificial ones. When soil, grass, and forests are replaced by asphalt, concrete, and rooftops, the surface properties change. Artificial surfaces often have low albedo, so they absorb more sunlight instead of reflecting it. They also store heat efficiently and release it slowly, keeping cities warm even after the sun has set.

Many urban surfaces are non-porous, so rainwater runs off rather than soaking into the ground. This reduces evaporation, which normally helps cool the air. At the same time, cities usually have less vegetation and open water, meaning less shade and less natural cooling. Cities also produce heat through their “urban metabolism” — the energy needed to keep the city running. Heating and cooling of buildings, traffic, industry, and infrastructure release heat into the atmosphere; this added warmth is known as anthropogenic heat flux.

In summer, urban heat islands are mainly driven by absorption and storage of solar radiation. In winter, especially in cold and Arctic cities, urban heat islands are often dominated by human-generated heat, even when there is little or no sunlight.

What are the consequences?

Urban heat islands can have important impacts on people and the environment. Higher temperatures increase the risk of heat stress, heat exhaustion, and heat-related illness, especially for older people, children, and those with existing health conditions. Warmer cities often require more energy for cooling, which can increase emissions and energy costs.

Urban heat islands also affect the urban environment. Higher temperatures can worsen air quality, increase water demand, and place stress on vegetation. Trees and other plants may suffer from heat and drought, reducing their ability to cool the city and support biodiversity. These changes can reinforce one another, making cities even more vulnerable during heatwaves.

How do we measure urban heat islands?

One common way to measure urban heat islands is by using meteorological stations. At least two stations are needed: one located inside the city and one outside the city in a nearby rural area. The temperature difference between these stations represents the intensity of the urban heat island. This simple method works well in many regions, but not everywhere; in small cities and Arctic areas, suitable rural reference stations may be limited, and temperature patterns can vary strongly with season.

Satellite data provide another important tool. Remote sensing allows us to study urban heat islands across entire cities and over long time periods. Satellites measure land surface temperature, similar to the warmth you feel when touching asphalt or a wall on a sunny day. This is called the surface urban heat island. Surface temperature is different from air temperature, but it is very useful for identifying hot and cool spots within cities, such as roads, rooftops, parks, and water bodies.

Urban heat islands in cold and Arctic cities

Urban heat islands are well studied in large cities and warm climates, but much less is known about how they behave in cold climates and small cities. In cold regions, urban heat islands exist all year round, but for different reasons. In summer, cities store heat from the sun, much like cities elsewhere. In winter, when sunlight is weak or absent, cities remain warmer than their surroundings because of heat released from buildings, heating systems, and infrastructure. The city becomes a source of heat in an otherwise cold landscape.

Studies of Arctic cities show large variation from place to place. Geography, climate, snow cover, wind, and city layout all influence how urban heat islands develop. Each city has its own urban heat island “fingerprint”, reflecting its particular combination of physical setting, built form, and energy use.

Hot islands, cool islands, and urban diversity

Cities are heterogeneous. They are made up of many different surfaces, building types, and land uses, which create multiple local climates within the same city. The “hot islands” are usually found in the built environment: dense building blocks, transport corridors, industrial zones, and paved surfaces.

The “cool islands” are the city’s natural assets: parks, gardens, forests, wetlands, rivers, lakes, and coastal areas. These spaces provide shade, support evaporation, and help moderate temperatures, while also offering cultural and recreational value. In many cities, there is a clear imbalance: the land area covered by hot islands can be more than twice as large as the area covered by cool islands within the main urban core. This imbalance highlights the importance of protecting and expanding green and blue spaces.

Why urban heat islands matter

Urban heat islands show that cities do not just respond to climate — they actively shape it. By understanding why urban heat islands form, how they affect people and ecosystems, and how they vary across seasons and regions, cities can make better decisions. Urban planning, building design, and energy systems all play a role in either amplifying or reducing urban heat.

Designing cities with more vegetation, reflective and permeable materials, and reduced waste heat can help create healthier, more resilient urban environments in both warm and cold climates. Examples include planting trees along streets, creating parks and green roofs, using high-albedo roofing and paving materials, and improving building insulation and energy efficiency. Together, these measures can lower urban temperatures, reduce energy demand, and improve the quality of life for city residents.