Urban heat island against the backdrop of the setting sun in summer

What is an urban heat island?

Referred to as an urban heat island, the climate in large cities is the result of the increased temperatures in urban areas compared to their rural surroundings.

This phenomenon is the result of the unintended modification of the climate by human activities and the impact of the energy balance that an area can have on the regional climate.

Why is the air warmer in the city?

First discovered in the early 1800s in London, this urban heat island was identified through statistical analysis of surface air temperatures.

On average, a city is warmer than the countryside due to differences in energy gains and losses in each region. A number of factors can contribute to the relative warmth of cities, such as the thermal properties of buildings, heat from industrial activity and water evaporation.

The heat produced by the cooling of urban buildings, heating and transport (planes, trains, buses and cars) contributes to the warming of cities. In addition, asphalt, brick and concrete retain heat better than natural surfaces.

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Why is the air colder in rural areas?

Water evaporation plays an important role in the magnitude of the urban heat island. In rural areas, solar energy absorbed near the ground causes water to evaporate from vegetation and soil. Thus, heat is reduced to some extent by evaporative cooling during evapotranspiration.

Does this heat island affect cloud cover?

According to a study based on satellite data, cities such as London and Paris appear to be able to generate their own clouds. In spring and summer, these megacities are consistently cloudier in the afternoons and evenings than their rural neighbours, due to heat trapped in buildings, which causes movement that leads to cloud formation.

On average, large cities tend to have up to 10 % more cloud cover than the surrounding rural areas.

Grenoble, city of tomorrow | What is an urban heat island?