The Hot City: Heatwaves and the Urban Heat Island

An urban heat island is an urban area that is markedly warmer than its rural surroundings. Causes:

• Sealed, dark surfaces (asphalt, concrete, roofs) absorb solar radiation during the day and release it at night.
• Little vegetation and open soil → less cooling through evaporation and shade.
• Dense buildings trap heat and block wind ("urban canyons").
• Waste heat from traffic, air conditioning and industry.

Daytime differences between city and countryside are usually small. The big difference appears at night, when rural areas cool quickly and the city does not. Swiss cities can be up to 5–7 °C warmer at night than their surroundings (MeteoSwiss).

In summer 2019, Kanton Zürich ran an urban-climate measurement network:

• On tropical nights, central squares were on average 4–5 °C warmer than rural sites.
• Zürich-Affoltern (green, at the city edge) was about 4 °C cooler than the Kaserne and main station (HB).

Tropical nights in summer 2019:

Site	Tropical nights
Paradeplatz	23
Sechseläutenplatz	18
Hardplatz	17
Platform at Zurich HB	16
Rural sites (e.g., Oberembrach, Elsau)	0–3

Over 1991–2020, the city-centre station Zürich-Kaserne recorded about 8 tropical nights per year, compared with 1 at Fluntern on the hillside. In a 3 °C world, Climate CH2025 projects about 41 per year at Kaserne. The warmest night of the year there would rise from 22.3 °C to 25.6 °C.

On clear summer nights, cool air forms over the forested slopes of the Zürichberg, Käferberg and Uetliberg and flows downhill into the city. According to modelling, this cold air reaches about 75% of the built-up area.

Buildings placed across these flow paths can block them. That is why the canton publishes climate maps (Klimakarten) — now at 5 × 5 m resolution — showing heat islands, cold-air flows and a future scenario with +2.5 °C summer warming. Planners use them to keep ventilation corridors open.

The City of Zurich adopted its heat-reduction plan (Fachplanung Hitzeminderung) in 2020. Its three goals:

1. Avoid overheating across the whole city.
2. Relieve vulnerable areas — dense neighbourhoods with schools and care homes — with a network of cool, shaded paths and places.
3. Protect the cold-air system flowing from the green slopes.

In its first four years, about 40 measures were implemented or started.

The toolbox

Measure	How it cools
Plant and protect trees	Shade + evaporation; a large street tree cools its surroundings by 4–7 °C
Unseal surfaces	Open soil stores water and cools by evaporation
Green roofs and façades	Lower surface temperatures, retain rainwater
Water features & keeping rain on site	Evaporative cooling (sponge city, Module 5)
Shade structures	Pergolas, sails, arcades
Light, reflective materials	Higher albedo → less absorbed heat
Keep air corridors open	Lets cold air flow into the city at night

The tree challenge

The Fachplanung Stadtbäume aims for at least 25% tree canopy cover by 2050 in the built-up area. Today it is only about 15%, and canopy has been lost, mostly on private land. The least green districts, such as Zürich-West, would need twice as many trees.

Heat is the natural hazard that kills the most people in Switzerland: 1,402 heat-related deaths in summer 2003, 747 in 2015 and 556 in 2023. In 2024, 85% of heat deaths were people aged 75 or older.

• In Kanton Zürich, climate change caused about 1,700 heat-related deaths between 1969 and 2018 — nearly a third of all heat deaths in that period (Oxford/Bern study, 2025).
• Globally, about 37% of warm-season heat deaths (1991–2018) are attributable to human-caused climate change (Vicedo-Cabrera et al., Nature Climate Change, 2021).

Module 5 looks at how Zurich protects vulnerable people during heatwaves.