Thermal imaging of urban trees in Cardiff

Step outside on a hot summer's day and you'll always find people sheltering under trees. The cool shade of the canopy in times of extreme heat is a well-known benefit of trees, and with heatwaves becoming fiercer and more frequent, providing shade in urban areas is now a matter for public health policy.

A study has estimated that London’s trees helped to avoid 153 heat-related deaths between 2015–22. This is around 16% of all heat-related deaths during this period.

To see the difference for ourselves, in June 2023 we commissioned Ti Thermal Imaging Ltd to record the surface temperature in the shade of urban trees in Cardiff during a heatwave. They also recorded the surface temperature next to the trees in direct sunlight.

The thermal imaging was carried out in Adamsdown, a district in the south of Cardiff with most neighbourhoods rated as 'highest priority' for action on tree equity (owing to low tree cover and other socio-economic and environmental factors). Adamsdown also has a temperature around 4.4 degrees C above the average for Cardiff – perfect for our study.

In Constellation Street (pictured above) Ti Thermal Imaging Ltd recorded the surface temperature under and around an urban tree. The results were astonishing – the difference between the shade and direct sunlight was around 20 degrees C.

The different colours in the thermal image illustrate the heat impacts on different locations. Sun-exposed areas – especially if they're dark – will soak up more heat and store it, then release it at night, a phenomenon known as the urban heat island effect.

The image above, taken in Pontcanna, shows the same pattern of an almost 20 degrees C difference between the shade of the trees and direct sunlight. This time, however, the entire road is lined with trees, which significantly reduces the urban heat island effect.

Tree equity can improve urban temperatures

You don’t need to be a tree expert to see that planting more trees in Adamsdown could help to reduce temperatures during future heatwaves, as well as provide the community with all the other benefits of urban trees. But it isn't just about tree planting – well-managed trees with large, healthy canopies provide the most shade, so caring for and protecting existing trees is vital too.

You can explore the local impact of urban tree canopy cover on temperatures using Tree Equity Score UK. The tool calculates a tree equity score out of 100 for thousands of neighbourhoods in towns and cities by combining data on tree canopy cover with information on air pollution, income, age, employment, health and temperatures. The tool measures 'heat disparity' – the extent to which a neighbourhood is above or below the local average temperature during a heatwave.

Tree equity could save lives

The Office for National Statistics showed that during the record-breaking heat periods between 1 June–31 August 2022 in England and Wales, there was a marked increase in deaths due to extreme heat. With heatwaves and drought conditions set to increase as we continue feeling the effects of climate change, improving tree cover in urban areas needs to be a top priority for our elected leaders.