We shared the questions we blogged about on ‘CIBSE weather data 2025 – have we lost sight of urban heat islands?‘ with CIBSE for their feedback. They kindly got back to us and below is their full response:
Thank you for your interest in CIBSE Weather Data 2025 and sharing your observations.
Your report was shared with our experts at University of Exeter who have supported CIBSE in preparing this response put together by the CIBSE Technical Team.
1. Rationale for Removing LWC Weather Station in CIBSE 2025 Data
CIBSE have released two weather files in the 2025 release that cover the Southeast of England – Z1, representative of the majority of Greater London and Z2 (rest of the region). The 2016 version of weather data provided 3 locations London Weather Centre (LWC), London Heathrow, and London Gatwick.
Producing weather data files requires 30 years of continuous observational data (i.e., actual measured data from weather stations), which can no longer be provided by LWC as it ceased collecting data in 2010. In the 2025 Weather Data files for Zone 1 Northolt Weather Station is used instead and all the DSYs use years after 2010 specifically 2019, 2020 and 2022.
It’s also relevant to highlight that all other weather stations used in both the 2016 and 2025 datasets are located outside city centres. LWC’s inclusion in the 2016 release was a unique exception and had its disadvantages (i.e., its rooftop location exposed it to higher wind speeds, making its data less representative of typical urban conditions). The lack of observational data collected over a 30-year period is a limitation to creating weather data.
2. Explaining Cooler Night-Time Temperatures in Zone 1 CIBSE Weather Data 2025
You noted that night-time temperatures are around 2°C cooler in the 2025 Zone 1 files compared to the 2016 LWC data. This reduction will be partly due to the use of different weather station (Northolt vs LWC) and related to the updated climate projections included in the update. Xie et al. (2024 – https://doi.org/10.1177/01436244241291203) investigated this by applying the new projections to the same historical baseline used in 2016, isolating the impact of the update to the climate projections alone.
3. How Urban Heat Islands Are Accounted for in the New CIBSE Zones
Urban Heat Islands are accounted for in CIBSE Weather Data 2025 but there are limitations due to the lack of observational data from city centres and the shift to making CIBSE Weather Data more representative of geographical areas (i.e., zones) as opposed to one specific site location.
Below is an explanation of how CIBSE’s climate zones were developed for the 2025 Weather Data release, including how Urban Heat Island (UHI) effects were considered using London as an example. The full methodology is detailed in Xie et al. (2023) https://doi.org/10.1016/j.apenergy.2023.122549.
The climate zone approach was based on clustering UKCP18 regional projections (12km resolution) using temperature, wind, and solar radiation to group similar sites across the UK. This created 14 initial climate zones, which were further segmented using max, min, and mean temperatures to identify 28 microclimates. These final zones, released by CIBSE, account for observed urban heat island (UHI) effects. For example, the Southeast was split into Zone 1 (Greater London) and Zone 2 (the surrounding region).
The next step was to identify what specific weather station location within a given microclimate zone best matches the wider climate of the microclimate zone. In the case of London this was Northolt. However, this approach has limitations: few weather stations are located in cities with robust 30-year datasets. Most are at airports or former military sites on the outskirts of cities. Additionally, urban areas often span only one or two grid squares at the 12km resolution, making them harder to represent accurately and more likely to appear as outliers within the broader zone.
We acknowledge that the current approach has limitations in accurately representing Urban Heat Island (UHI) effects, largely due to the lack of long-term observational data from city centres. However, this challenge also applied to the previous 2016 CIBSE Weather Data with London being the exception.
If you have further questions or would like to discuss this further, please get in touch and we appreciate your continued efforts in examining the new CIBSE Weather Data 2025.
