The City of London Thermal Comfort Criteria was developed to combine the effects of pedestrian wind and solar analysis to improve public space design. By understanding these factors, planners can strategically position parks, outdoor markets, cafes, and other public areas to enhance comfort and wellbeing. Thermal comfort analysis also helps pinpoint optimal locations for new developments, ensuring outdoor spaces remain enjoyable and accessible throughout the year.
The City of London Thermal Comfort Criteria is based on the Universal Thermal Climate Index (UTCI) metric for predicting thermal comfort. The metric uses this thermal comfort scale to determine the appropriateness of spaces for different activities. Unlike other thermal comfort metrics that display results for a single season or annually, the City of London criteria is a cross-seasonal, weighted analysis.
The metric sets thresholds for the percentage of time that the space must be thermally “comfortable” (between 0°C to 32°C on the UTCI scale) during different seasons in order to achieve the given usage rating. By calculating UTCI for every hour and computing the percentage of hours that are comfortable for each season, the system evaluates which criteria are satisfied and which rating a specific location should be given.
Please visit our HelpSite post for more details about UTCI.
| Usage Category | % of hours with Acceptable UTCI | Description | Colour (HTML Colour Code) |
|---|---|---|---|
| All Season | ≥90% in each season | Appropriate for use year-round (e.g. parks). | Green (#378C4B) |
| Seasonal | ≥90% spring-autumn AND ≥70% winter | Appropriate for use during most of the year (e.g. outdoor dining). | Purple (#C86EBE) |
| Short-term | ≥50% in all seasons | Appropriate for short duration and/or infrequent sedentary uses (e.g. unsheltered bus stops or entrances) year-round. | Cyan (#1EFFFF) |
| Short-term Seasonal | ≥50% spring-autumn AND ≥25% winter | Appropriate for short duration and/or infrequent sedentary uses during most of the year. | Orange (#FAB92D) |
| Transient | Appropriate for public spaces where people are not expected to linger for extended period (e.g. pavements, cycle paths). | Red (#DE2D26) |
The City of London requirements define the following seasons:
- Spring: March – May
- Summer: June – August
- Autumn: September – November
- Winter: December – February
The guidelines also recommend that meteorological records should be clipped to reflect the usage hours of the defined spaces. Typically, the hours between 8:00 am and 8:00 pm GMT are recommended, with thermal comfort being assessed for each hour.
It is important to differentiate between the definition of the calculations behind the City of London thermal comfort metric and the definition of the simulation process defined by the City of London regulatory guidelines. While Orbital Stack implements the calculation of the City of London wind and thermal comfort criteria, Orbital Stack offers many different levels of simulation, some of which may not meet the regulatory requirements of the City of London. For example, many studies will need both CFD and wind tunnel testing for City of London submission, CFD evaluations require very specific simulation settings, and AI simulations are not currently an accepted approach. Please refer to the official City of London documentation for a comprehensive review of all requirements.
City of London Color Legend
What is it measuring?
The color scale shows the usage category associated with each color of the City of London’s thermal comfort criteria, ranging from all season usage to transient usage.
What options can I specify?
The Season and Time of Day ranges.
*Note: The City of London criteria are also used in Birmingham and Leeds but are not referred to by that name in those locations.
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