Thermal comfort (UTCI) map

The «Thermal comfort» tab combines the weather file, the wind simulation, building and tree shade and ground cover to compute UTCI at each point, and draws it as a map in stress categories.

Overview

The thermal comfort map is for deciding, at concept stage, whether an outdoor space is a place where people can sit in summer. This question is usually studied with specialist tools such as SOLWEIG or ENVI-met. Most of the inputs are results that other tabs have already produced.

How to use

1. Choose the site and the radius
2. On the «Thermal comfort» tab, choose the period (summer, spring, autumn, winter or annual)
3. Choose the time (10:00, 14:00 or 18:00)
4. Press «Compute thermal comfort»

Times are Greenwich Mean Time without British Summer Time: in summer, 14:00 GMT is 15:00 on the clock. If a wind simulation for the same site and period exists, the calculation takes a few seconds; otherwise the wind simulation runs first (30 seconds to 3 minutes by radius).

UTCI and categories

UTCI (Universal Thermal Climate Index) combines air temperature, humidity, wind speed and radiation into one felt temperature, in °C. It is defined as the air temperature of a reference environment (no wind, shade, 50% humidity) that produces the same physiological response. It was developed in COST Action 730.

Cold categories (0–9 °C slight cold stress; below that moderate, strong, very strong and extreme) are defined the same way and are used in winter.

Method

These give the mean radiant temperature at each point, which is combined with temperature, humidity and wind speed to compute UTCI. The UTCI formula (a 6th-order polynomial with 210 terms) is taken by script from a validated public implementation, and the 9 cases of the public validation table are checked in the code each time (tolerance 0.1 °C).

Tree shade

Tree crowns come from the GLA London Public Realm Trees list (borough-managed trees). For shade, every crown is given an assumed height of 8 m. A crown lets 15% of direct radiation through (transmittance 0.15), and under a crown the sky view is capped at 0.30. The screen note always states the 8 m assumption.

Reading the result

A UTCI value at a single point is not guaranteed by this tool. The result is for comparing places: a shaded street against a sunlit square, or the change where a row of trees stops. Compare periods and times, and compare massing options on the same site.

Limitations

Known biases; taken together the result tends to read cooler than reality:

• Pedestrian-level wind is not resolved. UTCI uses wind at 10 m, while the lowest fluid cell of the simulation is 4–8 m (centre 6 m). Real wind at 1.5 m is weaker, so UTCI reads lower: biased cool
• Trees not in the GLA list (private gardens, private garden squares) give no shade: biased warm. This partly offsets the point above, in unknown places and amounts
• Surface properties (albedo, emissivity) are literature values, not measurements
• Building heights are the LiDAR heights used for the wind simulation (2016–2022). Where LiDAR gives less than 2 m (for example on buildings completed after the survey), the height falls back to the OpenStreetMap height, then OpenStreetMap storeys × 3 m, then 3 m

«Save PNG» keeps a note: tree shade assumes 8 m, surface properties are literature values, compare places without quoting the numbers. «Save plate only» drops it; add the caption yourself.

Data sources

Related guides

• Wind
• Trees
• Sun hours
• Data sources

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