Thermal comfort (UTCI)
The «Thermal comfort (UTCI)» section combines the weather file, the wind simulation, the shadows of buildings and trees, and the surfaces into a UTCI value at every point, and draws it as a map in stress classes.
Overview
The thermal comfort map serves the design-stage question of whether an open space is a place to stay in summer. This question is otherwise examined with specialist tools such as SOLWEIG or ENVI-met. Most inputs are results that other sections have already produced. The simulation is part of the Pro pass (Pricing).
How to use
- Choose the site and radius
- In the «Thermal comfort (UTCI)» section, choose the period («Summer», «Spring», «Autumn», «Winter» or «Annual»)
- Choose the time (10:00, 14:00 or 18:00)
- Start the calculation
All times are standard time (CET, UTC+1) without daylight saving time. From late March to late October the clock shows one hour more: 12:00 CET is 13:00 CEST. If a wind simulation already exists for the same site and period, the calculation takes a few seconds; otherwise the wind simulation runs first (30 seconds to 3 minutes depending on the radius).
UTCI and classes
UTCI (Universal Thermal Climate Index) combines air temperature, humidity, wind speed and radiation into a felt temperature in °C. It is defined as the air temperature of a reference environment (calm, shaded, 50% relative humidity) that produces the same physiological response. It was developed in COST Action 730.
| UTCI | Class |
|---|---|
| 9–26 °C | No thermal stress |
| 26–32 °C | Moderate heat stress |
| 32–38 °C | Strong heat stress |
| 38–46 °C | Very strong heat stress |
| above 46 °C | Extreme heat stress |
The cold classes (0–9 °C slight cold stress; below that moderate, strong, very strong and extreme) are defined in the same way and are used in winter.
Method
| Input | Data | Processing |
|---|---|---|
| Temperature, humidity, solar radiation | EPW weather file of the city (the same file as the wind rose) | 8,760 hours, grouped by month × hour |
| Wind speed | Result of the wind simulation | Reused if available for the same site and period |
| Shade and sky view | Building height map (the same as for wind), terrain and trees | Shadow from the sun position and sky view factor (SVF) |
| Surfaces | Green space ∪ tree crowns | Grass (albedo 0.20) heats up less than asphalt (albedo 0.12) |
From this follows the mean radiant temperature at every point, which is combined with temperature, humidity and wind speed into the UTCI. The UTCI formula (a 6th-order polynomial with 210 terms) was taken over by script from a validated public implementation; the 9 cases of the public test table are checked in the code every time (tolerance 0.1 °C).
Tree shade
| City | Trees in thermal comfort |
|---|---|
| Berlin | Tree inventory of Geoportal Berlin (street and park trees, crown from the register) and woodland |
| Cologne | Tree register of the City of Cologne (public trees only, crown from the register) and woodland |
| Stuttgart, Karlsruhe | Crowns estimated from the LGL surface model nDOM1; walls, roofs and masts from 3 m upwards can be included |
Tree height is unknown; for the shadow, every crown is assumed to be 8 m high. A crown lets 15% of direct radiation through (transmissivity 0.15). The note on the screen always states the 8 m assumption.
Reading the result
This tool does not guarantee a UTCI value at any single point. The result is for comparing places: a shaded street against a sunlit square, or the change where a row of trees ends. Periods and times are compared, as are massing options on the same site.
Limitations
Simulation for orientation only, not legally binding. The map has no measurement and no validation. An outdoor thermal comfort assessment calculates many days and hours with a validated model and surveyed trees.
Known deviations. The first two make the result warmer than in reality (less cold in winter):
- Wind at pedestrian height comes out too weak. The wind simulation gives weaker wind near the ground than in reality. On the Seoul and Tokyo sample sites, 81–89% of cells use the weakest wind the UTCI formula accepts (0.5 m/s), so the cooling effect of wind is almost absent. Open, windy places can come out warmer than in reality (less cold in winter): deviation towards the warm side
- Trees missing from the data (private trees in Cologne) cast no shadow: deviation towards the warm side
- Surface properties (albedo, emissivity) are literature values, not measurements
- Building heights are the same as in the wind simulation (LoD2 3D building models)
«Save PNG» keeps a note: tree height assumed at 8 m, surface properties from the literature, compare places and do not quote numbers. «Save plate only» leaves it out; the caption is then added by hand.
Data sources
| Data | Use |
|---|---|
| EPW/TMYx weather files (climate.onebuilding.org): Berlin-Tegel, Köln/Bonn, Stuttgart, Karlsruhe | Temperature, humidity, radiation, cloud cover (8,760 hours) |
| Wind simulation | Wind speed |
| Building height map (LoD2 3D building models) | Building shadows and sky view |
| DGM1 of the federal states (outside it, Copernicus DEM GLO-30) | Terrain |
| Berlin tree inventory · Cologne tree register · LGL nDOM1 | Tree shade and surfaces |
| COST Action 730; public UTCI implementation and test table | Definition and formula |