Urban heat and outdoor comfort from a weather file
Heatwave frequency and length, the day-night temperature swing that marks an urban heat island, outdoor thermal comfort hours and hours cool enough for night ventilation, for any site and year. For heat action plans, cool-roof and tree-canopy cases, and outdoor design.
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Sample result: twelve cities
Actual year 2023 at each city's airport or main station. Heatwaves are runs of 3 or more days above that year's 90th-percentile daily maximum; the day-night range is the annual mean; comfortable hours have no outdoor heat or cold stress (UTCI 9 to 26 °C); night-flush hours are below 20 °C.
| City | Heatwaves | Longest | Day-night range | Comfortable outdoors | Hours below 20 °C |
|---|---|---|---|---|---|
| Phoenix | 4 | 18 days | 11.9 °C | 44 % | 1,354 |
| Dallas | 5 | 15 days | 10.7 °C | 43 % | 1,558 |
| Houston | 3 | 23 days | 10.3 °C | 49 % | 1,319 |
| New York | 5 | 7 days | 7.3 °C | 45 % | 2,250 |
| Chicago | 5 | 5 days | 8.6 °C | 38 % | 2,384 |
| Paris | 6 | 9 days | 8.1 °C | 40 % | 2,725 |
| Madrid | 5 | 8 days | 13.1 °C | 48 % | 2,386 |
| Athens | 2 | 18 days | 9.1 °C | 63 % | 1,833 |
| New Delhi | 4 | 12 days | 10.1 °C | 41 % | 1,039 |
| Tokyo | 4 | 8 days | 6.3 °C | 42 % | 1,816 |
| Singapore | 2 | 6 days | 5.4 °C | 9 % | 0 |
| Dubai | 6 | 5 days | 8.2 °C | 41 % | 219 |
Singapore has the smallest day-night swing (5.4 °C) and not a single hour below 20 °C, so night ventilation does nothing there; dry inland cities such as Madrid and Phoenix swing 12 to 13 °C, which is where night cooling pays off most.
What you get
Heatwave count, total days and longest run; monthly and annual day-night temperature range; outdoor comfort (UTCI) hours; night-ventilation hours. It runs on any EnergyPlus weather file (EPW): a past year, a typical year, a 2050 future-climate file, or a street-level urban file for 53 US cities, so you can compare a city site with a rural one or today with 2050.
Frequently asked questions
How is a heatwave defined here?
Three or more consecutive days with a daily maximum above the file's own 90th percentile. It flags the hottest stretches of that year at that site; it is not a comparison with a long-term normal.
Why does the day-night temperature range matter?
Cities hold heat at night, so a small day-night swing is the classic urban heat island signature. Comparing a city file with a rural one for the same year shows how much cooling the city loses at night.
What is the outdoor comfort figure?
The share of hours with a UTCI (Universal Thermal Climate Index) between 9 and 26 °C, the band with no thermal stress, from temperature, humidity, wind and sun.
Can I get street-level urban temperature?
In 53 US cities, yes: a single-year weather file can be made with air temperature from citizen weather stations near the site instead of the airport. It is an option on the single-year form.
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