Invisible heat days
Days when heat and humidity together crossed the NIOSH work limit, but India's temperature-based heatwave rule would not have raised an alarm. 2021–2025, 12 cities.
4,372
city-days with WBGT in the shade above the NIOSH limit for moderate work (28.2 °C) and no heatwave under the recreated rule — 97% of all such days. 2,327 of them fell outside March–June, when heatwave bulletins do not run.
The water side
The same five years held 90 city-days of heavy rain (IMD "heavy", 64.5 mm or more in a day) and 68 warm nights, when the body cannot cool down after a hot day. Pehra adds both to its plans.
- A minimum: ERA5 (~25 km) smooths humid extremes.
- IMD rule recreated on one grid cell per city; not IMD's official record.
- Shade WBGT; sun counts are higher and listed separately.
- Warm nights: IMD rule recreated (only on days >= 40 C, min temp >= 4.5 C above normal).
- Heavy rain: ERA5 daily total >= 64.5 mm (IMD 'heavy' category) at one grid cell.
By city
| City | Invisible heat days | Outside Mar–Jun | Heatwave days (recreated rule) | Warm nights | Heavy-rain days | |
|---|---|---|---|---|---|---|
| Kolkata | 676 | 365 | 44 | 0 | 3 | |
| Bhubaneswar | 656 | 293 | 7 | 0 | 8 | |
| Patna | 571 | 378 | 28 | 6 | 4 | |
| Chennai | 512 | 184 | 0 | 0 | 12 | |
| Lucknow | 460 | 326 | 36 | 10 | 4 | |
| Guwahati | 342 | 270 | 0 | 0 | 13 | |
| Delhi | 341 | 237 | 30 | 15 | 4 | |
| Ahmedabad | 338 | 140 | 27 | 14 | 9 | |
| Kharghar, Navi Mumbai | 223 | 30 | 99 | 7 | 22 | |
| Jaipur | 136 | 89 | 15 | 15 | 1 | |
| Nagpur | 112 | 15 | 10 | 1 | 8 | |
| Hyderabad | 5 | 0 | 0 | 0 | 2 |
How it was counted
Invisible heat day: WBGT in shade, 06-19 IST, above the NIOSH REL for moderate work on at least one hour, with no heatwave under the recreated IMD rule. In the sun, 14,040 city-days crossed the same limit.
Source: ERA5 reanalysis via Open-Meteo archive (CC BY 4.0). WBGT by the Liljegren method (thermofeel, Apache-2.0). Code:
scripts/run_audit.py. Model card.