Heat is a frontier hazard
Ten years of reanalysis data put Bikaner above 40 °C on 74 days a year and above 45 °C on 11. In the humid east the danger arrives at lower air temperatures. What the heat categories mean for a patrol, and what to do when someone collapses.
- From 2016 to 2025, ERA5 puts Bikaner above 40 °C on 74 days a year on average and above 45 °C on 11. Jaisalmer: 66 and 7. Barmer: 62 and 3.
- 2024 was the decade's worst year for days above 45 °C at Bikaner (20), Jaisalmer (11), Barmer (9) and Attari (8).
- Air temperature understates risk where it is humid. In Raksha's outlook for 21 to 27 October, Dhubri's air is about 5 °C cooler than Jaisalmer's on each town's worst day, but its heat stress is higher: WBGT 32.4 °C against 31.4 °C.
- Above a WBGT of 32.2 °C, US Army guidance for moderate work, such as patrolling with a 14 kg load, is 20 minutes of work and 40 minutes of rest each hour.
- For heat stroke, India's national guidance is to cool first and transport second.
How hot the western border gets
We counted hot days at eight towns near the Pakistan border using ERA5, the European reanalysis of the global climate, through the Open-Meteo archive (Open-Meteo). For each town and each year from 2016 to 2025 we counted days when the daily maximum air temperature reached 40 °C and 45 °C.
| Town | Days ≥ 40 °C a year | Days ≥ 45 °C a year | Hottest day | Date |
|---|---|---|---|---|
| Bikaner | 73.7 | 11.2 | 49.4 °C | 19 May 2016 |
| Jaisalmer | 65.5 | 6.9 | 48.9 °C | 19 May 2016 |
| Barmer | 62.1 | 3.4 | 48.9 °C | 27 May 2024 |
| Sri Ganganagar | 39.0 | 1.9 | 46.3 °C | 21 May 2016 |
| Fazilka | 38.3 | 2.1 | 46.4 °C | 10 Jun 2019 |
| Ferozepur | 34.0 | 2.2 | 46.4 °C | 10 Jun 2019 |
| Attari | 32.1 | 2.9 | 47.9 °C | 4 Jun 2017 |
| Bhuj | 22.9 | 0.0 | 44.4 °C | 22 May 2018 |
The thresholds are not arbitrary. In the plains, the India Meteorological Department declares a heat wave when the maximum reaches 45 °C whatever the normal, and a severe heat wave at 47 °C. It also declares one when the maximum is at least 40 °C and 4.5 °C or more above normal, at two or more stations in a subdivision for two days in a row (IMD definitions).
Years differ a lot. At Bikaner, days at or above 45 °C ran 11, 11, 14, 12, 11, 2, 13, 3, 20 and 15 from 2016 to 2025. The worst year of the decade was 2024 at Bikaner, Jaisalmer, Barmer and Attari. A unit that plans for an average year will be caught out by a year like 2024.
These counts are conservative. A reanalysis grid cell is about 25 km across and smooths local extremes, so a thermometer at a post can read higher.
Why air temperature is not enough
The body cools by sweating, and sweat evaporates poorly in humid air. The wet bulb globe temperature, or WBGT, folds air temperature, humidity, wind and sunshine into one number used for work and rest limits. Raksha estimates it from forecasts with the Liljegren model (Liljegren et al., 2008).
Raksha's forecast for 21 to 27 October 2026 shows what this means. On each town's day of highest heat stress, air at Jaisalmer is forecast about 5 °C hotter than at Dhubri, yet Dhubri's heat stress is higher. Nagapattinam, on the humid Tamil Nadu coast, reaches category 5 with air below 29 °C.
| Town | Peak day | Air max | WBGT max | Category |
|---|---|---|---|---|
| Sabroom, Tripura | 27 Oct | 33.8 °C | 34.5 °C | 5 (black) |
| Nagapattinam, Tamil Nadu | 23 Oct | 28.9 °C | 32.6 °C | 5 (black) |
| Mankachar, Assam | 21 Oct | 33.2 °C | 32.5 °C | 5 (black) |
| Dhubri, Assam | 24 Oct | 30.9 °C | 32.4 °C | 5 (black) |
| Barmer, Rajasthan | 21 Oct | 36.1 °C | 31.8 °C | 4 (red) |
| Jaisalmer, Rajasthan | 21 Oct | 36.2 °C | 31.4 °C | 4 (red) |
Source: Raksha advisory generated 21 Oct 2026, 14:43 IST, from Open-Meteo forecasts. Peak day is each town's day of highest forecast WBGT; air max is that day's maximum air temperature. Raksha reissues the advisory with every forecast run, so its latest file may hold a newer run than this table. Forecast WBGT is a planning estimate with an error of about 1 to 2 °C; measure WBGT on site before and during work.
What the categories mean for a patrol
US Army guidance, which Raksha follows, sets five heat categories by WBGT and gives work, rest and water for each (TB MED 507, 2022). For moderate work, such as patrolling with a 14 kg load:
| Category | WBGT | Work and rest each hour | Water each hour |
|---|---|---|---|
| 1 (white) | 25.6 to 27.8 °C | No limit | 0.7 L |
| 2 (green) | 27.8 to 29.4 °C | No limit | 0.7 L |
| 3 (yellow) | 29.4 to 31.1 °C | No limit | 0.7 L |
| 4 (red) | 31.1 to 32.2 °C | 50 min work, 10 min rest | 0.7 L |
| 5 (black) | above 32.2 °C | 20 min work, 40 min rest | 0.95 L |
The guidance assumes acclimatised people in a standard uniform; heavier loads, body armour or protective suits shorten safe work time. It also caps water at about 1.4 L an hour and 11.4 L a day, because drinking far more than the body loses causes its own emergency.
When someone collapses
Confusion, collapse or hot skin after exertion can be heat stroke. India's national guidance is plain: cool first, transport second. Start active cooling at once and continue it on the way to hospital; immersion in cold or iced water is the most effective method for exertional heat stroke, with a target core temperature of 38 to 39 °C (NCDC, Emergency Cooling for Severe Heat-Related Illnesses, 2024). Call 112.
What Raksha does about it
Raksha gives each reference town a seven-day hourly outlook of WBGT, heat index, wind chill, lightning potential and air quality, turns the heat category into work, rest and water guidance, and finds the coolest windows for patrols. The free heat advisory carries the same outlook for the public. Both are advisory: follow IMD warnings, NDMA and state disaster management authorities, and your unit's medical officer.
Method
One request to the Open-Meteo archive API for ERA5 daily maximum 2 m temperature at eight town centroids, 1 January 2016 to 31 December 2025, dates in IST. We counted days at or above each threshold per calendar year and averaged over ten years. The request, retrieval time and licence are in the manifest; the counts are in heat_days.json; the script that made them is build_heat.py and rebuilds them offline from a cached response. ERA5 is a model of the atmosphere constrained by observations, not a station record.
Sources
- Copernicus Climate Change Service, ERA5 reanalysis, via the Open-Meteo Historical Weather API (CC BY 4.0), retrieved 21 Oct 2026. open-meteo.com
- India Meteorological Department, definitions of heat wave and severe heat wave. mausam.imd.gov.in
- Liljegren, J. C. et al. (2008), "Modeling the Wet Bulb Globe Temperature Using Standard Meteorological Measurements", Journal of Occupational and Environmental Hygiene 5(10). doi.org/10.1080/15459620802310770
- US Army TB MED 507 / AFMAN 48-152(I), Heat Stress Control and Heat Casualty Management, 12 Apr 2022. govinfo.gov
- National Centre for Disease Control (MoHFW), Emergency Cooling for Severe Heat-Related Illnesses, March 2024. heathealth.info
- Trinetra Raksha, heat and cold advisory for frontier towns, generated 21 Oct 2026; methods and thresholds. advisory.json, methods.json