Quick answer: At 40 °C (104 °F) and 30% humidity the heat index is about 43.1 °C (109.6 °F), in the NWS "danger" band; at 35 °C and 60% it is about 45 °C.
How it's calculated
The NWS first applies Steadman's simple formula, HI = 0.5 × (T + 61 + 1.2 × (T − 68) + 0.094 × RH) with T in °F. If the result averaged with T is 80 °F or more, it uses the Rothfusz regression, a nine-term fit to Steadman's tables, and adjusts it for very dry air (RH under 13%) or very humid air (RH over 85%) at moderate temperatures.
The heat index is how hot the air would feel at a reference humidity. Humid air slows sweat evaporation, so the body cools less: 35 °C feels like about 45 °C at 60% humidity, which is common on the Gulf coast in summer.
The levels are the NWS bands: caution 27–32 °C (80–90 °F), extreme caution 32–39 °C (90–103 °F), danger 39–51 °C (103–124 °F) and extreme danger above 51 °C (125 °F). The values are for shade; direct sun feels hotter still.
Worked examples
- 90 °F, 50% (NWS table: 95 °F): HI = 94.6 °F
- 100 °F, 40% (NWS table: 109 °F): HI = 109.3 °F
- 35 °C, 60%: HI = 45.1 °C
NWS heat index levels
| Heat index | Level | Possible effect with long exposure or activity |
|---|---|---|
| 27–32 °C (80–90 °F) | Caution | Fatigue |
| 32–39 °C (90–103 °F) | Extreme caution | Heat cramps, heat exhaustion |
| 39–51 °C (103–124 °F) | Danger | Heat exhaustion likely, heat stroke possible |
| 51 °C+ (125 °F+) | Extreme danger | Heat stroke highly likely |
Frequently asked questions
Is the heat index the same as the "feels like" temperature?
In hot weather, yes: weather apps usually show the heat index as "feels like". In cold weather they show the wind chill.
Why can dry desert heat score lower than the air temperature?
In very dry air sweat evaporates quickly, so the body cools well and the index can be slightly below the air temperature. The sun and hot ground still add heat load.
Does the heat index include sunshine?
No. It assumes shade and a light wind. Full sunshine can raise the effective temperature by up to about 8 °C (15 °F).
How do I stay safe in extreme heat?
Drink water before you're thirsty, keep hard outdoor work out of the midday hours and take breaks in shade or AC. If you feel dizzy, sick, confused or get a pounding headache, stop and cool down straight away. Saudi Arabia and the UAE also ban outdoor work at midday through the summer.
Why does 35 °C feel worse in Jeddah or Dubai than in Riyadh?
The coastal air is much more humid, so sweat can't evaporate and your body can't cool itself. Riyadh's dry heat at the same temperature is easier to bear.
What is WBGT?
The wet-bulb globe temperature. Unlike the heat index, it also counts direct sun and wind, so it's what employers and sports bodies use to set heat limits.