Two different Middle Easts, two hundred kilometres apart

On an August afternoon Riyadh sits at 45 °C with a dewpoint near 5. Dammam, three hours' drive east on the Gulf coast, sits at 38 °C with a dewpoint of 30. The thermometer says Riyadh is the hotter place. Every physiologist, every occupational health regulation and every construction site in the Kingdom says the opposite.

Riyadh, interior

45 °C

air temperature

24 °C

wet-bulb — survivable, uncomfortable

Dammam, Gulf coast

38 °C

air temperature

33 °C

wet-bulb — outdoor work must stop

What the wet-bulb is

Wrap a thermometer bulb in a wet cloth and blow air over it. Water evaporates, the evaporation takes heat, and the thermometer settles below the air temperature — by a lot in dry air, by almost nothing in saturated air. That reading is the wet-bulb temperature, and it is the lowest temperature anything can reach by evaporating water into the air around it.

A human being cools by sweating, which is exactly that process. So the wet-bulb is not a meteorological curiosity here; it is the temperature a human body can cool itself down to, and no further. Above a wet-bulb of about 35 °C, a healthy adult at rest in the shade with unlimited water cannot shed metabolic heat at all. Core temperature rises regardless of what they do. Six hours is generally taken as fatal.

< 25 °CNormal working conditions.

27 °CSustained heavy outdoor labour becomes hazardous. Most Gulf states' summer midday work bans exist because of numbers around here.

31 °CThe standard work/rest cycles stop working — the rest intervals required exceed the working intervals. Acclimatised, hydrated, fit people are still at risk.

35 °CThe theoretical limit of human thermoregulation. No amount of shade, rest or water is sufficient.

Why the Gulf, specifically

Three things line up in the Persian Gulf and almost nowhere else on earth.

The water is absurdly warm. The Arabian Gulf is shallow — averaging about 35 metres, less than 20 in much of the southern half — nearly enclosed, and sits under one of the strongest solar loads on the planet. By August its surface reaches 34–36 °C. It is the warmest large body of open sea in the world. A sea at 35 °C puts a saturated air mass at 35 °C directly against a populated coast.

The land next to it is a furnace. The Arabian interior in summer is one of the largest, hottest, driest surfaces on earth, and the heat low it generates draws that saturated marine air onshore every afternoon.

The coast is where everyone lives. Dubai, Abu Dhabi, Doha, Manama, Kuwait City, Dammam, Bandar Abbas, Basra — the region's population is concentrated on exactly the strip where the two meet.

Weather stations at Dhahran and on the Iranian coast at Bandar Abbas and Jask have recorded wet-bulb readings brushing 35 °C for an hour or two at a time in the last decade. The Persian Gulf and the Indus valley are the only two places on the planet where that has happened. Both are projected to see it more often.

Read it live: the wet-bulb map shows the whole region at once, and the ramp's colours break at 27, 31 and 35 rather than at round numbers, so the bands you can see are the bands that mean something. The front page ranks the region's cities by wet-bulb every time it loads.

How this site computes it

From the model's own 2 m temperature and 2 m dewpoint, using the empirical fit in Stull (2011), which is good to a few tenths of a degree across −20 to +50 °C and 5–99 % relative humidity — comfortably inside the spread of the models it is applied to, and one array pass rather than an iterative psychrometric solve per pixel. It is computed on the model's native grid in native units, before anything is resampled or quantised.

Note that this is the thermodynamic wet-bulb of a shaded, ventilated thermometer. It is not WBGT — the wet-bulb globe temperature that occupational standards actually specify — which adds direct solar load and radiant heat from surfaces, and which in Gulf sunshine runs several degrees higher. Treat the map as the physiological floor, not as a compliance instrument.

The other heat fields on this site

Apparent temperature — the US National Weather Service heat index, in Celsius. A different way of combining the same two numbers, and the one most readers have seen before. On the Gulf coast in August it runs 10 to 15 degrees above the thermometer; in the Nejd it runs below it, because the dry-air correction is real and fires across the entire Arabian interior.
Dewpoint — the cleanest single line between the two Middle Easts. A Gulf coast dewpoint of 30 and a Nejd dewpoint of 0 on the same afternoon, 200 km apart.
Sea surface temperature — drawn on a scale that goes to 38 °C, because a ramp that stopped at 30 would render the entire Gulf as one colour all summer.
Air temperature — on a −20 to +54 °C scale. Both ends of that get used: Erzurum in eastern Türkiye has been below −35, and Kuwait's Mitribah recorded 53.9 °C.