The solar updraft tower is famous: warm air rises through a chimney, spins turbines. But its inverted twin — the evaporative downdraft tower — is arguably more useful, because it drops cold air directly onto a hot city while generating power on the way down. Israeli physicist Dan Zaslavsky proposed the concept in the 1970s. Let's build one for a desert megacity like Riyadh.

The physics. Hot desert air at the top of a tall shaft is misted with water. Some water evaporates, drawing about 2.26 MJ per kilogram from the air. The air cools from, say, 40°C to 25°C — a wet-bulb-limited drop. Cool air is denser than hot air, so it plummets down the shaft, hits turbines at the base, and exits as an artificial cool breeze at street level.

Numbers, please. Consider a hollow cylinder 1,200 m tall, 400 m in diameter, cross-section A = π(200)² ≈ 1.26×10⁵ m². Desert ambient: 40°C, density 1.128 kg/m³. Post-evaporation: 25°C, density 1.184 kg/m³. Density difference: Δρ ≈ 0.056 kg/m³.

Buoyant pressure at the base:

ΔP = Δρ · g · H = 0.056 × 9.81 × 1200 ≈ 660 Pa

Ignoring turbine load, terminal velocity in the shaft would be v = √(2ΔP/ρ) ≈ 33 m/s. With turbines loading the flow to about half that, we get roughly 16 m/s. Volume flow: Q ≈ 2×10⁶ m³/s. Available fluid power:

P_avail = ΔP · Q ≈ 660 × 2×10⁶ ≈ 1.3 GW

Real turbines extract maybe 60% of this after friction losses. Call it ~750 MW gross. That is the output of a mid-size nuclear reactor, from a hollow concrete tube and some sprinklers.

The water bill. Cooling 2.4 million kg/s of air by 15°C requires:

ṁ_water = (ṁ_air · c_p · ΔT) / L_vap
        = (2.4×10⁶ × 1005 × 15) / 2.26×10⁶
        ≈ 16,000 kg/s ≈ 16 m³/s

That's 1.4 million m³ per day — the entire domestic water demand of a city of 5 million. Pumping it 1,200 m up costs mgh ≈ 190 MW, or about 25% of gross output. Net: ~560 MW electrical, plus a hurricane of 25°C air spilling into the streets.

Fresh water is a non-starter in Arabia. But seawater works — you just accept that the tower doubles as the world's largest salt spray humidifier, and downstream deposition ruins any downwind farmland. Better: co-locate reverse-osmosis desalination and use the brine, or run the misters with polished RO water and vent brine to the Gulf.

Materials. The tube itself is mostly hollow — the wall carries wind and self-weight but no buoyancy load. Prestressed concrete cooling-tower construction already goes to 200 m. Scaling to 1,200 m means about a 12 m wall thickness at the base and hyperboloid geometry to resist buckling under 150 km/h desert gusts. Total concrete: ~30 million m³, roughly six Three Gorges Dams' worth. Cost: probably $20–30 billion, comparable to a large nuclear plant per kilowatt but with no fuel cycle.

The bonus. Cool air exits at 200 m/s equivalent momentum through radial diffusers, feeding a district cooling ring. Riyadh currently spends 60% of summer electricity on air conditioning. Free cold air at the base could offset a second reactor's worth of AC load — meaning the effective footprint is closer to 1 GW-equivalent.