Air conditioning is brute-force thermodynamics: pump heat from inside to outside using compressors that burn ~10% of global electricity. But there's a colder reservoir we ignore — outer space, sitting at 3 K. Between us and it lies an atmospheric transmission window from 8–13 μm where our atmosphere is nearly transparent to infrared. A surface tuned to radiate strongly in that band, while reflecting incoming sunlight, can dump heat directly into the cosmic void. No compressor. No electricity. Even at noon.

The physics is set by Planck's law and Kirchhoff's law. A blackbody at 300 K (room temp) radiates peak emission near 10 μm — right in the sky window. Stanford's Fan group demonstrated this in 2014 with a photonic stack of HfO₂ and SiO₂ layers, achieving ~5 °C below ambient in direct sunlight. Subsequent metasurfaces and polymer films (PDMS on silver) have pushed practical cooling power to ~100 W/m² during the day and ~150 W/m² at night under clear skies.

The household math. A typical U.S. single-family home has a peak cooling load of about 3.5 kW (a 1-ton AC unit). At 100 W/m²:

Area needed = 3,500 W / 100 W/m² = 35 m²

That's roughly a third of a 100 m² roof. Plausible — until you account for the COP problem. A modern AC has COP ≈ 4, so it delivers 3.5 kW of cooling for ~900 W of electricity. The sky cooler delivers cooling for free, but it can't deliver it where you want without a heat exchanger and a coolant loop. Add a small circulating pump (~50 W) and you're still beating AC by a factor of 15 in electricity.

Scaling to a city. New York City peaks around 10 GW of summer AC demand. To replace it:

Sky-cooler area = 10 × 10⁹ W / 100 W/m² = 10⁸ m² = 100 km²

NYC's land area is 778 km². You'd need ~13% of the city covered in radiative film — essentially every flat rooftop. The rooftop area of NYC is estimated at ~100 km². The numbers just barely close.

Where the physics bites back:

• Humidity destroys the window. Water vapor absorbs across 8–13 μm. In Houston at 80% RH, cooling power drops to ~40 W/m². Miami and Mumbai are nearly hopeless during monsoon.
• Clouds close the door entirely. An overcast sky radiates back at near-ambient temperature. Net cooling collapses to single watts.
• You can't beat ambient by much. The ΔT below air temperature is bounded by the radiative flux divided by the convective heat transfer coefficient (~10 W/m²·K). At 100 W/m² net flux, that's ~10 °C max sub-ambient — enough for comfort cooling, not refrigeration.
• Storage is the real problem. Peak cooling demand is at 3 PM. Peak radiative cooling is at 3 AM (no sun, dry air). You need a thermal mass — a chilled-water tank in the basement, ~20 m³ for a house — to time-shift it.

The honest answer: replace 60–80% of AC load in dry climates (Phoenix, Madrid, Denver), 20–30% in humid ones. Globally, that's still a few hundred terawatt-hours of electricity recovered annually — and the only moving parts are the photons leaving Earth.