A series of experiments by Bristol Stickney and Steve Baer (October 2002) using matched 12″ × 12″ aluminum plates — one powered by a 9.5-watt heating pad, the other unpowered — to measure heat loss coefficients and demonstrate that night-sky radiation can pull a heated surface below ambient air temperature.
Twin 12″ × 12″ × ⅛″ aluminum plates, always tested side by side in identical orientations. One plate has a 9.5-watt (32.4 BTU/hr) electric heating pad behind it; the other has none. They started with black-painted plates but later switched to white paint to reduce temperature rise in direct sun.
| Configuration | ΔT (heated − unheated) | Derived U-value |
|---|---|---|
| Vertical (wall) at night | ~20°F | 32.4 ÷ 20 = 1.6 BTU/°F·ft²·hr |
| Horizontal at night | ~15°F | 32.4 ÷ 15 = 2.16 BTU/°F·ft²·hr |
The horizontal surface loses heat faster (higher U-value) because it has an unobstructed view of the sky dome — more radiative loss to space.
A great surprise is to find that during clear dry nights the heated horizontal plate temperature does not rise as high as ambient air temperature. Night radiation to the sky can exceed 9.5 watts per square foot.
This is the key finding: even with 9.5 W/ft² of electric heat pumping into it, the plate cannot reach ambient on a clear, dry New Mexico night. The sky is pulling heat away faster than the heater can supply it. This gives direct empirical support to the Cool Cell premise — that roof-mounted water radiators can dump a building's heat to the sky without any mechanical refrigeration.
Baer warns against extrapolating linearly. At 9.5 W/ft² the plate rises ~15–20°F. At full sun (~93 W/ft²), a naive calculation predicts 132°F above ambient — but "I have never seen such a large temperature rise with black paint. Heat flows more readily as the flux increases, and also as temperatures rise." He proposes testing 100-watt panels alongside 10-watt and 0-watt panels to demonstrate this non-linearity. This connects directly to the e-folding essay's theme: thermal processes are exponential, not linear.
Graphs from the experiments show that wind narrows the gap between heated and unheated plates by sweeping convective heat away. This is why the archive emphasizes calm, clear nights as the ideal operating condition for radiative cooling systems.