Evaporative cooling collapses in humid air — wet-bulb physics caps it to hot-dry climates

The problem

The cooling a wet-terracotta module can deliver is bounded by the air's wet-bulb temperature. Air can be cooled towards its wet-bulb point but never below it, and the maximum useful drop equals the wet-bulb depression (dry-bulb minus wet-bulb). That depression is large in hot, dry air and small in hot, humid air.

At 35 °C dry-bulb and 50% relative humidity, the wet-bulb temperature is ~26 °C — so 26 °C is the absolute floor an ideal evaporative cooler could reach, with the practical drop smaller still. Raise the humidity and the floor rises with it until, in muggy tropical heat, there is almost no depression left to exploit.

Deployment implications

• Strong fit: hot-dry and hot-arid climates (interior India, the Gulf, North Africa, the US Southwest, inland Mediterranean) — where the technology can deliver 6–15 °C drops.
• Weak fit: humid subtropical and tropical-coastal cities (Singapore, Mumbai coast, the US Gulf Coast, much of West Africa) — where the same hardware yields little, and shade, trees, or refrigerant cooling are more honest answers.

bloc°'s team claims forced solar airflow can recover some performance in higher-humidity Central Europe, but that remains an unproven field assertion, not a workaround for the underlying thermodynamics.

What a resolution needs

A climate-screening rule — e.g. a design-day wet-bulb-depression threshold below which the technology is not recommended — plus honest siting guidance so municipalities avoid installing evaporative modules in climates where they cannot perform. Indirect evaporative designs, which cool without adding moisture, are a partial route into milder-humidity zones and warrant separate evaluation.