Why European Cathedrals Are Cool Without Air Conditioning

European stone cathedrals stay cool even in the sweltering summer without air conditioning, thanks to their unimaginably thick stone walls. The Girona Cathedral in Spain has walls that average 1.2 meters thick, and up to 2.5 meters in some places. The key isn't that the stone reflects heat, but that it absorbs a massive amount of it. While the midday heat spends all its time trying to warm up these massive stone blocks, the sun has already set outside. Add to this the 35-meter high ceilings that hide the hot air near the top. It's even more surprising that measurements over six years showed the indoor temperature remained at a stable 24-25 degrees Celsius during the summer.