Why does heat feel more exhausting here than elsewhere?

The short answer

Radiant heat from every surface, invisible dehydration from low humidity, and elevated UV at 450 metres altitude. Multiple heat stressors operating simultaneously.

The city sits on a clay and limestone plain that reflects solar radiation upward. Walls absorb heat all morning and re-radiate it through the afternoon. You're heated from above, below, and both sides. Effective temperature can be ten degrees above what a thermometer reads.

Below 20% humidity, sweat evaporates so fast you don't feel wet. Dehydration arrives before you notice. At 450 metres, UV is measurably stronger than at sea level.

Two litres of water a day is a minimum. Three is better. People who've been here a while drink before they're thirsty.