A tropical night is meteorologically defined as one in which the minimum temperature does not fall below 20 °C. It is a key indicator of nocturnal heat stress because it prevents the body's physiological recovery: the body needs ambient temperatures to drop at night to reduce the core temperature accumulated during the day.
Health impact
Tropical nights have a more severe health impact than daytime maximum temperatures: mortality during heat waves correlates more closely with nocturnal minima than with daytime maxima. The body cannot complete its cooling cycle if the temperature does not drop enough, producing cumulative fatigue, dehydration, and cardiovascular failure in vulnerable populations (elderly, chronically ill).
In Spain, tropical nights have increased dramatically in recent decades. Coastal cities such as Valencia, Barcelona, Malaga, and Palma de Mallorca now record 30-60 tropical nights per year, double the 1980s figures. In extreme episodes, equatorial nights (minimum >25 °C) are recorded, increasingly common on the Mediterranean coast during July and August.
The urban heat island effect aggravates the problem: cities retain more heat than the rural surroundings, amplifying tropical nights by an additional 2-5 °C. Climate change projects a 50-100 % increase in tropical nights for the Iberian Peninsula before 2050. See also: wet-bulb temperature, heatstroke.