Planning to travel to Mombasa over the holidays? You may want to prepare for hotter nights. A new global study has found that nighttime heat in Mombasa is rising and becoming more frequent, making evenings more humid and uncomfortable across the coastal city.
The study shows that the gap between daytime and nighttime temperatures is shrinking. In simple terms, heat that builds up during the day is no longer cooling off at night; instead, it lingers, offering little relief.
The report, titled ‘Extreme heat and the shrinking diurnal range: A global evaluation of oppressive air mass character and frequency,’ was released by Climate Resilience for All.
It analysed 30 years of weather data from 1994 to 2024, focusing on two types of weather conditions most dangerous to human health: moist tropical (MT) and dry tropical (DT) air masses.
Mombasa is listed among cities worldwide experiencing this trend under both moist tropical (hot and humid) and dry tropical (hot and dry) weather patterns.
Other affected cities include Upington in South Africa, Bamako in Mali, Abidjan in Côte d'Ivoire, Cairo in Egypt, Agadir in Morocco, Dakar in Senegal and Brazzaville in Congo.
Traditionally, heat exposure has been measured using daytime high temperatures. However, the research warns that rising nighttime temperatures may carry equal, if not greater, health risks.
Hotter evenings disrupt sleep and increase heat stress, particularly for vulnerable groups such as the elderly, children, pregnant women and outdoor workers.
Rising nighttime heat can also worsen pre-existing medical conditions, increase electricity demand for cooling and strain water resources.
“It is critical for us to understand how the heat of summer, that sends people to the emergency room, is shifting and what we are overlooking when we talk about it,” said Larry Kalkstein, climatologist and chief heat science advisor at Climate Resilience for All.
The findings come as world leaders gather in Brazil for the COP30 climate summit, where heat adaptation is among key discussion points.
According to the study, 83 per cent of cities analysed globally are now experiencing sustained higher nighttime temperatures.
In Africa, 13 out of 15 cities show rising nighttime heat in moist tropical conditions. Meanwhile, 10 out of 14 African cities show rising nighttime heat in dry tropical conditions, which includes Mombasa under both weather types.
Kathy Baughman McLeod, CEO of Climate Resilience for All, said the analysis should serve as a wake-up call to policymakers.
“We want this analysis to mobilise city and health leaders to urgently broaden their view of what is a 24-hour heat crisis. This research uncovers a critical blind spot in our understanding of extreme heat.”
The report stressed that when nighttime temperatures stay high, the human body struggles to cool down, increasing risks of dehydration, heat exhaustion and cardiovascular stress. Disrupted sleep further reduces the body’s ability to recover from daytime heat exposure.
The researchers warned that many existing heat-warning systems are still focused mainly on daytime temperatures and fail to account for rising overnight heat.
They are calling on health officials and city planners to strengthen heat early-warning systems, improve public awareness on nighttime heat risks, ensure heat adaptation plans protect vulnerable groups and prepare for multi-day heatwaves that offer little nighttime relief.
