Characterizing Heat Extremes in Uganda: A Multi-Metric analysis of Heatwave Trends

Background: Extreme temperatures are a key indicator of climate change, and the frequency and intensity of deadly heatwaves are projected to rise. Heat already poses a major public health threat, with over a billion people exposed globally, and yet Africa is warming faster than the global average. Tropical countries such as Uganda are expected to experience hotter conditions and growing cooling needs. However, extreme heat remains under-studied in tropical climates, especially in Africa. Although there is no single universal definition of heatwaves, current evidence generally identifies heatwaves as periods of unusually high temperatures relative to local climates.

Objective: This study aims to assess the spatio-temporal characteristics of heatwaves in Uganda and their trends.

Methods: We analyzed daily temperature data from 31 meteorological stations across Uganda over a 34-year period to assess national and regional trends. Heat extremes were characterized using three complementary heatwave metrics: the 90th percentile of maximum temperature (TX90), the Heat Index (HI), and the Excess Heat Factor (EHF). To describe temperature extremes and warming rates, the mean Tmax was calculated per year, month, and zone. The hottest and coolest years and months were identified by their respective Tmax values, while trends in monthly mean Tmax were estimated using linear regression. The slopes were expressed as °C per decade, and the months with the fastest warming and cooling rates were reported alongside their corresponding p-values. Stations were classified according to the ten climatological zones and zonal aggregation was performed to summarize station-level heatwave metrics into broader climatic regions, thereby capturing spatial patterns while reducing sensitivity to local variability.

Results: Uganda has warmed by an average of 1.7°C over the past three decades. February consistently emerges as the hottest month, while July and November are generally the coolest, with some regional variation. The year 2016 was the hottest on record, with the highest mean maximum temperature of 30.35°C recorded in northern Uganda. Across all stations, the proportion of days classified as heatwave days increased significantly, rising by an average of 0.2 percentage points per year between 1990 and 2023.

Heatwave characteristics exhibit strong spatial and temporal variability. Annual heatwave prevalence at individual stations ranges from years with no heatwaves to years exceeding 20–30%, with notable peaks in 1998, 2003, 2010, and 2023. The annual average number of heatwave events fluctuates widely, from 0 to more than 15 events per year at some stations. After 2015, more stations experienced frequent and intense episodes, indicating a clear intensification of heatwaves. Seasonally, heatwaves occur most often from June to August, with additional clusters in February –April and October–November. Sparse heatwave prevalence in the 1990s contrasts sharply with the dense occurrence of heatwave days in recent years.

Conclusion: Uganda is experiencing significant warming and a marked increase in both the frequency and intensity of heatwaves. This multi-metric characterization provides a robust foundation for understanding heat risk in a tropical African context, informs the design of heat-related policies and early warning systems, and offers a methodological framework applicable to other countries assessing evolving heatwave hazards.