Dear Editor:
He et al. [1] present an extensive analysis indicating that all-cause, cardiovascular, and respiratory mortality increases on days characterized by rare rainfall events with return periods exceeding five years. The observed effect diminishes in the days following such events. While He et al. [1] suggest several plausible mechanisms, such as impaired access to healthcare due to transportation challenges or enhanced microbial growth driven by elevated humidity, these hypotheses remain speculative as a direct causal relationship between heavy rainfall and mortality has not been conclusively demonstrated.
It is crucial to recognize that heavy rainfall days also exhibit deviations in other meteorological variables. For instance, such events are less frequent during winter and are often associated with elevated temperatures and humidity during summer—conditions necessary for heavy rainfall. Thus, the observed mortality increases may be attributed to the hot and humid conditions preceding heavy rain rather than the rainfall itself, suggesting that the rainfall might mitigate these stressors instead.
Regrettably, He et al. [1] provide mortality data exclusively for days following heavy rainfall, omitting data from antecedent days. This omission limits the ability to discern whether heavy rainfall increases mortality compared to preceding days or, conversely, reduces mortality by alleviating a period of oppressive heat and humidity. For the conclusions drawn by He et al. [1], mortality trends on the days leading up to heavy rainfall are more critical than those observed afterward.
A second line of reasoning further questions the plausibility of heavy rainfall as the direct cause of increased mortality. Heavy rainfall requires substantial atmospheric moisture and instability to develop, conditions that peak later in the day due to evapotranspiration and surface warming [2,3]. Consequently, heavy rainfall events show a pronounced diurnal variation, with a minimum occurrence between 07:00 and 10:00. Using the 18264 heavy rain events with a return period larger than 5 years by [4] shows that only 12% of the events started during these three hours. The probability of heavy rain to start peaks between 15:00 and 20:00 (36%)and then slightly decreases until midnight. Furthermore, only a small fraction (4%) of heavy rainfall events lasted less than two hours. This temporal distribution implies that most heavy rainfall occurs late during the day when individuals are likely indoors and shielded from direct exposure.
Given these temporal constraints, it is implausible that, within a few hours remaining after rainfall onset, restricted access to medical care could result in a pronounced mortality peak. Similarly, the rapid microbial growth hypothesized by He et al. [1] would unlikely significantly impact mortality within the same timeframe, especially among individuals already indoors.
Another critical consideration is the discrepancy between precipitation measurement protocols and calendar days used in health records. In many regions, precipitation data are recorded once daily during morning observations (between 07:00 and 09:00 local time, depending on the country) [5], coinciding with the diurnal rainfall minimum. Consequently, up to 30–36% of heavy rainfall events that began after midnight may still be attributed to the previous calendar day in meteorological records. Such misalignment challenges the attribution of rainfall to observed mortality increases.
In conclusion, the association between heavy rainfall and increased mortality likely reflects coincidental alignment with adverse meteorological conditions preceding the rainfall event. Rather than being a causative factor, heavy rain may provide relief from these stressors, which is the opposite of the interpretation by He et al. [1]. Their findings should be interpreted cautiously, and additional analyses incorporating mortality data from days preceding rainfall events are needed to better understand this relationship.
Sincerely,
Karl Auerswald
References
1. He C, Breitner-Busch S, Huber V, Chen K, Zhang S, Gasparrini A et al. Rainfall events and daily mortality across 645 global locations: two stage time series analysis. BMJ 2024; 387: e080944. doi:10.1136/bmj-2024-080944
2. Dayan U, Nissen K, Ulbrich U. Review Article: Atmospheric conditions inducing extreme precipitation over the eastern and western Mediterranean. Natural Hazards and Earth System Sciences 2015; 15, 2525-2544. doi:10.5194/NHESS-15-2525-2015.
3. Frei C, Schär C, Lüthi D, Davies H. Heavy precipitation processes in a warmer climate. Geophysical Research Letters 1998; 25. doi:10.1029/98GL51099.
4. Fischer FK, Winterrath T, Auerswald K. Temporal- and spatial-scale and positional effects on rain erosivity derived from point-scale and contiguous rain data. Hydrol Earth Syst Sci 2018; 22, 6505–6518. doi:10.5194/hess-22-6505-2018
5. Burt S. Observing hours and time standards. In: The Weather Observer’s Handbook. Cambridge University Press; 2024:303-313.
Author
Auerswald Karl, Dr, is a retired professor at the School of Life Sciences, Technical University of Munich ([email protected]).
Is Rare Rainfall Friend or Foe to Health? Re: Rainfall events and daily mortality across 645 global locations: two stage time series analysis