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Heatwaves - We're Doing it Wrong (Sometimes)

In short. Australian heatwaves are determined by the degree to which the current and forecast maximum and minimum temperatures differ from both recent and long term observations. Sole reliance on ambient temperature to determine heatwave forecasting seems appropriate for most climatic regions excluding those of the tropics and when other regions experience combined elevated ambient temperature and relative humidity. This has been anecdotally reported during 2024 east-coast low-intensity heatwaves, and using a real-world example, we demonstrate how heatwave classifications would differ through user of apparent temperature (that accounts for relative humidity) instead of ambient temperature. Furthermore, recent research supports the inclusion of relative humidity in heatwave calculations for Northern tropical regions (Nairn et al., 2022). Collectively, the evidence undermines the relevance of one-size-fits-all National heatwave warning system, such that caution should be exercised when classifying the risk of heat stress based upon one variable or metric, especially where heat and humidity are combined. 


Figure 1. A forecast map depicting low-intensity, severe and extreme heatwaves across Northern Australia (Bureau of Meteorology)


For many years, we've had concerns regarding how heatwaves are classified in humid regions of Australia, particularly in the tropics (Oppermann et al., 2017). Before we detail our concerns and use recent east-coast heatwaves along with published research to make our point, let's review how heatwaves are determined by the Bureau of Meteorology (BoM). 


According to BoM, the forecast maximum and minimum temperatures for each three-day period in the coming week (e.g. Monday–Wednesday, Tuesday–Thursday) is compared to the 'normal' temperatures expected for that location at that time of year, and to observed temperatures over the last 30 days. Again, according to BoM, this comparison identifies regions where ambient temperatures are unusually high in relation to the local long-term climate and the recent past. The outcome of these comparisons is a metric called the Excess Heat Factor (EHF) that is used to determine the existence of a heatwave and its intensity as per BoM's classifications below.


1.  Low-intensity heatwaves are the most common—most people are able to cope with this level of heat.

2. Severe heatwaves are less frequent and are challenging for vulnerable people such as the elderly—particularly those with pre-existing medical conditions.

3. Extreme heatwaves are the rarest kind. They affect the reliability of infrastructure, like power and transport, and are dangerous for anyone who does not take precautions to keep cool—even those who are healthy. People who work or exercise outdoors are particularly at risk.


We don't have an issue with use of the EHF. Our issue is related to the use of ambient temperature as the sole variable to calculate EHF. Of course, ambient temperature plays a major role in thermoregulation and heat stress more broadly. But, without consideration of other factors such as relative humidity, ambient temperature provides an incomplete summary of thermal conditions (Oppermann et al., 2017). For example, December maximum temperatures of 33ºC for Darwin, NT and Adelaide, SA are not as comparable once relative humidity is considered. So, why have BoM adopted an EHF based upon ambient temperature as a one-size-fits-all approach to heatwave classification in Australia?


Presumably, the answer is that ambient temperature EHF is appropriate for the vast majority of the population for the vast majority of the time. Until it isn't (anecdotally), such as during the recent east coast “low intensity” heatwaves. 


Prior to discussing these heatwaves, it's important to acknowledge the growing field of research that describes associations between ambient temperature EHF and public health outcomes, predominantly in temperate and subtropical climates (Bhatta et al., 2023). This work is critical to identify the heat-health burden and has seen increased research funding as a result. We are strong supporters of such research but a heatwave service that is good for most people, most of the time, is not good enough in our opinion.


The recent east coast heatwaves have been generally classified as low intensity, below the threshold of severe or extreme, see Figure 2 for an example.


Figure 2. A recent heatwave for approximately half of QLD (Bureau of Meteorology). Elevated humidity during this heatwave was limited to coastal regions.


The issue with Figure 2 was that this heatwave (based upon ambient temperature EHF) coincided with elevated relative humidity for coastal regions. The feedback we received from various work sites in the affected area was that the combined elevated heat and relative humidity conditions were oppressive, beyond that typically represented by a low intensity heatwave. We agreed based upon our observations from various work sites. In our opinion, it's worth considering the benefits of reporting EHF based upon combined heat and relative humidity (EHF Heat Index or apparent temperature) for regions with "humid" summers according to Figure 3.


Figure 3. Australian climate zones (Bureau of Meteorology)


Those in favour of the current (EHF ambient temperature) system cite that elevated minimum temperatures are indicative of elevated relative humidity and therefore, relative humidity is currently accounted for. We disagree, as does a key research paper. Led by Dr John Nairn, lead author of the first paper to describe the EHF, his research team examined the current ambient temperature EHF and heat index EHF (includes relative humidity) for representative cities within the six climate zones of Figure 3. They reported that "results support ongoing use of a local temperature-only percentile-based heatwave index for detection of both dry and humid severe heatwaves in five of Australia’s six climate zones. This differs in Australia’s hot and humid tropical climate zone. This region experiences rare, very dry and very humid heatwaves. A comprehensive heatwave service in the tropics needs to operate the temperature-only and humidity-included versions of Australia’s EHF in order to capture these unusual heatwave events for an effective warning service" (Nairn et al., 2022).


So, their data supports inclusion of relative humidity for Northern tropical regions (Figure 1) but not necessarily all of the recently heatwave impacted east-coast regions. Thanks to Christine Killip and the team from Weather Intelligence (a Katestone Company), we share a comparison of EHF ambient temperature v EHF apparent temperature for one site. You can read more about what the WI team intend to do with this new work by incorporating it into their Kite Weather Risk Forecasting platform here.


Figure 3. Heatwave severity for Western Brisbane (Nov 2023 - Feb 2024) calculated Comparison of Excess Heat Factor based upon ambient temperature (blue line and markers) or apparent temperature (orange line and markers) 


Analysis of all weather stations is well beyond the scope of this article, so data from Western Brisbane during recent months is included. The graph of Figure 3 represents heatwave severity using EHF based on ambient temperature (blue line and markers) compared to EHF based on apparent temperature (orange line and markers). The graph proves our point (and the anecdotal reports from workers) correct - the low intensity heatwaves of late January were classified as severe based upon apparent temperature. Sure, it’s a small sample, but this example coupled with the evidence from Northern Australia (Nairn et al., 2022) is enough to caution individuals and organisations against classifying the risk of heat stress based upon one variable or metric, especially where heat and humidity are combined. 


References

Bhatta M, Field E, Cass M, Zander K, Guthridge S, Brearley M, Hines S, Pereira G, Nur D, Chang A, Singh G. Examining the Heat Health Burden in Australia: A Rapid Review. Climate. 2023;11(12):246.


Nairn J, Moise A, Ostendorf B. The impact of humidity on Australia’s operational heatwave services. Climate Services. 2022;27:100315.


Oppermann E, Brearley M, Law L, Smith JA, Clough A, Zander K. Heat, health, and humidity in Australia's monsoon tropics: a critical review of the problematization of ‘heat’ in a changing climate. Wiley Interdisciplinary Reviews: Climate Change. 2017;8(4):e468.


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