Moisture ELevated Temperature (MELT) index: A novel index to capture dry and humid heatwaves

Quagraine, Kwesi Twentwewa; Quagraine, Kwesi Akumenyi

doi:https://doi.org/10.5194/egusphere-2025-1574

Preprints

https://doi.org/10.5194/egusphere-2025-1574

Preprints

30 Apr 2025

| 30 Apr 2025

Moisture ELevated Temperature (MELT) index: A novel index to capture dry and humid heatwaves

Kwesi Twentwewa Quagraine and Kwesi Akumenyi Quagraine

Abstract. In this study, we introduce a novel heatwave characterization metric: the Moisture ELevated Temperature (MELT) index. MELT integrates anomalies in temperature and relative humidity to quantify moist heatwaves and more accurately assess physiological heat stress. Traditional heatwave metrics predominantly rely on temperature alone, often underestimating the compounded effects of humidity on human health and thermoregulation. To address this gap, the MELT index offers improved accuracy for public health risk assessment and response strategies. To validate MELT's effectiveness and versatility, we applied it to analyze three significant, record-breaking heatwave events from recent decades: the 2021 Pacific Northwest (PNW), 2016 South Korea, and 2019 Western Europe heatwaves. Our analysis demonstrates that MELT clearly distinguishes between humid and dry heatwave conditions, accurately identifying the moisture characteristics specific to each region. Specifically, the PNW and South Korea events exhibited notably higher humidity levels, influenced by atmospheric rivers and increased convective activities, respectively. Conversely, the Western Europe heatwave was characterized by drier conditions resulting from Saharan dry-air intrusions. MELT's reliance on widely accessible datasets of temperature and humidity ensures its global applicability and consistency, addressing limitations inherent in temperature-only indices. Furthermore, its flexible use of climatological percentile thresholds allows adaptation to varying climates and future scenarios. Given anticipated increases in heatwave frequency and intensity due to climate change, MELT provides a critical tool for evaluating emerging risks, informing climate adaptation policies, and guiding targeted mitigation measures.

Received: 02 Apr 2025 – Discussion started: 30 Apr 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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Kwesi Twentwewa Quagraine and Kwesi Akumenyi Quagraine

Status: closed

RC1:
'Comment on egusphere-2025-1574', Jonathan Buzan, 03 Jun 2025

The MELT index, Moisture Elevated Temperature, is designed to evaluate potential heatwaves and diagnose dry and moist heat events. Categories range from 0 (no heat event), <0.5 (dry enhanced), to ≥1 (moist elevated) scale. The intent is for use to help meteorologists and decision makers to determine the threat to health outcomes. The manuscript then uses 3 recent heatwaves that were recently scrutinized by multiple researchers, to showcase the metric. Two of the events, the 2021 Pacific Northwest Heatwave and the 2016 South Korean heatwave were consistent with elevated moist heatwaves. The 3^rd event, the 2019 European heatwave, was identified as a dry heatwave. Overall, a straightforward synoptic scale analysis was conducted, and the metric shows promise in meteorological applications.

From the scientific objectives and demonstration, the manuscript is well written and straightforward. I appreciate the authors efforts in readability! From this standpoint, the paper is good, and some minor adjustments which I detail below would be fine.

However, there are 3 a fundamental ‘elephant-in-the-room’ aspects to the paper from an editorial point of view that perhaps should be addressed.
1) the authors discuss physiological responses to moist heat, and that their metric can identify a moist or dry heatwave. But without showing human responses to the moist or dry heatwave examples, it can be hard to determine whether their classification would correctly address the societal outcomes from these heatwaves.
2) There are 100s of heat stress metrics. The authors mention that metrics are not quite universal or are region specific. But this gets into the nuances heat stress: there are 100s of heat stress metrics because humans respond to heat in a myriad of ways. A dry heatwave can be just as deadly as a wet heatwave. Exposure, duration, activity, health status, age, etc., all interplay into heat stress. It is very difficult to generalize heat stress on humans to an individual index. Fundamentally, this is because the thermo-physiological system is complex, and there are many ‘paths’ that lead to negative outcomes, many of which are non-linear. So, I am skeptical that this index would be universally applied. What is interesting is the interplay with meteorology, which is clearly demonstrated from the authors’ synoptic analysis.
3) Lastly, there is the aspect of applying MELT to future climate change. Buzan and Huber, 2020 show that relative humidity scales negatively with global mean temperature change. This does not mean that future heatwaves would switch from moist to dry in the future. There are a lot of issues with relative humidity as a basis for a metric, and one of those is that absolute humidity increases non-linearly with temperature. Even if extreme relative humidity goes down in the future, the danger from the heatwave is enhanced due to the unusually elevated moisture.

To me, what I discuss in the 3 paragraphs above is an editorial decision on the status of the manuscript, because, as stated before, the paper is well written and is self-contained with clear scientific objectives, construction, and application. Below are the minor comments that should be addressed.
Best regards,
-Jonathan R. Buzan

Line 10: may want to remove the mentioning of accurately assessing physiological heat stress. The paper does not compare with health data. Emphasize the meteorological applications instead.

Line 42—50: Buzan et al., 2015 shows that batteries of heat stress metrics cover a larger swath of societal outcomes. Furthermore, the manuscript also comes up with methods that address the dry vs wet heat through this battery of metrics. The utility of using multiple metrics also allows for broader applications, such as the interplay of infrastructure, climate change, and heat stress (Parkes et al., 2022). Additionally, Ivanovich et al., 2024 also created a new index called ‘stickiness’ that also goes into splitting heatwaves into dry and wet classifications. These manuscripts should be mentioned and discussed.

Line 59-66: WBGT and UTCI go one step further than temperature-humidity covariance, they also include radiation… as long as they are calculated correctly (Cvijanovic incorrectly calculates WBGT, even with the assumptions about “radiation free” environment). Buzan, 2024 highlights the temperature-humidity-radiation relationship.

Line 86: The reference period includes the climate events. Does this change when choosing a different reference period? Or climate change?

Line 100-106: the temporal resolution of RH is not stated, and daily maximums is not stated for temperature. I recommend making each step explicit on what data is used. I found it confusing. Buzan and Huber, 2020 and Buzan, 2024 demonstrate that changes in precision can change the outcomes.

Figure color bars: use less colors, especially with the MELT figures. The patterns should become easier to see. For example, Figures 4 and 6. The color bars here becomes important. There are a lot of sign changes for specific humidity, but it looks like the elevated specific humidity correspond with the wet heat in the MELT. I was a little confused by this. It will likely become clearer with less color steps.

Citation: https://doi.org/10.5194/egusphere-2025-1574-RC1
- CC2:
  'Reply on RC1', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC2
  - AC2: 'Reply on RC1', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC2-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC2
- AC2: 'Reply on RC1', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC2
CC1:
'Comment on egusphere-2025-1574', Kristie Ebi, 05 Jun 2025

There are more than 150 heat metrics, including many metrics based on synoptic classifications. The first heatwave early warning system was based on synoptic classifications; many others followed worldwide. Please provide a compare and contrast of how the proposed metric differs from synoptic classifications in use.
Relative humidity is associated with temperature. Explanations would be helpful as to how this was considered and why absolute humidity was not considered.
The manuscript needs to make a compelling case that a new metric provides novel insights into heatwave characterization. The manuscript also needs to provide a compelling case that the proposed metric better characterizes physiological heat stress, considering that low and high humidity affect human physiological response to exposure to high ambient temperatures.
It is not accurate to state that epidemiological studies have focused primarily on temperature. Many publications used temperature-humidity indices.
The metric was tested in three extreme heatwaves, but no criteria were provided for how those heatwaves were selected over the hundreds of other heatwaves that occurred over the past decade.
Better characterization of heatwaves does not necessarily translate into improved responses. The 2021 PNW heatdome was accurately forecast, but was so extreme that authorities did not believe it would occur as forecast.
Many low-resource settings often have limited available temperature and humidity data at the scales needed for decision-making.
The cutting-edge research on heatwave early warning and response systems is developing impact-based forecasts. It would be helpful to discuss the value of forecasts using the new metric vs. impact forecasts.
KL Ebi

Citation: https://doi.org/10.5194/egusphere-2025-1574-CC1
- CC3:
  'Reply on CC1', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC3
  - AC3: 'Reply on CC1', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC3-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC3
- AC3: 'Reply on CC1', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC3
RC2:
'Comment on egusphere-2025-1574', Tom Matthews, 05 Jun 2025
Review of Moisture ELevated Temperature (MELT) index: A novel index to capture dry and humid heatwaves

By Kwesi Twentwewa Quagraine and Kwesi Akumenyi Quagraine
This is a clearly presented and succinct paper on an interesting topic. However, in my opinion there are some major issues that must be addressed before the paper is considered for publication. I am in full agreement with concerns already raised by the other reviewers, and I provide my own major and minor issues below. The key point (relevant to all my major issues – and raised by the other reviewers) – is that the virtutes of the new humid-heat metric are not at all clear. This is very problematic because using relative humidity (RH) over an absolute measure of humidity (like the absolute or specific humidity, or the vapour pressure) is not the obvious choice from an impacts’ perspective; it also challenges interpretation of ‘dry’ and ‘moist’ heat events. The lack of context regarding how the MELT index is an improvement on other work also does not help convince of the benefits provided by the new index.
Major issues
There is a misrepresentation of the extent to which previous work has engaged with this topic. For example, please see Matthews et al. (2022) and Ivanovitch et al. (2024). The former discusses the use of equivalent temperature (and the ‘latent’ temperature) as a physical quantity to characterise humid heat; the latter presents a novel new metric to communicate the extent to which humidity (and temperature) contribute to a specific level of humid heat (e.g., a given wet-bulb or equivalent temperature, noting that equivalent temperature is (MSE-gz)/Cp). How does the manuscript advance on such work (noting that the above are not empirical metrics/tailored for regional applications)?

The MELT index might, at least in theory, miss extreme humid events due to the initial dependence on dry-bulb temperature to identify a heatwave. For example, Tmax might not be that extreme yet, because of very high RH, the wet-bulb temperature could be.

Despite the claims in the paper, I don’t think you can compare/characterise ‘humid’ and ‘dry’ heatwaves using MELT. Under the authors’ definition, a heatwave could be interpreted as dry (e.g., MELT<1 due to relatively low RH) – or at least drier than during a heatwave in another region -- even if the specific humidity was extreme due to very high dry-bulb temperature (and hence higher saturation specific humidity). The same issue is there if we compare heatwaves at a single site. To illustrate, imagine two heatwaves (a and b) in the same place with identical specific humidity. If b had higher dry-bulb temperature than a, its MELT would be lower (because saturation specific humidity, and hence RH, would decline). Yet, from a human heat stress perspective, we would expect the cooling potential via sweating to be very similar between heatwaves a and b because that depends on specific/absolute (and not relative) humidity. There are arguments for using RH rather than an absolute measure of humidity; please elaborate if this remains the choice for the MELT index.

Minor issues
Not all references appear correctly (i.e., only some are hyperlinked). Please check.

"If an event has a MELT index < 0.5 then, there is a relatively dryer [sic] heatwave with humidity lower than 50% of the climatological relative humidity"... Not quite, if I understand correctly – it’d be lower than the 47.5^th % percentile? (Because the RH is 50 % of the 95^th percentile?). Please correct or explain.

Correct 'dryer' to 'drier' (I think it only occurs in one place (above), but please check).

References
Matthews, T., Byrne, M., Horton, R., Murphy, C., Pielke Sr, R., Raymond, C., Thorne, P. and Wilby, R.L., 2022. Latent heat must be visible in climate communications. Wiley Interdisciplinary Reviews: Climate Change, 13(4), p.e779.
Ivanovich, C.C., Sobel, A.H., Horton, R.M. and Raymond, C., 2024. Stickiness: A new variable to characterize the temperature and humidity contributions toward humid heat. Journal of the Atmospheric Sciences, 81(5), pp.819-837.
Citation: https://doi.org/10.5194/egusphere-2025-1574-RC2
- CC4:
  'Reply on RC2', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC4
  - AC1: 'Reply on RC2', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC1-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC1
- AC1: 'Reply on RC2', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC1

Status: closed

RC1:
'Comment on egusphere-2025-1574', Jonathan Buzan, 03 Jun 2025

The MELT index, Moisture Elevated Temperature, is designed to evaluate potential heatwaves and diagnose dry and moist heat events. Categories range from 0 (no heat event), <0.5 (dry enhanced), to ≥1 (moist elevated) scale. The intent is for use to help meteorologists and decision makers to determine the threat to health outcomes. The manuscript then uses 3 recent heatwaves that were recently scrutinized by multiple researchers, to showcase the metric. Two of the events, the 2021 Pacific Northwest Heatwave and the 2016 South Korean heatwave were consistent with elevated moist heatwaves. The 3^rd event, the 2019 European heatwave, was identified as a dry heatwave. Overall, a straightforward synoptic scale analysis was conducted, and the metric shows promise in meteorological applications.

From the scientific objectives and demonstration, the manuscript is well written and straightforward. I appreciate the authors efforts in readability! From this standpoint, the paper is good, and some minor adjustments which I detail below would be fine.

However, there are 3 a fundamental ‘elephant-in-the-room’ aspects to the paper from an editorial point of view that perhaps should be addressed.
1) the authors discuss physiological responses to moist heat, and that their metric can identify a moist or dry heatwave. But without showing human responses to the moist or dry heatwave examples, it can be hard to determine whether their classification would correctly address the societal outcomes from these heatwaves.
2) There are 100s of heat stress metrics. The authors mention that metrics are not quite universal or are region specific. But this gets into the nuances heat stress: there are 100s of heat stress metrics because humans respond to heat in a myriad of ways. A dry heatwave can be just as deadly as a wet heatwave. Exposure, duration, activity, health status, age, etc., all interplay into heat stress. It is very difficult to generalize heat stress on humans to an individual index. Fundamentally, this is because the thermo-physiological system is complex, and there are many ‘paths’ that lead to negative outcomes, many of which are non-linear. So, I am skeptical that this index would be universally applied. What is interesting is the interplay with meteorology, which is clearly demonstrated from the authors’ synoptic analysis.
3) Lastly, there is the aspect of applying MELT to future climate change. Buzan and Huber, 2020 show that relative humidity scales negatively with global mean temperature change. This does not mean that future heatwaves would switch from moist to dry in the future. There are a lot of issues with relative humidity as a basis for a metric, and one of those is that absolute humidity increases non-linearly with temperature. Even if extreme relative humidity goes down in the future, the danger from the heatwave is enhanced due to the unusually elevated moisture.

To me, what I discuss in the 3 paragraphs above is an editorial decision on the status of the manuscript, because, as stated before, the paper is well written and is self-contained with clear scientific objectives, construction, and application. Below are the minor comments that should be addressed.
Best regards,
-Jonathan R. Buzan

Line 10: may want to remove the mentioning of accurately assessing physiological heat stress. The paper does not compare with health data. Emphasize the meteorological applications instead.

Line 42—50: Buzan et al., 2015 shows that batteries of heat stress metrics cover a larger swath of societal outcomes. Furthermore, the manuscript also comes up with methods that address the dry vs wet heat through this battery of metrics. The utility of using multiple metrics also allows for broader applications, such as the interplay of infrastructure, climate change, and heat stress (Parkes et al., 2022). Additionally, Ivanovich et al., 2024 also created a new index called ‘stickiness’ that also goes into splitting heatwaves into dry and wet classifications. These manuscripts should be mentioned and discussed.

Line 59-66: WBGT and UTCI go one step further than temperature-humidity covariance, they also include radiation… as long as they are calculated correctly (Cvijanovic incorrectly calculates WBGT, even with the assumptions about “radiation free” environment). Buzan, 2024 highlights the temperature-humidity-radiation relationship.

Line 86: The reference period includes the climate events. Does this change when choosing a different reference period? Or climate change?

Line 100-106: the temporal resolution of RH is not stated, and daily maximums is not stated for temperature. I recommend making each step explicit on what data is used. I found it confusing. Buzan and Huber, 2020 and Buzan, 2024 demonstrate that changes in precision can change the outcomes.

Figure color bars: use less colors, especially with the MELT figures. The patterns should become easier to see. For example, Figures 4 and 6. The color bars here becomes important. There are a lot of sign changes for specific humidity, but it looks like the elevated specific humidity correspond with the wet heat in the MELT. I was a little confused by this. It will likely become clearer with less color steps.

Citation: https://doi.org/10.5194/egusphere-2025-1574-RC1
- CC2:
  'Reply on RC1', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC2
  - AC2: 'Reply on RC1', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC2-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC2
- AC2: 'Reply on RC1', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC2
CC1:
'Comment on egusphere-2025-1574', Kristie Ebi, 05 Jun 2025

There are more than 150 heat metrics, including many metrics based on synoptic classifications. The first heatwave early warning system was based on synoptic classifications; many others followed worldwide. Please provide a compare and contrast of how the proposed metric differs from synoptic classifications in use.
Relative humidity is associated with temperature. Explanations would be helpful as to how this was considered and why absolute humidity was not considered.
The manuscript needs to make a compelling case that a new metric provides novel insights into heatwave characterization. The manuscript also needs to provide a compelling case that the proposed metric better characterizes physiological heat stress, considering that low and high humidity affect human physiological response to exposure to high ambient temperatures.
It is not accurate to state that epidemiological studies have focused primarily on temperature. Many publications used temperature-humidity indices.
The metric was tested in three extreme heatwaves, but no criteria were provided for how those heatwaves were selected over the hundreds of other heatwaves that occurred over the past decade.
Better characterization of heatwaves does not necessarily translate into improved responses. The 2021 PNW heatdome was accurately forecast, but was so extreme that authorities did not believe it would occur as forecast.
Many low-resource settings often have limited available temperature and humidity data at the scales needed for decision-making.
The cutting-edge research on heatwave early warning and response systems is developing impact-based forecasts. It would be helpful to discuss the value of forecasts using the new metric vs. impact forecasts.
KL Ebi

Citation: https://doi.org/10.5194/egusphere-2025-1574-CC1
- CC3:
  'Reply on CC1', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC3
  - AC3: 'Reply on CC1', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC3-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC3
- AC3: 'Reply on CC1', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC3-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC3
RC2:
'Comment on egusphere-2025-1574', Tom Matthews, 05 Jun 2025
Review of Moisture ELevated Temperature (MELT) index: A novel index to capture dry and humid heatwaves

By Kwesi Twentwewa Quagraine and Kwesi Akumenyi Quagraine
This is a clearly presented and succinct paper on an interesting topic. However, in my opinion there are some major issues that must be addressed before the paper is considered for publication. I am in full agreement with concerns already raised by the other reviewers, and I provide my own major and minor issues below. The key point (relevant to all my major issues – and raised by the other reviewers) – is that the virtutes of the new humid-heat metric are not at all clear. This is very problematic because using relative humidity (RH) over an absolute measure of humidity (like the absolute or specific humidity, or the vapour pressure) is not the obvious choice from an impacts’ perspective; it also challenges interpretation of ‘dry’ and ‘moist’ heat events. The lack of context regarding how the MELT index is an improvement on other work also does not help convince of the benefits provided by the new index.
Major issues
There is a misrepresentation of the extent to which previous work has engaged with this topic. For example, please see Matthews et al. (2022) and Ivanovitch et al. (2024). The former discusses the use of equivalent temperature (and the ‘latent’ temperature) as a physical quantity to characterise humid heat; the latter presents a novel new metric to communicate the extent to which humidity (and temperature) contribute to a specific level of humid heat (e.g., a given wet-bulb or equivalent temperature, noting that equivalent temperature is (MSE-gz)/Cp). How does the manuscript advance on such work (noting that the above are not empirical metrics/tailored for regional applications)?

The MELT index might, at least in theory, miss extreme humid events due to the initial dependence on dry-bulb temperature to identify a heatwave. For example, Tmax might not be that extreme yet, because of very high RH, the wet-bulb temperature could be.

Despite the claims in the paper, I don’t think you can compare/characterise ‘humid’ and ‘dry’ heatwaves using MELT. Under the authors’ definition, a heatwave could be interpreted as dry (e.g., MELT<1 due to relatively low RH) – or at least drier than during a heatwave in another region -- even if the specific humidity was extreme due to very high dry-bulb temperature (and hence higher saturation specific humidity). The same issue is there if we compare heatwaves at a single site. To illustrate, imagine two heatwaves (a and b) in the same place with identical specific humidity. If b had higher dry-bulb temperature than a, its MELT would be lower (because saturation specific humidity, and hence RH, would decline). Yet, from a human heat stress perspective, we would expect the cooling potential via sweating to be very similar between heatwaves a and b because that depends on specific/absolute (and not relative) humidity. There are arguments for using RH rather than an absolute measure of humidity; please elaborate if this remains the choice for the MELT index.

Minor issues
Not all references appear correctly (i.e., only some are hyperlinked). Please check.

"If an event has a MELT index < 0.5 then, there is a relatively dryer [sic] heatwave with humidity lower than 50% of the climatological relative humidity"... Not quite, if I understand correctly – it’d be lower than the 47.5^th % percentile? (Because the RH is 50 % of the 95^th percentile?). Please correct or explain.

Correct 'dryer' to 'drier' (I think it only occurs in one place (above), but please check).

References
Matthews, T., Byrne, M., Horton, R., Murphy, C., Pielke Sr, R., Raymond, C., Thorne, P. and Wilby, R.L., 2022. Latent heat must be visible in climate communications. Wiley Interdisciplinary Reviews: Climate Change, 13(4), p.e779.
Ivanovich, C.C., Sobel, A.H., Horton, R.M. and Raymond, C., 2024. Stickiness: A new variable to characterize the temperature and humidity contributions toward humid heat. Journal of the Atmospheric Sciences, 81(5), pp.819-837.
Citation: https://doi.org/10.5194/egusphere-2025-1574-RC2
- CC4:
  'Reply on RC2', Kwesi Quagraine, 10 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-CC4-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-CC4
  - AC1: 'Reply on RC2', Kwesi Quagraine, 14 Jun 2025
    
    The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC1-supplement.pdf
    
    Citation: https://doi.org/10.5194/egusphere-2025-1574-AC1
- AC1: 'Reply on RC2', Kwesi Quagraine, 14 Jun 2025
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1574/egusphere-2025-1574-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2025-1574-AC1

Kwesi Twentwewa Quagraine and Kwesi Akumenyi Quagraine

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Latest update: 09 Oct 2025

Short summary

In a changing climate, hot days and humidity are expected to rise, increasing heatwave frequency and impact. Most heatwave indices focus only on temperature or are region-specific, limiting their ability to assess human thermal comfort. To address this, we developed a novel index that integrates temperature and relative humidity—two readily available variables—to detect heatwaves. We test it on record-breaking heatwaves, evaluating its ability to assess humidity conditions.


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