Review on "Response of urban heat stress to heat waves in Athens (1960-2017)" by George Katavoutas and Dimitra Founda.

Dear authors,

thanks for submitting your work to Atmosphere. The manuscript is well written, interesting and relevant. The overall quality is very good. I therefore only have few minor comments:

General comments:
- Please decide for one notation of HD classes and keep this consistent throughout the text. either with ";" or with "-". If you decide for "-" please add some whitespaces before and after the dash.
- Please use consistent precision for all parameters (e.g. ll 169). You are switching between 0 and 1 decimal digit for Temperatures, as well as for percentages. Decide for one and use consistently!

- ll 42: missing ")" after quotes
- ll 127 / ll 128: quote HD and UTCI directly after first occurrence
- ll 143: Table 1 is not addressed in this sentence. The reference therefore should be moved or the table should be referenced in the text.
- ll 143: Missing "s" after cover.
- Table 1: Separation between HD and UTCI classes should be stronger. In the present form Table 1 e.g. implies that HD class "No discomfort" compares to UTCI class "Moderate heat stress". Maybe consider splitting into two tables?
- Equation 4: Either there is something is missing at the end of the equation or the comma, as well as the period should be removed.
- Figure 2: The figure shows absolute numbers, not frequencies. Please correct labels and caption.
- Figure 3: The years in the upper part of the figure are too small. Please consider increasing the width of the graph to get sufficient room for larger labels.
- ll 227/228: Maybe add a reference to table 1 here?
- Figure 7: The x-axis is getting in the way of your data here. Maybe consider moving the ticks and labels of the x-axis to y=-2?
- ll 351: Wrong quotation style for Kalkstein et al. 2008

Author Response

Response to comments of Reviewer #1

The authors are grateful to Reviewer 1 for his/her time devoted for this paper and the useful suggestions aiming at its improvement. All suggestions and comments were particularly helpful and were taken seriously into consideration by the authors. The following provides an item-by-item response to Reviewer 1 comments. We also attach the revised version of the paper with all the adjustments highlighted (track-changes).

Comments and Suggestions for Authors

Review on "Response of urban heat stress to heat waves in Athens (1960-2017)" by George Katavoutas and Dimitra Founda.

Dear authors,

thanks for submitting your work to Atmosphere. The manuscript is well written, interesting and relevant. The overall quality is very good. I therefore only have few minor comments:

General comments:

- Please decide for one notation of HD classes and keep this consistent throughout the text. either with ";" or with "-". If you decide for "-" please add some whitespaces before and after the dash.

Thank you for your comment and attention. We used the notations “Great discomfort – avoid exertion” and “Dangerous – heat stroke possible” throughout the text and in the Table 1.

- Please use consistent precision for all parameters (e.g. ll 169). You are switching between 0 and 1 decimal digit for Temperatures, as well as for percentages. Decide for one and use consistently!

Thank you very much for pointing out this inadvertence. We now use one decimal point for the values of air temperature, percentages, values of UTCI and HD throughout the text. The only case that we do not follow this, is when we are referring on the thresholds of indices.

- ll 42: missing ")" after quotes

Thank you for spotting this. Noted and it was revised accordingly.

- ll 127 / ll 128: quote HD and UTCI directly after first occurrence

Thank you for this comment. We have made relevant modifications.

- ll 143: Table 1 is not addressed in this sentence. The reference therefore should be moved or the table should be referenced in the text.

Thank you for spotting this. Noted and it was revised accordingly. We removed the reference to the Table (in the initial submission Table 1, in the revised version Table 2) and the citation from the sentence you pointed out and therefore, we added a new sentence describing the new Table 2 (UTCI classification) in the revised version of the manuscript.

- ll 143: Missing "s" after cover.

Thank you for your comment and attention. Noted and it was revised accordingly.

- Table 1: Separation between HD and UTCI classes should be stronger. In the present form Table 1 e.g. implies that HD class "No discomfort" compares to UTCI class "Moderate heat stress". Maybe consider splitting into two tables?

Many thanks for your suggestion. We split the initial Table 1 into two separate tables in order to avoid causing confusion to the readers. In the revised version of the manuscript, Table 1 presents the HD classification and Table 2 shows the classification of UTCI. The necessary amendments have been made to the text as well as to the captions of Figure 4 and Figure 5.

- Equation 4: Either there is something is missing at the end of the equation or the comma, as well as the period should be removed.

We thank the reviewer for this comment. Hoping that we follow exactly this comment, we would like to note the following. At the end of all equations (4 in total) there is the punctuation mark of comma (,). We put the comma mark after the end of an equation following the template of the Journal. You could verify this if you download the Microsoft word template (“atmosphere-template.dot”) from the website of Journal in the section “Instruction for authors”. We also checked all equations and we did not find anything that is missing at the end of all equations. As far as your comment about the “period” is concerned, we assume that with the word “period” you are meaning the punctuation mark of “dot” or “full stop”. Only in equation 3 there is such a symbol indicating the mathematical symbol of multiplication and thus, we deleted this symbol from equation 3.

- Figure 2: The figure shows absolute numbers, not frequencies. Please correct labels and caption.

Thank you very much for pointing this out. We have changed “Frequency of HWs” to “Number of HWs” in the label of y-axis in Figure 2. The caption of Figure 2 was revised accordingly.

- Figure 3: The years in the upper part of the figure are too small. Please consider increasing the width of the graph to get sufficient room for larger labels.

Thank you for the suggestion. We have increased the width of Figure 3 from 11.5 cm (initial submission) to 13 cm (revised version of the manuscript).

- ll 227/228: Maybe add a reference to table 1 here?

Thank you for the suggestion. This information was added in the sentence you pointed out. In addition, following the meaning of your suggestion, we also added a reference of the new Table 2 (UTCI classification) in the next paragraph when we are commenting on UTCI results.

- Figure 7: The x-axis is getting in the way of your data here. Maybe consider moving the ticks and labels of the x-axis to y=-2?

Thank you for pointing this out. Noted and it was revised accordingly. We put the labels of time at a lower level. The labels of time on x-axis do not hide the plots anymore.

- ll 351: Wrong quotation style for Kalkstein et al. 2008

Many thanks for spotting this. Noted and it was revised accordingly.

We hope that you would find the revised version has been significantly improved and your comments have been adequately addressed.

Yours sincerely,

The Authors

Reviewer 2 Report

Dear Authors,

I read very carefully this paper devoted to the analysis of heat waves occurred in Athens during the last 60 years. I found the topic very interesting in terms of potential effects on the wellbeing of people to reduce thermal risks factors in an era of climate change.

The manuscript is well structured, based upon a robust methodology and I think it could provide interesting information in the field (several Mediterranean urban areas exhibit climatic conditions similar to those of Athens). Notwithstanding this, an in-depth analysis of the manuscript reveals some criticalities on which you should work a little.

I am confident that my comments will be useful to revise the present version of the paper and I will be glad to be invited as reviewer again.

Here below observations.

Best regards

1. Introduction

L44-L45. It would be necessary stressing that in literature hundred and hundreds of bioclimatic indices have been formulated in the past, but only few of them are formulated on rational and physiological basis accounting for the minimum dataset of input variables necessary for a reliable assessment. This has been stressed in:

· C. R. de Freitas, E. A. Grigorieva. (2015) A comprehensive catalogue and classification of human thermal climate indices. Int J Biometeorol 59:109–120 DOI 10.1007/s00484-014-0819-3.

· d’Ambrosio Alfano F.R., Palella B.I., Riccio G. (2011). Thermal Environment Assessment Reliability Using Temperature-Humidity Indices. Industrial Health, vol 49(1), pp. 95-106 ISSN: 0019- 8366, doi:10.2486/indhealth.MS1097.

2. Material and methods

L 125-128. All limitations of temperature humidity indices have to be stressed. In particular: a) HD is an empirical index; b) its limit values have been calibrated for people living in temperate region of Canada; c) does not account for mean radiant temperature and air velocity (e.g. neither radiation nor convection are considered); d) it is not suitable for assessing heat stress neither indoor nor outdoor as stressed in d’Ambrosio Alfano F.R. et al. (2011). In the field of the ergonomics of thermal environments the word “stress index” should only be used thermal indices used for the assessment of working condition being reliable and validated on thousand and thousand experimental data and standardized worldwide. This is the case of WBGT (screening index as claimed by ISO 7243) and PHS (more robust method which allows the evaluation of DLE). This is the reason why you should use the term “bioclimatic index” instead of “stress or comfort index”.

Please observe that WBGT is being considered a potential tool for the assessment of regional risk maps as stressed by Hyatt et al. (2010).

Useful references:

o ISO 7243 (2017) Ergonomics of the thermal environment – Assessment of heat stress using the WBGT (wet bulb globe temperature) index

o d'Ambrosio Alfano F.R., Palella B.I., Riccio G., Malchaire J., (2016). On the Effect of Thermophysical Properties of Clothing on the Heat Strain Predicted by PHS Model. Annals of Occupational Hygiene, vol. 60(2), pp. 231-251. ISSN:0003-4878, doi: 10.1093/annhyg/mev070.

o ISO 7933 (2004) Ergonomics of the thermal environment – Analytical determination and interpretation of heat stress using calculation of the predicted heat strain. International Organization for Standardization,Geneva.

o Olivia M. Hyatt, Bruno Lemke and Tord Kjellstrom, Regional maps of occupational heat exposure: past, present, and potential future, Glob Health Action. 2010; 3: doi 10.3402/gha.v3i0.5715.

L148. As all bioclimatic indices, UTCI does not consider metabolic rate as input value, being calibrated on a typical metabolic rate of 2.2 met (an activity level corresponding to a walk).

3. Results and Conclusions

It should be stress that all limitations affecting the analysis by means of HD are mainly related to the inability to account for radiative and convective phenomena (and also to limit values thought for Canada). In addition it should be useful (in a future work) a comparison WBGT/UTCI: this is because WBGT is well known and accepted by the whole community of Occupational Health experts.

Author Response

Response to comments of Reviewer #2

The authors are grateful to Reviewer 2 for his/her time devoted for this paper and the useful suggestions aiming at its improvement. All suggestions and comments were particularly helpful and were taken seriously into consideration by the authors. The following provides an item-by-item response to Reviewer 2 comments. We also attach the revised version of the paper with all the adjustments highlighted (track-changes).

Comments and Suggestions for Authors

Dear Authors,

I am confident that my comments will be useful to revise the present version of the paper and I will be glad to be invited as reviewer again.

Here below observations.

Best regards

1. Introduction

· C. R. de Freitas, E. A. Grigorieva. (2015) A comprehensive catalogue and classification of human thermal climate indices. Int J Biometeorol 59:109–120 DOI 10.1007/s00484-014-0819-3.

We thank the reviewer for this comment. We have added an explanatory paragraph in the “Introduction” chapter discussing the issues you pointed out. The new paragraph is: “To date, a plethora of bioclimatic indices have been developed worldwide aiming to assess the human thermal environment [20-22]. The type, quality and formulation of all developed indices vary considerably. Their differences refer to the number or type of variables taken into consideration, the mechanisms of heat exchange between the human body and the atmosphere as well as the rationale of the thermo-physiological processes on the human body, but are not limited to these [20-22]. Most of the existing indices are simple two-parameters indices, neglecting significant energy fluxes between the human body and the thermal environment. Only few indices are formulated on rational and physiological basis taking into account the minimum dataset of input variables for a reliable appraisal [20-22]”.

2. Material and methods

Please observe that WBGT is being considered a potential tool for the assessment of regional risk maps as stressed by Hyatt et al. (2010).

Useful references:

o ISO 7243 (2017) Ergonomics of the thermal environment – Assessment of heat stress using the WBGT (wet bulb globe temperature) index

o Olivia M. Hyatt, Bruno Lemke and Tord Kjellstrom, Regional maps of occupational heat exposure: past, present, and potential future, Glob Health Action. 2010; 3: doi 10.3402/gha.v3i0.5715.

We thank the reviewer for this comment and for the feedback. The limitations of HD were further stressed in several parts of the sub-chapter “Heat stress assessment” following the suggestions. Moreover, the term ‘bioclimatic indices’ has been now used in our study.

L148. As all bioclimatic indices, UTCI does not consider metabolic rate as input value, being calibrated on a typical metabolic rate of 2.2 met (an activity level corresponding to a walk).

Thank you for your comment. This sentence was now rephrased in order not to be misleading. The new sentence is: “The adaptive clothing model [48] integrated in UTCI-Fiala model [47] determines the clothing insulation, while the metabolic rate is set at 135 Wm^-2 (2.3 met) and the walking speed at 1.1 ms^-1 [50]”.

3. Results and Conclusions

We thank the reviewer for this comment. We fully agree that HD subjects to several limitations and this is further highlighted in the revised version. In the course of the revision and combining answers to all reviewers’ comments, we have added an extensive paragraph at the end of the ‘Discussion & Conclusions’ session in which –in addition to other information- more information on the performance and limitations of HD are is also reported.

We also thank the reviewer for the suggestion about the comparison between UTCI and WBGT which looks very interesting and we hope to be able to make a relevant study in the near future.

We hope that you would find the revised version has been significantly improved and your comments have been adequately addressed.

Yours sincerely,

The Authors

Reviewer 3 Report

This work studies the frequency and severity of heatwaves in Athens, Greece. It has attempted to evaluate the effect of heatwaves using two well-known indices, namely, the UTCI and heat-humidity index. There are also further comparisons between an urban and a rural weather station. Overall a number of results are presented which can be simplified to:

1) HWs are becoming more frequent

2) Recent HWs tend to be more intense.

3) HWs rank significantly hotter than similar periods out of the HW periods

4) There are clear differences between diurnal cycles for HW and non HW events (people are likely to be under greater heat stress for more of the time).

5) The diurnal cycle depends on the metric used to assess its severity.

6) There are clear differences between the rural and urban locations.

Unfortunately the novelty of any of these findings is not particularly high nor are they surprising.

The observed weather has been processed into indices which are related to potential heat stress but that is as far as it has gone. While there is no methodological weakness to the work as presented it doesn’t provide much insight into the nature of hot events in Athens beyond what is relatively well-known. For example this study assumes that the UTCI and HD capture the heat stress in Athens, it would be more interesting to know the extent to which that is true for people living in Athens.

Overall, I have limited comments about the manuscript itself (it is very well written). The aims of the work are described well, and these have been achieved. It just doesn’t go far enough in terms of questioning the extent to which the metrics are appropriate, or interpreting what the results mean. The most interesting is that HWs are one level (in the heat stress metrics) higher than non HW events, but again the actual significance is lacking - can we observe such differences in practice for example? Do these trends exist at other locations or are these unique to Athens?

Author Response

Response to comments of Reviewer #3

The authors are grateful to Reviewer 3 for his/her time devoted for this paper and the useful suggestions aiming at its improvement. All suggestions and comments were particularly helpful and were taken seriously into consideration by the authors. The following provides an item-by-item response to Reviewer 3 comments. We also attach the revised version of the paper with all the adjustments highlighted (track-changes).

Comments and Suggestions for Authors

1) HWs are becoming more frequent

2) Recent HWs tend to be more intense.

3) HWs rank significantly hotter than similar periods out of the HW periods

4) There are clear differences between diurnal cycles for HW and non HW events (people are likely to be under greater heat stress for more of the time).

5) The diurnal cycle depends on the metric used to assess its severity.

6) There are clear differences between the rural and urban locations.

Unfortunately the novelty of any of these findings is not particularly high nor are they surprising.

We thank the reviewer for the aforementioned comments and the suggestions. Hoping that we follow exactly these comments and suggestions, we would like to note the following.

The authors added a whole paragraph in the “Discussion and Conclusions” chapter in order to discuss the performance of the used thermal indices (UTCI, HD) with the actual human thermal responses. Please see the penultimate paragraph in the chapter “Discussion and Conclusions”. The new paragraph is: “A crucial question that arises when bioclimatic indices are applied is if these metrics can efficiently capture and imprint the thermal stress in different thermal conditions compared to the actual human thermal responses. Indices have been designed aiming to achieve this, from the simpler to the more comprehensive indices, regardless of whether they achieve it and to what extent. In addition, even the rational indices which take into consideration the mechanisms of heat exchange and the thermo-physiological process on the human body as well as behavioral factors do not take into account psychological and expectations factors. The latter factors seem to affect actual thermal responses of people, playing an important role in thermal comfort [65,66]. Nevertheless, recent studies have shown that UTCI satisfactorily predicts the outdoor thermal comfort which is based on questionnaires in China [67], in Hong Kong [68] and in Brazil [69]. In the city of Athens, as the area of interest in this research, the surveys of Pantavou et al. [70,71] have shown strong association between the predicted classes of UTCI and the thermal sensation votes, while UTCI presents the best applicability in terms of predictions and thermal sensation votes among 54 indices. In addition, the thermal sensation votes are predicted fairly well by the UTCI in the heat stress categories of its assessment scale and the UTCI is the index which best simulates the thermal sensation votes among the studied indices [70,71]. Surprisingly, the simple HD index, which is also employed in our study, predicts relatively well the actual human thermal responses and it is ranked first among indices designed for warm weather conditions [70]. Nevertheless, the shortcomings in HD formulation are mainly related to the inability to account for convective and radiative phenomena and the fact that it is initially designed for temperate climates [22]”.

Moreover, the authors also added a section in the third paragraph of “Discussion and Conclusions” chapter to note and emphasize that other cities exhibit an increase in HWs the recent decades. The new section is: “Recent research studies have shown a significant increase in both the number of HW days and the number of long HWs in other Mediterranean or eastern Europe cities like Madrid, Palma, Nicosia, Rome, Sofia, Zagreb and Ljubljana [19,60,61]”.

Finally, regarding the novelty of our research we would like to note that although the studies on HWs or heat stress metrics are now numerous worldwide, only limited studies focus on the different response of urban and non-urban sites to HWs. Even these studies analyze the interaction between HWs and UHI intensity (e.g. Founda and Santamouris 2017/ https://doi.org/10.1038/s41598-017-11407-6; Li et al. 2015 / https://doi.org/10.1088/1748-9326/10/5/054009; Zhao et al. 2018 / https://doi.org/10.1088/1748-9326/aa9f73) while – to our knowledge – the different response of urban and non-urban sites to severe HWs based on heat stress metrics (and not UHI intensity) has not been analyzed so far. We believe that this kind of analysis (showing also consistent and robust results between HD and UTCI) adds novel aspects in our research.

We hope that you would find the revised version has been significantly improved and your comments have been adequately addressed.

Yours sincerely,

The Authors

Round 2

Reviewer 3 Report

The document has been improved. But there are still some issues.

Your comments in your response with regards to the novelty make much more sense than what is currently written in the main document. . I (and any reader) would want you to state this upfront. With it the work has more purpose and is likely to have greater interest. Most of the results still have low novelty as they are as expected but they quantify the effects which you would expect to see. As I said before I found the difference in heat stress level for Athens between HW and not HW to be the most interesting. I still think this work lacks some further depth rather than just reporting the differences. Perhaps relating back to the average heat island intensity (the standard metric reported by similar studies) could be a simple way of showing the impact.

The extra paragraph from line 378 in the discussion and conclusion doesn't add much as it doesn't tie in well to the points made above. say something like 'this is similar to recent studies where...'

The paragraph around 424 is necessary and does provide justification around the UTCI. The only factor missing is the idea of adaptation to conditions (the same conditions in a temperate climate would be even more severe) but reference 70 covers this to some extent.

On a technical note. you can not state percentage difference in metrics that are on an arbitrary temperature scale (line 288 for example) as it doesn't make any physical sense. A temperature of 2C is 100% increase over 1C but this isn't a significant difference. Just report the absolute change.

Author Response

Response to comments of Reviewer #3

We thank the reviewer for these comments and the additional time he/she devoted to our manuscript. The following provides an item-by-item response to Reviewer 3 comments. We also attach the revised version of the paper with all the adjustments highlighted (track-changes).

The document has been improved. But there are still some issues.

We thank the reviewer for this comment. We agree that this specific analysis and aim of the study should have been more emphasized in the manuscript and stated upfront. Thus, we added relevant text in the Introduction as follows:

‘Moreover, the study explores the different response of heat stress to HWs between urban and non-urban sites. Although the studies on HWs or heat stress metrics are numerous worldwide, only limited studies focus on the different response of urban and non-urban sites to HWs. However, even these studies are focused on the interaction between HWs and the magnitude of UHI [24-29] while, to our knowledge, the different response of urban and non-urban sites to severe HWs based on heat stress metrics (and not UHI intensity) has not been analyzed so far’.

We also included some more comments with regard to similar findings in other studies based on the standard metric of UHI intensity. A direct quantitative comparison is not feasible since bioclimatic indices involve additional meteorological parameters and not only air temperature used in UHI metrics. In this respect, the contrast of thermal discomfort (and not only air temperature) between urban and non-urban sites during HWs is better captured through the use of bioclimatic indices.

We have added the following text in the end of the Results section (sub-chapter 3.3):

‘….This is not surprising given the fact that severe HWs are commonly associated with large scale atmospheric anomalies and persistent anticyclonic conditions that predominate over more local or mesoscale phenomena like UHIs and this is particularly the case for the extreme HW of 2007 in Greece [34,59]. Although bioclimatic indices involve additional atmospheric variables apart from the air temperature, the observed findings are in qualitative agreement with relevant studies based on the standard metric of UHI intensity. For example, exceptionally hot conditions in Athens have been found to exacerbate significantly the nocturnal UHI intensity (based on the daily minimum air temperature) in contrast to the daytime UHI intensity (based on the daily maximum air temperature) which remained almost constant or even declined with respect to normal summer conditions [59]. Other relevant studies also report stronger synergistic interactions between UHIs and HWs during nighttime, possibly related to increased heat storage and anthropogenic heat release (use of cooling systems) at the urban versus non-urban areas [24,25,27]. Other key contributors for the interaction between UHIs and HWs concern different evaporating rates and unsymmetrical changes in latent and sensible heat fluxes between urban and non-urban sites [24-26]. Advective phenomena and local circulations like sea breezes along with increased evaporation rates at coastal non-urban stations have been also found to cause exacerbation of UHI magnitude during HWs, which is more pronounced in daytime hours [26]’.

The extra paragraph from line 378 in the discussion and conclusion doesn't add much as it doesn't tie in well to the points made above. say something like 'this is similar to recent studies where...'

We thank the reviewer for this comment and we fully agree with the suggestion. We have revised the text accordingly, using the following: ‘These findings for Athens are similar with the results reported in other Mediterranean or eastern Europe cities. For instance, a significant increase in both the number of HW days and the number of long HWs have been recently reported in Madrid, Palma, Nicosia, Rome, Sofia, Zagreb and Ljubljana [19,61,62]’.

We thank the reviewer for this comment. Adaptation to conditions is very important as well as suggested by numerous recent studies. We added a relevant sentence in the text as following: ‘It is also worthy to add at this point, that adaptation factor is also important in the perception of the thermal environment and thus, the impact of the same weather conditions in a different temperate climate could possibly be more severe [66,73-75]’.

We thank the reviewer for this comment and we fully agree with the suggestion. We have made appropriate adjustments in the subchapter 3.2 and in the chapter “Discussion and Conclusions”.

We hope that you would find the revised version has been significantly improved and your comments have been adequately addressed.

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Response of Urban Heat Stress to Heat Waves in Athens (1960–2017)

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