carries merit where the index is applied to health
sectors, as the human physiology has evolved to
cope with this historical temperature. However,
when applied to other sectors, such as infrastructure, a historical measure may be less meaningful
such as when considering material properties, for
example.
Indices based on absolute thresholds are identified as being not suitable in some regions [16];
however, absolute thresholds are sometimes necessary when considering the impacts of heatwaves
[16] and can be easier for policy makers and the
general public to interpret [31]. Some indices
focus on summer season thresholds (such as the
HWA90), annual thresholds (such as the EHF and
HWN90) (as discussed in Perkins and Alexander
[16]), or calendar-based thresholds (TX90pct) to
account for heatwaves occurring at different
times. Existing research has not discussed the
benefits or drawbacks of the application of these
different classes of thresholds to the performance
of the indices.
A suggested reason for the difference between
the findings on the impact of heatwaves on human
health may be due to the selection of indices
chosen to study the impacts. EEHD and EHF
indices used by Trigo et al. [32] and Scalley
et al. [22], respectively, measure the intensity of
the heatwave events, as compared to those used by
Tong et al. [18], which only determine temperatures on days are, or are not, “heatwave” days. It is
stipulated that the intensity then of heatwave
events is important and not just the heatwave/not
heatwave condition. This comparison highlights
the complexity and importance of selecting an
appropriate heatwave index.
Karl and Knight [68] identify that while urbanization tends to increase temperature, it also tends
to decrease moisture supply and thus possibly
reducing T a (a humidity-adjusted temperature
measure), shown in Table 1. This reflects the
complexity in selecting indices and incorporating
required information when evaluating heatwave
impacts in a changing environment.
Perkins and Alexander [16] acknowledge the
inability of many (or all) indices to capture the
range of heatwave characteristics and attributes
properly. This, paired with the diversity of sectors
in which heatwave analysis must be undertaken,
leads to the admission that there is likely no “ultimate” index which is globally applicable for the
assessment of heatwave impacts. As an alternative, it is suggested that a “toolbox” of indices is
created, with the indices which have been verified
for individual sectors noted. The use of different
hazard-characterizing variables (here indices) for
the creation of vulnerability curves was coherent
with the analysis of natural disaster risk in the
alpine setting presented by Papathoma-Köhle
et al. [14].
Considerations must be made for the availability and practicality of heatwave indices which rely
on specific data (such as the PMV or PET)
[16]. Apparent temperature is identified as one
index which balances data requirements and specificity of index results well [16].
It is important to be aware that caution needs to
be taken when analyzing return periods for studied heatwaves. Due to the complexity of the
nature in which heatwaves propagate, estimates
in the form of “1 in N year” events may assume
climate and urban structure stationarity while also
failing to miss vital heatwave drivers [2].
Future Directions
The occurrence of heatwaves is of concern to a
wide range of sectors. Significant research has
been undertaken in studying the impacts of
heatwaves on health. In accordance with this,
many indices which have been developed to measure heatwaves are, directly or indirectly, centered
around the human condition. As heatwaves
worsen in duration and severity, impacts will
increasingly be felt in non-health domains, such
as infrastructure and electricity. The applicability
of existing heatwave indices in monitoring and
predicting impacts on these sectors is of question,
and developing a standardized approach to evaluating heatwave impacts is vital for planning and
responding to future extreme temperature events.
In addition to the application of heatwave indices to non-health domains (whether through creation of new indices or modification/application
of existing ones), the creation of a standardized
472
Measuring Heatwaves and Their Impacts
sectors, as the human physiology has evolved to
cope with this historical temperature. However,
when applied to other sectors, such as infrastructure, a historical measure may be less meaningful
such as when considering material properties, for
example.
Indices based on absolute thresholds are identified as being not suitable in some regions [16];
however, absolute thresholds are sometimes necessary when considering the impacts of heatwaves
[16] and can be easier for policy makers and the
general public to interpret [31]. Some indices
focus on summer season thresholds (such as the
HWA90), annual thresholds (such as the EHF and
HWN90) (as discussed in Perkins and Alexander
[16]), or calendar-based thresholds (TX90pct) to
account for heatwaves occurring at different
times. Existing research has not discussed the
benefits or drawbacks of the application of these
different classes of thresholds to the performance
of the indices.
A suggested reason for the difference between
the findings on the impact of heatwaves on human
health may be due to the selection of indices
chosen to study the impacts. EEHD and EHF
indices used by Trigo et al. [32] and Scalley
et al. [22], respectively, measure the intensity of
the heatwave events, as compared to those used by
Tong et al. [18], which only determine temperatures on days are, or are not, “heatwave” days. It is
stipulated that the intensity then of heatwave
events is important and not just the heatwave/not
heatwave condition. This comparison highlights
the complexity and importance of selecting an
appropriate heatwave index.
Karl and Knight [68] identify that while urbanization tends to increase temperature, it also tends
to decrease moisture supply and thus possibly
reducing T a (a humidity-adjusted temperature
measure), shown in Table 1. This reflects the
complexity in selecting indices and incorporating
required information when evaluating heatwave
impacts in a changing environment.
Perkins and Alexander [16] acknowledge the
inability of many (or all) indices to capture the
range of heatwave characteristics and attributes
properly. This, paired with the diversity of sectors
in which heatwave analysis must be undertaken,
leads to the admission that there is likely no “ultimate” index which is globally applicable for the
assessment of heatwave impacts. As an alternative, it is suggested that a “toolbox” of indices is
created, with the indices which have been verified
for individual sectors noted. The use of different
hazard-characterizing variables (here indices) for
the creation of vulnerability curves was coherent
with the analysis of natural disaster risk in the
alpine setting presented by Papathoma-Köhle
et al. [14].
Considerations must be made for the availability and practicality of heatwave indices which rely
on specific data (such as the PMV or PET)
[16]. Apparent temperature is identified as one
index which balances data requirements and specificity of index results well [16].
It is important to be aware that caution needs to
be taken when analyzing return periods for studied heatwaves. Due to the complexity of the
nature in which heatwaves propagate, estimates
in the form of “1 in N year” events may assume
climate and urban structure stationarity while also
failing to miss vital heatwave drivers [2].
Future Directions
The occurrence of heatwaves is of concern to a
wide range of sectors. Significant research has
been undertaken in studying the impacts of
heatwaves on health. In accordance with this,
many indices which have been developed to measure heatwaves are, directly or indirectly, centered
around the human condition. As heatwaves
worsen in duration and severity, impacts will
increasingly be felt in non-health domains, such
as infrastructure and electricity. The applicability
of existing heatwave indices in monitoring and
predicting impacts on these sectors is of question,
and developing a standardized approach to evaluating heatwave impacts is vital for planning and
responding to future extreme temperature events.
In addition to the application of heatwave indices to non-health domains (whether through creation of new indices or modification/application
of existing ones), the creation of a standardized
472
Measuring Heatwaves and Their Impacts