Heat and cold — extreme heat

Tropical nights

Definition

The tropical nights index is defined as the annual number of days with a minimum night temperature of at least 20 °C. The minimum temperature threshold adopted is considered suitable for the pan-European perspective. However, other thresholds could be used depending on the local climate.

Index factsheet (ETC/CCA Technical Paper): Tropical nights

Relevance

The index is relevant mainly for the health sector. The index’s simple definition means that it is easy to apply and interpret.

Past and projected changes

The number of tropical nights has increased throughout Europe  since the 1980s, but with significant regional variation. Further increases are projected to be largest in southern Europe, where up to 100 tropical nights per year could occur by the end of the century under the high-emissions scenario (representative concentration pathway (RCP)8.5).

Note that the climate models underlying the projections described here do not account for urban heat island effects, which can increase night-time temperatures in urban regions substantially more than their surroundings.

Further information (European Climate Data Explorer): Tropical nights, 2011-2099 

Hot days

Definition

The hot days index represents the total number of days in a year in which maximum daily temperatures above a fixed threshold are registered. Here, 30 °C is considered a suitable threshold for the pan-European level, but higher thresholds can be considered depending on regional climate conditions.

Index factsheet (ETC/CCA Technical Paper): Hot days

Relevance

The index can easily be interpreted and used for a wide range of applications and monitoring purposes, such as in health or agriculture. The absolute threshold used here is considered suitable for Europe, but it could be adapted based on the specific application and local climate features.

Past and projected changes

Hot days have increased in Europe since the 1980s, and this increase is expected to continue. The number of hot days is expected to double and almost quadruple during the 21st century under the medium- and high-emissions scenarios (RCP4.5 and RCP8.5), respectively, with more limited increases expected under the low-emissions scenario (RCP2.6). The highest occurrence of hot days and the largest absolute increase are projected for southern Europe under all scenarios, whereas the largest relative increases are projected for central Europe.

Warmest three-day mean temperature

Definition

The warmest three-day mean temperature index is based on the highest daily mean temperature in a year averaged over a three-day window. In variations of this index, the length of the time window over which the moving average of temperature is computed could vary depending on the specific application.

Index factsheet (ETC/CCA Technical Paper): Warmest three-day period

Relevance

The index measures heatwave intensity by using a simple definition, which enables the easy interpretation of changes in heat-related hazards for various applications, especially in health and agriculture.

Past and projected changes

The warmest three-day mean temperature has increased throughout Europe since the 1980s, and this increase is projected to continue. It is projected to increase by between 1.5 °C, for the low-emissions scenario (representative concentration pathway (RCP)2.6), and around 6.5 °C, for the high-emissions scenario (RCP8.5), which is a larger increase than that projected for annual mean temperature. The warmest three-day mean temperature may reach more than 35 °C in some areas of southern Europe during this century.

Annual warmest three-day mean temperature for European land area and sub-regions

Notes:   The black lines show the annual values for 1950-2020 from reanalysis data, and the dashed horizontal lines show the means for 1986-2005. Solid grey, blue, yellow and red lines represent the ensemble medians of model simulations for the historical period and under low-, medium- and high-emissions scenarios (RCP2.6, RCP4.5 and RCP8.5, respectively) (smoothed by a 20-year moving average). Shaded areas show the 15th and 85th percentile ranges of the model ensembles. The size of the model ensemble used for each scenario is shown by the coloured numbers in the top-right corner.

Source:  ERA5 and bias-adjusted CMIP5 data.

Current situation and projected changes in annual warmest three-day mean temperature in Europe

Heatwave days based on apparent temperature

Definition

This index reports the total number of heatwave days per year. Heatwaves are defined here as periods of at least two consecutive days during summer (June to August) with a maximum apparent temperature and a minimum air temperature exceeding a local reference threshold derived from the 90th percentile in a particular region and at a particular time of year. The apparent temperature is the temperature perceived by humans and combines the effects of air temperature and relative humidity. This heatwave definition permits the comparison of changes in unusually warm conditions at different locations, independently of their local climates. Alternative heatwave definitions have been developed at the national and sub-national levels that reflect regional climate conditions more accurately.

Index factsheet (ETC/CCA Technical Paper): Heatwave days based on apparent temperature

Relevance

This index is most relevant for the health sector. The inclusion of relative humidity allows the detrimental effects of humid heatwaves, during which the effectiveness of transpiration is limited, on human health to be accounted for. This index is more informative for health-specific applications than simpler indices (e.g. the warmest three-day mean temperature index; see above), but its interpretation can be more difficult because of its more complex definition.

Past and projected changes

The duration of extreme humid heat conditions is projected to increase substantially across Europe under all emissions scenarios considered. Changes are projected to be largest over southern Europe, where the number of days with extreme humid heat conditions could increase by more than 50 days per year during the 21st century under the high-emissions scenario (representative concentration pathway (RCP)8.5).

Further information (ECDE): Health heatwave (high temperature and humidity), 1971-2099

Climatological heatwave days 

Definition

The climatological heatwave days index counts the number of days in a year within prolonged periods of unusually high temperatures. In contrast to the heatwave days based on apparent temperature index (see above), this index considers only maximum daily temperature, not humidity. A heatwave is defined here as a period of at least three consecutive days when the daily maximum temperature exceeds the 99th percentile of daily temperature values from May to September in a given location over the reference climate period. The percentile-based definition permits the assessment and comparison of changes in unusually warm days at different locations, independently of the baseline climate.

Index factsheet (ETC/CCA Technical Paper): Climatological heatwave days

Relevance

This index is widely applicable in the health and energy sectors. The somewhat complex definition involving percentile-based thresholds makes the index more informative but also more difficult to interpret than simpler heat-related indices (e.g. the warmest three-day mean temperature index).

Past and projected changes

Prolonged episodes of extreme heat conditions show an increasing trend throughout Europe, but with significant variations depending on latitude. The largest increases are projected for southern Europe, where the number of climatological heatwave days could increase from about 1 to more than 30 per year by the end of the century under the high-emissions scenario (representative concentration pathway (RCP)8.5).

Further information (European Climate Data Explorer): Climatological heatwave (high temperature), 1971-2099 

Days with Universal Thermal Climate Index (UTCI) above a threshold

Definition

This index accounts for the total number of days in a year with exceptional heat-stress conditions measured by means of the Universal Thermal Climate Index (UTCI). The UTCI describes how the human body experiences atmospheric conditions and was developed as a universal heat-related health risk index for Europe. It is computed from air temperature, humidity, wind speed and solar radiation. The proposed threshold of a maximum daily UTCI value above 32 °C, which corresponds to ‘strong heat stress’, is considered suitable for a pan-European perspective, but other thresholds could be considered depending on the local climate.

Index factsheet (ETC/CCA Technical Paper): Days with UTCI above a threshold

Relevance

The index can be used for health-related applications considering human thermal stress and discomfort in outdoor conditions. The UTCI, from which this index is computed, is recognised as one of the most informative bioclimatic indicators. However, the complex definition and computation of the UTCI currently hinder the calculation of this index for future projections.

Past and projected changes

UTCI values during summer months have increased across Europe in recent decades. The largest increase has generally occurred in southern Europe, but large interannual variability has prohibited the identification of a robust spatial pattern. Robust future projections based on the UTCI over Europe are not currently available.

Further information (European Climate Data Explorer): Thermal comfort indices — Universal Thermal Climate Index, 1979-2019