SUMMER DAYS

    Descrizione 1
    Update date
    Authors

    Piero Fraschetti, Francesca Lena, Walter Perconti, Emanuela Piervitali, Giulio Settanta

    Abstract
    Immagine
    Abstract

    The indicator describes the trend in extreme heat events in Italy, specifically representing the number of days with a maximum daily air temperature exceeding 25 °C. In 2023, an increase of approximately 19 summer days was observed compared to the climatological average for the 1991–2020 reference period.

    Description

    The occurrence of extreme temperature events and the presence of significant trends are assessed through the analysis of absolute minimum and maximum daily air temperature values. Specifically, the indicator “summer days”, as defined by the CCL/CLIVAR Working Group on Climate Change Detection, refers to the number of days when the daily maximum air temperature exceeds 25 °C.

    Purpose

    The annual time series of the average number of summer days, expressed as the deviation from a climatological baseline, allows for the estimation of the frequency of extreme heat events and the assessment of any significant trends over time.

    Policy relevance and utility for users
    It is of national scope or it is applicable to environmental issues at the regional level but of national relevance.
    It can describe the trend without necessarily evaluating it.
    It is simple and easy to interpret.
    It provides a representative picture of environmental conditions, environmental pressures, and societal responses
    It provides a basis for international comparisons.
    Analytical soundness
    Be based on international standards and international consensus about its validity;
    Be theoretically well founded in technical and scientific terms
    Presents reliability and validity of measurement and data collection methods
    Temporal comparability
    Spatial comparability
    Measurability (data)
    Adequately documented and of known quality
    Updated at regular intervals in accordance with reliable procedures
    Readily available or made available at a reasonable cost/benefit ratio
    An “adequate” spatial coverage
    An “appropriate” temporal coverage
    Main regulatory references and objectives

    The indicator is not directly linked to any regulatory or legislative framework.

    DPSIR
    State
    Impact
    Indicator type
    Descriptive (A)
    References

    https://scia.isprambiente.it; 

    APAT-OMS, 2007, Cambiamenti climatici ed eventi estremi: rischi per la salute in Italia;

    ISPRA, 2022, I normali climatici 1991-2020 di temperatura e precipitazione in Italia;

    ISPRA, 2018, Variazioni della temperatura in Italia: estensione della base dati e aggiornamento della metodologia di calcolo; 

    ISPRA, 2016, Controlli di qualità delle serie di temperatura e precipitazione; ISPRA, 2015, Valori climatici normali di temperatura e precipitazione in Italia; 

    ISPRA, 2014, Focus on "Le città e la sfida ai cambiamenti climatici"; 

    ISPRA, 2013, Variazioni e tendenze degli estremi di temperatura e precipitazione in Italia; 

    ISPRA, 2012, Elaborazione delle serie temporali per la stima delle tendenze climatiche; 

    Peterson T.C ., Folland C , Gruza G, Hogg W, Mokssit A e Plummer N., 2001, Report on the activities of the Working Group on Climate Change Detection and Related Rapporteurs 1998-2001. World Meteorological Organization, Rep. WC DMP-47, WMO -TD 1071, Geneva, Switzerland, 143 pp.; 

    SNPA, 2024, Il clima in Italia nel 2023; 

    Toreti A. e Desiato F., 2007, Changes in temperature extremes over Italy in the last 44 years, Int. J. Climatology, DO I 10.1002/joc.1576.

     

    Further actions

    Improvements in the estimation of summer days in Italy could result from extending the indicator calculation system to include additional sources of raw temperature data, with the objective of increasing the availability of historical temperature series that meet the requirements of temporal length, continuity, and completeness.

    Frequenza di rilevazione dei dati
    Annuale
    Fonte dei dati
    ISPRA
    Data availabilty

    SCIA – Sistema nazionale per l’elaborazione e diffusione di dati climatici (https://scia.isprambiente.it)

    Spatial coverage

    Italy

    Time coverage

    1961-2023

    Processing methodology

    To determine whether a given calendar day can be classified as a summer day, the maximum daily air temperature is compared against a threshold value of 25 °C. Therefore, validated daily maximum temperature data must be available. The SCIA system (National System for the Processing and Dissemination of Climate Data) processes raw temperature data and provides a daily maximum temperature value, which is accepted as valid only if it passes predefined quality control procedures. If the data meet the validation criteria, it is compared to the 25 °C threshold to assess whether the day qualifies as a summer day.
    The annual indicator value is calculated by summing the number of summer days observed in a given year. This annual value is considered valid only if at least 75% of daily values for the year are available and have passed quality checks.
    The climatological reference period used to calculate anomalies has been updated to the most recent 30-year baseline, 1991–2020, in accordance with recent World Meteorological Organization (WMO) guidance recommending the regular decadal update of climate normals for operational climatology purposes.

    Update frequency
    Year
    Qualità dell'informazione

    The indicator provides a reliable representation of the trend in extreme heat events in Italy. The indicator is computed using a standardized methodology, in line with general guidelines issued by the World Meteorological Organization (WMO). The methodology ensures consistency over time and across geographical areas. Both the input data and the indicator itself are subject to quality assurance procedures, carried out by the institutions responsible for the primary data sources—CRA-CMA (Research Unit for Climatology and Meteorology applied to Agriculture), the Synoptic Network (AM and ENAV), and regional monitoring networks—as well as by SCIA (the National System for the Collection and Dissemination of Climate Data), managed by ISPRA.
    The use of national-scale average anomaly values ensures that the information needs related to this indicator are appropriately addressed. The meteorological stations used to calculate anomalies and assess trends meet strict requirements regarding time series length, continuity, completeness, and homogeneity.

    State
    Poor
    Trend
    Negative
    State assessment/description

    In 2023, an increase of approximately 19 summer days was observed compared to the average value calculated over the 1991–2020 reference period.

    Trend assessment/description

    Over the past 10 years, the number of summer days has consistently exceeded the 1991–2020 reference period average, with the exception of 2014 and 2016. Since the main international climate change strategies and policy programmes aim to mitigate ongoing warming in the climate system, the assessment of an unfavourable trend and the assignment of the corresponding icon can be interpreted as an indication of deviation from that objective.

    Comments

    The time series of annual mean anomalies in the number of summer days in Italy indicates that 2023, with an increase of approximately 19 days compared to the 1991–2020 climatological baseline, ranks as the second highest value in the entire historical dataset (Figure 1)

    Data
    Thumbnail
    Headline

    Figure 1: Time series of annual mean anomalies of the number of summer days in Italy compared to the 1991-2020 reference value.

    Data source

    ISPRA

    English