SUMMER DAYS

Update date
Authors

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

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 2024, an increase of approximately 6.9 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 applicable to environmental issues at the regional level but of national significance.
It is able to describe the trend without necessarily providing an evaluation of it.
It is simple and easy to interpret.
It provides a representative overview 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, Rapporti 77/2007

ISPRA, 2022, I normali climatici 1991-2020 di temperatura e precipitazione in Italia; Stato dell'Ambiente 99/2022

ISPRA, 2018, Variazioni della temperatura in Italia: estensione della base dati e aggiornamento della metodologia di calcolo; Stato dell’ambiente 81/2018;

ISPRA, 2016, Controlli di qualità delle serie di temperatura e precipitazione; Stato dell'Ambiente 66/2016

ISPRA, 2015, Valori climatici normali di temperatura e precipitazione in Italia; Stato dell'Ambiente 55/2015

ISPRA, 2014, Focus su "Le città e la sfida ai cambiamenti climatici"; Stato dell'Ambiente 54/2014

ISPRA, 2013, Variazioni e tendenze degli estremi di temperatura e precipitazione in Italia; Stato dell'Ambiente 37/2013

ISPRA, 2012, Elaborazione delle serie temporali per la stima delle tendenze climatiche; Stato dell'Ambiente 32/2012

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, 2025, Il clima in Italia nel 2024; Report ambientali SNPA n. 44/2025

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.
Data source

ISPRA 
Data collection frequency
Yearly
Data availabilty

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

Italy
Time coverage

1961-2024
Core SET
7° Programma di azione per l’Ambiente Europeo (7° EAP) - Dati sull’ambiente
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
Data quality

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.

Status
Poor
Trend
Negative
State assessment/description

In 2024, an increase of approximately 6.9 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 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 2024, with an increase of approximately 6.9 days compared to the 1991–2020 climatological baseline, ranks as the 11° highest value in the entire historical dataset (Figure 1)
Data
Data
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