VERY HOT DAYS

Descrizione 1
Update date
Authors

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

Abstract

The indicator describes the trend in extreme heat events in Italy, specifically representing the number of days with a daily maximum air temperature exceeding 35 °C. In 2023, an increase of approximately 7 extremely hot days was observed compared to the climatological average for the 1991–2020 reference period.

Description

The occurrence of extreme thermal events and the presence of significant trends are assessed through the analysis of absolute minimum and maximum air temperature values. Specifically, the indicator “very hot days”, as defined by the Expert Team on Sector-Specific Climate Indices (ET-SCI), represents the number of days when the daily maximum air temperature exceeds 35 °C.

Purpose

The annual time series of the average number of hot days, expressed as the deviation from a climatological baseline, allows for the estimation of the frequency of extreme heat events and the evaluation of 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

SNPA, 2024, Il clima in Italia nel 2023;
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.;
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;
APAT-OMS, 2007, Cambiamenti climatici ed eventi estremi: rischi per la salute in Italia.

Further actions

Improvements in the estimation of hot days in Italy could result from the extension of the indicator calculation system to include additional sources of raw temperature data, with the aim of increasing the availability of long-term temperature time series that meet requirements of duration, 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 specific calendar day qualifies as a “very hot day”, the daily maximum air temperature is compared with the threshold of 35°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 meteorological data and produces a daily maximum temperature value, which is accepted as valid only if it passes specific quality control procedures. If the daily maximum temperature is validated, it is compared with the 35°C threshold to determine whether the day can be classified as a very hot day. The annual indicator value is calculated by summing the number of very hot days recorded in a given year. This value is considered valid only if at least 75% of the daily data are available and validated for the year in question. The climatological baseline used for anomaly calculation has been updated to the most recent 30-year reference period, 1991–2020. This choice follows the latest guidance from the World Meteorological Organization (WMO), which recommends updating climatological baselines every ten years for operational climatology purposes.

Update frequency
Year
Qualità dell'informazione

The indicator adequately describes the trend of extreme heat events in Italy. The calculation of the indicator is conducted using a standardized methodology and follows the general criteria set by the World Meteorological Organization (WMO). The methodology is consistent over time and space. Both the input data and the indicator itself undergo validity checks carried out by the entities responsible for the raw data (CRA-CMA (Research Unit for Climatology and Meteorology Applied to Agriculture), Synoptic Network (AM and ENAV), Regional Networks) and by the SCIA system (National System for the Processing and Dissemination of Climate Data) of ISPRA. The use of average anomaly values across the entire national territory allows for an adequate response to the information needs related to this indicator.
The measurement stations from which the anomaly is calculated and the ongoing trend is estimated meet the requirements of duration, continuity, completeness, and homogeneity of the time series.

State
Poor
Trend
Negative
State assessment/description

In 2023, an increase of approximately 7 hot days was observed compared to the average value calculated for the reference thirty-year period (1991-2020).
Trend assessment/description

Over the last 10 years, the number of hot days has consistently been higher than the reference thirty-year period (1991-2020), except for 2014, 2016, and 2018. Given that the main international climate change strategies and policies aim to combat the ongoing warming of the climate system, the evaluation of an unfavorable trend and the assignment of the corresponding icon can be interpreted in terms of deviation from this goal.
Comments

The time series of annual average anomalies for the number of hot days in Italy indicates that 2023, with an increase of approximately 7 days compared to the average value calculated for the reference thirty-year period (1991-2020), ranks 3rd among the highest in the entire time series (Figure 1).

Data
Headline

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

Data source

ISPRA

English