Piero Fraschetti, Francesca Lena, Walter Perconti, Emanuela Piervitali, Giulio Settanta
The indicator describes the trend of intense cold weather in Italy, more precisely, it expresses the number of days with an absolute minimum air temperature less than or equal to 0°C. In 2025, a decrease of 15.4 days with frost was observed compared to the average value calculated in the thirty-year reference period (1991-2020).
The existence of extreme temperature events and the presence of any significant trends are analyzed by examining the absolute minimum and maximum air temperatures. Specifically, the "frost days" indicator, defined by the "CCL/CLIVAR Working Group on Climate Change Detection" for the analysis of temperature extremes, expresses the number of days with an absolute minimum air temperature less than or equal to 0°C.
The annual series of the average number of days with frost, expressed as a difference from a climatological baseline, allows us to estimate the frequency of intense cold events and evaluate any significant trends over the years.
The indicator has no direct references to regulatory elements
APAT-OMS, 2007, Climate change and extreme events: health risks in Italy - Reports 77/2007
ISPRA, 2022, 1991-2020 climate normals for temperature and precipitation in Italy - State of the Environment 99/2022
ISPRA, 2018, Temperature variations in Italy: extension of the database and update of the calculation methodology - State of the Environment 81/2018;
ISPRA, 2016, Quality controls of temperature and precipitation series; State of the Environment 66/2016
ISPRA, 2015, Climate normals for temperature and precipitation in Italy; State of the Environment 55/2015
ISPRA, 2014, Focus on "Cities and the challenge of climate change"; State of the Environment 54/2014
ISPRA, 2013, Variations and trends in temperature and precipitation extremes in Italy; State of the Environment 37/2013
ISPRA, 2012, Processing of time series for estimating climate trends; State of the Environment 32/2012
Peterson T.C., Folland C., Gruza G, Hogg W, Mokssit A and 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, 2026, The climate in Italy in 2025; SNPA environmental reports n. 48/2026;
Toreti A. and Desiato F., 2007, Changes in temperature extremes over Italy in the last 44 years, Int. J. Climatology, DO I 10.1002/joc.1576.
Improvements in the estimation of frost days in Italy could result from extending the indicator's calculation system to other elementary data sources, in order to increase the availability of historical temperature series that meet the requirements of duration, continuity, and completeness of the time series.
ISPRA (Higher Institute for Environmental Protection and Research)
SCIA – National System for the Processing and Dissemination of Climate Data (https://scia.isprambiente.it)
National
1961-2025
Average annual anomalies in the number of days with frost
To determine whether a given day of the year can be defined as a frost day, the minimum temperature for that day is compared to the 0°C threshold. Therefore, the daily minimum temperature must be available. The program used by the SCIA (National System for the Processing and Dissemination of Climate Data) system processes the elementary data and returns a daily minimum temperature value that is accepted as valid only if it has passed certain checks. If the daily minimum temperature is correct, the difference between it and the 0°C threshold is calculated and it is determined whether the day can be considered a frost day. The annual value of the indicator is calculated by adding the number of frost days present in a given year. This value is accepted as valid only if at least 75% of valid daily data is available in the year in question. The climatological basis for calculating anomalies has been updated to the most recent thirty-year period, 1991-2020. This choice stems from the World Meteorological Organization's recent recommendations to update the climatological base every ten years for operational climatology purposes.
In 2025, a decrease of 15.4 days with frost (average number of days with minimum temperatures less than or equal to 0°C) was observed compared to the average value calculated in the reference thirty-year period 1991-2020. For the average number of days with frost, 2025 ranks second among the lowest in the entire historical series (Figure 1).
Over the past 13 years, frost days have consistently been below average, with the exception of 2017. Since major international climate change strategies and policy programs aim to counteract ongoing warming in the climate system, the assessment of an unfavorable trend and the assignment of the corresponding icon can be considered in terms of a departure from this objective.
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Figura 1: Serie delle anomalie medie annuali del numero di giorni con gelo in Italia rispetto al valore normale 1991-2020 Fonte
ISPRA |
The index of days with frost (minimum temperature below 0 °C) recorded the second lowest value in the entire historical series, after 2014, with a decrease of 15.4 days compared to the average of the reference period 1991-2020 (Figure 1).