Temperature trends since 1900 in and around Trieste

Also including Udine and Gorizia

Using data from the European Centre for Medium-Range Weather Forecasts, we analysed 118 years of weather data in and around the city of Trieste. The area analysed also includes the surroundings of Trieste, which might include mountains or bodies of water, so that the temperatures shown here do not correspond exactly to the temperatures recorded by the weather stations of Trieste (see Methodology for details). This is what we found:

  • The temperature in and around Trieste between 2000 and 2018 was 1.2°C above the 20th century average.
  • The number of hot days (above 27°C over a 24-hour average) went from 0.2 days per year in the 20th century to 2.7 per year in the years since 2000.
  • The number of freezing days (below −1°C over a 24-hour average) went from 20.9 days per year in the 20th century to 9.8 per year since 2000.

Our analysis is based on a grid, where each area can contain more than one city. The results for Trieste also hold for nearby Udine and Gorizia. These three cities have been analysed together.

Changes in weather patterns

Temperature changes

Since 1900, the average temperature in and around Trieste increased from an average of 11.4°C between 1900 and 1999 to an average of 12.6°C between 2000 and 2018. The warmest years in and around Trieste were 2014, 2018, 2015, 2000, and 1994.

Temperature in and around Trieste from 1900 to 2018. (png|svg|eps)

Hot days

In the 20th century, the average number of hot days (days for which the 24-hour average temperature is above 27°C) per year was 0.2. Between 2000 and 2018, the average number of hot days were 2.7 per year.

A day is considered hot when its average temperature is over two standard deviations of the normal average.

Number of days when the average temperature was above 27°C in and around Trieste, each year. (png|svg|eps)

Freezing days

Temperature averaged −1°C or less for 20.9 days per year in the 20th century, on average. Between 2000 and 2018, the number of freezing days were 9.8 per year.

Number of freezing days in and around Trieste, each year. (png|svg|eps)

What does it mean for Trieste?

Health and heat waves

Higher temperatures lead to excess mortality. The heatwave of July and August 2003, for instance, killed over 52,000 people in Europe, according to the Earth Policy Institute (Larsen, 2006), a think-tank. The elderly and infants are most at risk.

Rising temperatures may also cause the number of deaths related to extremely cold weather to drop.

Rail buckling and tarmac softening

In high temperatures, asphalt exposed to the sun starts to soften. This causes delays and some roads have to be closed to traffic.

When temperatures rise above 30°C, rails exposed to the sun can move or buckle. This can cause trains to derail, as happened many times in Europe already, and forces them to run more slowly, causing major delays.

Tick and mosquito-borne diseases

Tick-borne encephalitis, and more recently ehrlichiosis have been spreading in the past decades, probably due to higher temperatures (Gray et al., 2009).

Trieste and its environs in context

Trieste and nearby cities

Here are the five locations closest to Trieste, among the 558 we analyzed:

LocationDistanceTemperature change
Rijeka63 km+1.3
Ljubljana72 km+1.1
Pula86 km+0.9
Pordenone92 km+1.0
Venice114 km+1.2

Cities of Italy

Trieste is one of 54 locations in Italy we have analyzed. This is how temperature has changed in the rest of them.

LocationTemperature change
La Spezia+1.2
Giugliano in Campania+1.0
Cities of Italy
Cities of Italy(png|svg|webp)


We analyzed two data sets from the European Centre for Medium-Range Weather Forecasts (ECMWF), ERA-20C for the period 1900–1979 and ERA-interim for the period 1979–2018.

Both data sets are re-analysis, which means that ECMWF scientists used observations from a variety of sources (satellite, weather stations, buoys, weather balloons) to estimate a series of variables for squares of about 80 kilometers in side width (125 kilometers for ERA-20C). While weather stations offer a much better record for immediate daily observations, using the ECMWF re-analyses is much more adequate for the study of long-term trends. Weather stations might move, or the city might expand around them, making their data unreliable when looking at centennial trends. However, the ECMWF data does not take into account micro-climates or “heat island” effects, so that the actual weather in the streets of Trieste was probably one or two degrees warmer than the values reported here (the trend, however, is the same).

Since the start of this project, ECMWF has adjusted the way historical temperatures are calculated, to give better estimates for e.g. coastal cities. Because of this, some figures published here in 2019 may differ slightly from corresponding figures published in 2018.

This report was produced by the European Data Journalism Network. Partners include OBC Transeuropa (Italy), J++ (Sweden), Spiegel Online (Germany), Vox Europe (France), Pod Crto (Slovenia), Mobile Reporter (Belgium), Rue89 (France), Alternatives Economiques (France), and El Confidencial (Spain).


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