The territory of Trieste is composed of several different climate zones depending on the distance from the sea and elevation. The average temperatures . On average, humidity levels are pleasantly low (~65%), while only two months (January & February) receive slightly less than 60 mm (2 in) of precipitation.
Trieste along with the Istrian peninsula has evenly distributed rainfall above 1,000 mm (39 in) in total; it is noteworthy that no true summer drought occurs. Snow occurs on average 0–2 days per year. Temperatures are very mild—lows below zero are somewhat rare and highs above 30 °C (86 °F) aren’t as common as in other parts of Italy. Winter maxima are lower than in typical Mediterranean zone (~ 5–11 °C) but with quite high minima (~2–8 °C). Two basic weather patterns interchange—sunny, sometimes windy but often very cold days frequently connected to an occurrence of northeast wind called Bora as well as rainy days with temperatures about 6 to 11 °C (43 to 52 °F). Summer is very warm with maxima about 28 °C (82 °F) and lows above 20 °C (68 °F), with the hot nights being influenced by the warm sea water. The absolute maximum of the last 30 years is 40.1 °C (104 °F) in 2020, whereas the absolute minimum is −7.9 °C (18 °F) in 1996.
The Trieste area is divided into 8a–10a zones according to USDA hardiness zoning; Villa Opicina (320 to 420 MSL) with 8a in upper suburban area down to 10a in especially shielded and windproof valleys close to the Adriatic sea.
The climate can be severely affected by the Bora, a very dry and usually cool north-to-northeast katabatic wind that can last for some days and reach speeds of up to 140 km/h (87 mph) on the piers of the port, thus sometimes bringing subzero temperatures to the entire city.