The climate of Trieste can be described as transitional Mediterranean.
Trieste is outside of the Po Valley and has a relatively mild climate: the average in January is 6 °C (43 °F). However, this city is particularly subject to the Bora, since cold air masses coming from eastern Europe accelerate when they are channeled through the mountain ranges. So the wind rushes to the Gulf of Trieste, with gusts above 100 kilometers (60 miles) per hour, and can also lower the temperature below freezing from December to early March.
In Trieste, snow is quite rare because the Bora is originally a dry wind, though sometimes its rarer version, called "dark Bora", can bring some snowfalls in winter and some cool and rainy days in summer.
Summer is hot and humid, with an average in July of 24 °C (75 °F), but tempered by the breezes.
Here are the average temperatures.
Trieste - Average temperatures
Min (°C)4471015182020171385
Max (°C)891217222528282318129
Min (°F)393945505964686863554641
Max (°F)464854637277828273645448

Trieste is a quite rainy city: 1,000 mm (40 in) of rain fall in a typical year, and there can be thunderstorms in late spring and summer. Precipitation ranges from 55 mm (2.2 in) in the driest month (February) to 110 mm (4.3 in) in the wettest (October). Here is the average precipitation.
Trieste - Average precipitation

On average, there are around 2,110 sunshine hours per year. Here are the average sunshine hours per day.
Trieste - Sunshine

The sea temperature ranges between 10 °C (50 °F) and 26 °C (79 °F). The sea is warm enough for swimming from July to September. Here are the average sea temperatures.
Trieste - Sea temperature
Temp (°C)121011141822252623201614
Temp (°F)545052576472777973686157


Best Time

The best periods to visit Trieste are spring and autumn, and in particular, from mid-April to mid-June and in the month of September. However, in both periods, some rainy days are possible. The best months to go to the beach are July and August. In June, the sea is still a little cool.