The Bora, measured

'How strong is it? How many days a year? In summer too? And where?' These are the Bora questions buyers ask us most often, especially those who come from abroad and know it only by reputation. People born in Trieste usually answer with a story. The stories are true, but choosing a home takes numbers.

For every home we offer we measure what can be measured: how many hours of sun the terrace gets, how high it stands, how warm the sea is below the house. This page does the same with the wind. It sets out what can honestly be said about Trieste's Bora, with numbers from weather stations, satellites and terrain, and the sources at the bottom. It also says what is not known.

We are not a weather service and do not want to look like one. The data come from the Friuli Venezia Giulia environmental agency (ARPA), from the American NOAA and from the European Copernicus programme;^1,2,4 the calculations are ours, and anyone can repeat them. We do it because anyone buying a home, from Trieste or from Vienna, should be able to see all the data that exist, not just the photographs.

days a year with at least three hours of Bora above 18 km/h²

147

days a year with Bora gusts of 60 km/h or more²

74

the year's strongest gust at the Molo, on average¹²

128km/h

ARPA's record at the Molo, 11 February 2012¹³

168km/h

The first two figures come from the Trieste synoptic station in the years when it measured consistently, 2006-2011 and 2013-2014; the other two from ARPA at the Molo F.lli Bandiera, since 1994.^2,3,12 The Bora blows all year round, more from October to March and less in May and June; when the wind in Trieste goes above 18 km/h, 91 times out of a hundred it is the Bora.² How much of it arrives depends a great deal on the place: in summer and autumn 2026 the Karst plateau had between 44 and 68% of the Molo's mean wind, the buoy in the middle of the gulf 98%.¹

The Bora forms when cold, dry air from central and eastern Europe crosses the Dinaric Alps and the Karst, ridges about a kilometre high, and flows down to the sea. It is pushed by high pressure to the north-east, or drawn in by low pressure over the Adriatic or the Mediterranean.⁹

Until the 1980s it was explained by the weight of cold air sliding down under gravity. We now know that strong Bora is something else: air passing over the mountains forms waves, the waves break, and beneath the breaking the flow accelerates downhill, like water going over a step and then jumping.⁹ Two numbers show that weight alone is not enough: 20 metres per second of wind from gravity alone would need air about 25 °C colder, and the strongest wind is not at the ground but around half a kilometre up.⁹

Above Trieste the Karst is a plateau between 350 and 500 metres that ends in an escarpment, about five kilometres from the Molo; from there the ground drops to the city and the sea within four kilometres. It is the last step the Bora crosses before the gulf.⁴

It does not come down evenly: where the mountains are lower it is funnelled into jets, with calmer zones in between. In the northern Adriatic there are four main jets; the strongest is at Senj, in Croatia, and the Trieste jet crosses the whole northern Adriatic.¹¹

It is a wind that comes in blows. At Senj, where it has been measured second by second, the gusts return every 3 to 11 minutes; we have not found such a measurement for Trieste.¹⁰ And there are two kinds: 'clear' Bora, with blue skies, linked to high pressure, the more common; and 'dark' Bora, with clouds and sometimes rain or snow, linked to low pressure.⁹

At the Trieste station, in the consistent years, hours of Bora above 18 km/h made up 29% of the hours in January and 27% in October; in May and June 10%.² Counting days: in January, October and December about fifteen with at least three hours of Bora, and nine or ten with gusts of 60 km/h or more; in June 8 days of Bora and 3 with gusts like that.²

So yes, there is Bora in summer: in July and August from 10 to 12 days a month, with three or four days of gusts of 60 km/h.² The episode with the strongest gust we measured between August and October 2026 was in midsummer, on the night of 7 to 8 August: 81 km/h at the Molo.¹

The time of day matters too. The Bora blows more often at night and in the morning, peaking around 07:00 (24% of hours), and less in the early afternoon: 13% at 15:00.²

An episode lasts 8 hours in the median; one in ten goes beyond 30. The longest, in the years considered, lasted 146 hours: about 6 days.⁷

ARPA has measured the wind at the Molo F.lli Bandiera since 1994, ten metres above the ground. The strongest gust of each year has averaged 128 km/h; the highest in the series is 168 km/h, on 11 February 2012, from the east.^12,13 From the same data ARPA estimates that a 153 km/h gust returns on average every ten years, a 176 km/h gust every fifty.¹²

Gusts make the news, the mean much less so. From 2001 to 2020, in Trieste, the mean wind at ten metres was 14 km/h; it blew from the north-east 16% of the hours, averaging 29 km/h. Episodes with an hourly mean above 35 km/h for more than five hours in a row, ARPA writes, are not rare.¹⁵

How many days of strong wind? At the Molo, from 1994 to 2026, on average 106 days a year with gusts above 54 km/h, 49 above 72 and 17 above 90, and 21 days with a daily mean wind above 36 km/h.¹² These counts include every direction, but in Trieste strong wind is almost all Bora: of the 130 months with a maximum gust of 100 km/h or more, 119 were Bora.¹² At the Trieste synoptic station, counting Bora only, days with gusts of 60 km/h or more were 74 a year, and days with 100 km/h or more 6.6.²

Other instruments in the city have measured more. On the roof of the Nautical Institute the University of Trieste recorded 52 metres per second, about 187 km/h, on 10 March 2010, while ARPA measured 152 km/h at the Molo: the same Bora strikes places a kilometre apart differently.^14,12 Higher figures also circulate, such as 171 km/h in 1954 or 212 km/h in 2010: we have not found them in an official source, and we do not use them here.

Duration has a record too: from 29 January to 12 February 2012 the Molo had fifteen days in a row with gusts above 100 km/h.¹³ And over time? In the twentieth century the Bora decreased slightly, with swings lasting decades: from 1903 to 1985 wind from north-north-east to east, light wind included, blew on average 42% of the time, the equivalent of 153 days a year.¹⁷ Over the last thirty years, at the Molo, days with a mean wind above 36 km/h have stayed almost the same: 22 a year in 1996-2005, 20 in 2006-2025.¹² Climate models project slightly less winter Bora in the northern Adriatic, with much uncertainty.^18,19

The Bora does not blow the same everywhere. Between 1 August and 23 October 2026 the Molo F.lli Bandiera had 9 Bora episodes, by the rule described in the note.⁸ Over the same hours we compared the other thirteen ARPA stations around Trieste that measure wind.¹

Out in the gulf, the Paloma buoy had 98% of the Molo's mean wind; Muggia 87%. In the city the Nautical Institute, thirty metres up among the buildings, had 69% of the mean but 89% of the strongest gust: among buildings the mean drops more than the gusts.¹ On the Karst plateau, around 270 metres, Prosecco had 68%, Sgonico 46%, Borgo Grotta Gigante 44%; Cattinara, on the city's hill, 53%.¹

The stations are not all at the same height above ground or in the same kind of place, and nine episodes in a summer and an autumn are not a winter: the ratios give an order of magnitude, not the wind under a particular window.¹

On the coast road between Barcola and Duino no station measures the wind on land. Satellite radars measure it over the sea, working back from the roughness of the water. Two different measurements say this:

Off Santa Croce the two measurements agree: the wind over the sea is between three quarters and four fifths of the wind off the Molo.^5,6 Off Barcola and Grignano they disagree: the 2003 radar, on six days, saw less wind than at the Molo; Sentinel-1, on 44 days, sees more.^5,6 They were made in different ways on different days, and for now the honest answer there is that we do not know. Towards Duino the two measurements come close again: off Sistiana they coincide, off Aurisina and Duino Sentinel-1 sees somewhat more wind than the radar.^5,6 One day is not the median: on 16 of the 44 days, off Santa Croce, the wind over the sea was stronger than at the Molo.⁶ In any case this is wind over the sea, one to three kilometres from the shore: not the wind in a garden.

In the 3D view at the top of the page, 'The Bora of 8 August 2026' replays a real episode, hour by hour. At the Molo it lasted 22 hours, from the evening of 7 August to the evening of the 8th.^1,8 The strongest moment came around two in the morning: 49 km/h mean wind and a gust of 81 km/h, from 53°.¹

In the same hour the buoy in the middle of the gulf had 45 km/h mean and a 68 km/h gust; Muggia 33 and 72; the Nautical Institute 30 and 65; Prosecco, on the plateau, 29 and 50; Borgo Grotta Gigante 22 and 45.¹ On the plateau, where the Bora comes from, the mean wind was between less than half and three fifths of the Molo's.

It brings cold, dry air and usually clear skies: the Karst has the lowest mean humidity in the region, partly because of it.¹⁶ It cleans the air: in winter 2019-2020, while pollutants stagnated in the rest of the region, in the Gulf of Trieste the Bora and the easterly winds diluted them.²³

In winter the cold feels sharper: at 0 °C with a 50 km/h mean wind the wind chill is about −8 °C, according to the US weather service formula.²⁴ In February 2012, the month of the longest Bora, Trieste had seven days when the temperature did not rise above freezing.¹³

In summer it brings cool water. On 24 June 2024 the Bora brought bottom water to the surface: in two days the mean sea temperature at two metres in Trieste fell from 25.5 to 19.1 °C, and was back at 26.8 °C six days later.²⁰

In winter it chills the sea to the bottom. Between late January and mid-February 2012 the Gulf of Trieste's daily mean heat losses exceeded a thousand watts per square metre, and at the bottom the water fell to 4 °C, denser than the water measured in 1929.²¹ Water of record density formed across the northern Adriatic.²²

And sometimes it stops the city: in 2020 the Barcolana regatta was called off because of strong Bora and rain.²⁷ Trieste has even given the Bora a museum, the Museo della Bora, with its Borarium in Opicina, open by appointment.²⁸

The data on this page allow a first classification by zone. It is experimental: it rests on a few stations, on one summer and one autumn, and on the sea seen by satellite where nobody measures on land. For each zone, the mean wind and the strongest gust of its stations in the 2026 Bora episodes, relative to the Molo F.lli Bandiera.^1,5,6 The same classification appears in each home's listing, under the map, in the box 'The Bora here'.

• City
• Muggia
• Coast road
• Hills and Karst
• Monfalcone and lagoon
• no zone
• ARPA station

Within each zone it is the house that makes the difference: the floor, the façades and terraces facing east-north-east, the buildings and trees around. None of this is seen by a station kilometres away. That is why in our listings we write about the Bora only what has been measured for that house or for the stretch of sea in front of it, and we do not write that a house is sheltered unless we have measured it.

The building code treats it separately too. Italy's 2018 Technical Standards for Construction divide the country into nine wind zones: the province of Trieste is a zone of its own, with a reference wind speed of 30 metres per second against 25 in the rest of Friuli Venezia Giulia, and the wind pressure used to design buildings is 44% higher.²⁵ It is a ten-minute mean expected once in fifty years, not a gust.

For residents, the Civil Protection advice for strong wind applies: secure pots and objects on balconies and window sills, check awnings and fabric structures, do not stand under trees and scaffolding.²⁶ The Bora comes from the east-north-east: terraces and façades on that side are the first to look at when you visit a house.

On the coast road, where nobody measures the wind on land, we want to install a station of our own, to be compared with the Molo F.lli Bandiera episode by episode. When it has seen enough Bora, its numbers will appear here: even if they say the Bora there is as strong as at the Molo.

The short answers, with the numbers from the chapters above.

• The Bora calendar comes from a single station, Trieste's, and from eight years in which it measured consistently; it is not the climate of the whole province, and it does not say whether the Bora is changing.^2,3
• The comparison between stations comes from nine episodes between August and October 2026: in winter the Bora is stronger and longer, and the ratios may change.^1,8
• Satellites see the wind over the sea, not on land.^5,6
• Gusts cannot be compared between different instruments, nor at the Molo before and after 2005, when ARPA changed how it measures them.¹²
• The terrain is the 30-metre Copernicus model: buildings and trees are included, smaller details are not.⁴
• Every calculation can be repeated: the programs and downloaded data are kept, and we send them to anyone who asks.