The Bora: the scourge of the Adriatic

Able to turn over a lorry and devastate a marina, yet without it there would be no famed Dalmatian prosciutto or sheep's cheese. According to the locals, only the Bora wind can dry out the local ham or shower saltwater onto the pastures of Pug Island. The Croatians have learned to live with it, but what about us sailors? Can or should we sail it? And how treacherous is it really? 

How is the Bora formed and when is it strongest?

The Bora is a falling (katabatic) wind. Such winds are formed in the mountains when a large amount of cold air accumulates over the peaks, and then gravity begins to pull it down the slopes to lower elevations — in this case, to the sea. By the time the cold air reaches the coast, it has picked up incredible speed. Here it begins to sink beneath the warmer, lighter air and picks up sea spray that the strong gusts then carry across the sea, land and islands. The Bora blows with more or less the same intensity day and night but if it eases a little, this is usually between 12 noon and 3 pm. The danger for sailors is that the Bora appears suddenly and hits with hurricane force — no warning and no time to prepare. It is also accompanied by very powerful gusts.

A severe Bora must always be taken into consideration, especially in autumn and winter. Although it often appears during the spring months as well. A winter Bora can last up to a week and reach hurricane force. In fact, it has been known to hit speeds of around 300 km/h. July and August used to be relatively safe "Bora-free" months, but due to climate change, this can no longer be counted on. In recent years a violent Bora can be encountered during the main sailing season, but in the summer, it usually ends within three days and is usually not so fierce.

Atmospheric pressure also has an effect on the Bora. The mass of cold air can be boosted by a jet stream from the mainland created where high and low pressure meet. This happens in two ways. Firstly, when high pressure occurs over the snow-capped Dinaric Mountains and a low pressure occurs over the warmer Adriatic. Secondly, when low pressure from the Black Sea region meets high pressure around the Balearic Islands. At their meeting point, a strong current is created which blows southwards from the mainland, bringing the cold air from the Dinaric Mountains with it.

Why does the Bora affect Croatia?

The Dinaric Mountains are a 700 km long set of several mountain ranges that stretch from Slovenia to Albania. The mountains line most of the coastline of Croatia, creating ideal conditions for a powerful bora. The main culprits are the three mountain ranges — Velika Kapela, Velebit and Biokovo.

The Velika Kapela Bora can be encountered if you sail from Pula or Rijeka between the islands of the Kvarner Bay — Krk, Cres, Rab, Prvić and  Goli Otok. The regular strong winds here have shaped the northeastern and eastern parts of the islands and also affect the towns of Crikvenica and Senj on the mainland. Be especially careful in the Vinodol Strait between the mainland and the island of Krk, where the terrain intensifies the Bora.

The Velebit Strait is one of the regions with the fiercest winds. Here, the Bora not only ravages the entire mainland, coastline and the nearby island of Pag (which is practically bare thanks to it), but also the Novigrad Sea, which is connected to the Velebit Strait by the Novska Strait. The latter serves as an acceleration zone for the Bora, making the wind even faster. 

The famous Makarska Riviera is in turn plagued by the Bora winds from the Biokovo mountains as well as surrounding boras which blow through river valleys or passes. These include Šibenik at the mouth of the Krka River, Split or Omis. Further south, we see the Bora from the mouth of the river Neretva, which reaches the Mljet Strait and the Pelješac Peninsula. Dubrovnik also experiences boras, but they weaken towards the south.

Sailing the Bora

The Bora won't upset your stomach as much as some of the other Mediterranean winds. It doesn't create very deep waves, but they are short and unsuitable for manoeuvring. Strong winds tend to drive the boat away from the mainland, which in the case of Croatia usually means to an island. Be sure not to sail on the windward side of the Bora, or the wind may blow you onto shoals or rocks.

You'd expect the rugged coastline to be safer and that you'd be able to hide in more sheltered bays. But the opposite is true — not only will such a coastline not protect you, but you will encounter unexpected gusts accelerated by the narrowed terrain (a kind of wind channel). The Bora also kicks up water spray, reducing visibility and limiting your ability to navigate.

Experienced crews can sail the weaker boras if they are confident, and enjoy a fast and somewhat adrenaline-filled ride. However, the Bora is unpredictable and it is the treacherous wind gusts that pose the greatest risk, as they can destroy sails and cause unwanted tacking.

Mooring during the Bora wind

The Bora makes it virtually impossible to moor in the northern and north-eastern parts of most Croatian islands. Some harbours here do not even operate during the Bora because they are not safe. The situation is better on the southern and western side of the islands. But we know from our own experience that this cannot be relied upon. In fact, the Bora can climb tens of metres when it is at full strength, easily crossing an island even with larger hills, and without losing any of its power. We've encountered strong wind gusts everywhere from Cres to Mljet and on the sides facing the open sea.

An annoying fact is that in high season the safer and more sheltered harbours tend to fill up quickly. Therefore, when planning your route, it's never a bad idea to have one or two other anchorages as a backup. More and more marinas allow you to book your berth online in advance.

Fortunately, Croatia's marinas are covered in detail by the legendary guide — 777 Harbours and Anchorages. The book also provides detailed information on the marinas affected by the Bora. The latest edition was published in 2017/18, which also takes into account climate change as it impacts the region.

Sheltering from the Bora

Western Istria is well protected from the Bora, so crossings from Croatia to Italy are usually trouble-free. Despite the Bora wind from the Biokovo Mountains, you'll find calmer waters in the south of the islands of Hvar, Brač, Vis and Solta. This is because they are further away from the mainland and the Bora is not as rough as in the Velebit Channel.

In the vicinity of Zadar, the best shelter is offered by the well-protected bays of Ugljan and Pasman. Dugi otok also benefits from its location further inland. Thanks to weaker bora winds from the Neretva estuary, the southern sides of the islands of Lastovo and Mljet are also relatively safe, which are also at least partially protected by the Pelješac peninsula.

The Bora can be predicted

It appears out of the blue in nice sunny weather and unlike the the Croatian Jugo/Sirocco, most often during high pressure, when the sky is clear and you don't expect the weather conditions to turn. Nevertheless, the Bora is signalled by large white clouds over the mountain tops. Isobaric maps can also tell us a lot. As we've said, the Bora will start when high and low pressure meet.

The most reliable method, of course, is to pay attention to the forecast and listen to local radio and TV broadcasts. They'll alert you to a major bora. Unfortunately, you may encounter localised smaller bora winds in the straits, caused not only by differences in pressure but also by differences in sea and land temperature. And, unfortunately, you may get no warning about those.