When a sudden, violent and cold gust of wind sweeps down from the mountains, you know the williwaws have arrived. And if bora winds are in the forecast, make sure to set your clothes out to dry along with your prosciutto.
Haven’t heard of williwaw and bora winds before? Maybe you know them by other names, such as Chinook, Santa Ana or zonda? They are all forms of katabatic winds, and in many cases, they can be very dangerous.
These winds are generally divided into warm and cold categories. The warm ones are known as foehn winds (such as Chinook, Santa Ana, and zondas). The cold ones are known as fall winds (such as williwaws and boras).
But don’t let the difference in temperatures fool you, because they can both reach hurricane-force intensity, leading to damage.
Williwaw and bora winds
“A williwaw is a sudden burst of wind descending from high terrain down to sea level, usually along coastlines at high latitudes,” Alan Shriver, a meteorologist with the Anchorage National Weather Service, tells CNN weather.
These winds can be especially dangerous for mariners if caught off guard by sudden rough waves, and for low flying pilots, they can generate significant turbulence, Shriver said.
Shriver says one of these extreme windstorms moved “across the eastern Bering north of Dutch Harbor” August 30-31, 2020. “We actually received a reported wind gust of 120 mph,” he said. “This extreme wind caused damage to a few buildings, overturned boats and tossed shipping containers from the port into the harbor.”
Wind gusts of around 80 mph were also reported along parts of the Alaska Peninsula, he added.
“This again was an extremely unusual event,” Shriver said, explaining that most windstorms that could be categorized as a williwaw event happen between the late fall and early spring.
Other areas of the world can also experience similar cold downsloping winds.
“One of the most well-known regional/local equivalents is the bora, which develops routinely during the winter season out of the northeast along the Croatian coastline of the Adriatic Sea,” Shriver says.
Just like williwaws, bora winds can be strong and dangerous.
“The highest gust we have measured is 248 kilometers per hour (154 mph),” said Kristian Horvath, a meteorologist and head of research and development for the Croatian Meteorological and Hydrological Service. “This is the highest officially recorded wind gust (in Croatia).”
Wind speeds of that magnitude are equivalent to an EF-3 tornado or a Category 4 hurricane.
“The instruments, which are not designed for such strong winds, just malfunction,” Horvath said. “But to date, the strongest is 248 kilometers per hour on the 21st of December in 1998 from Sveti Rock Tunnel location.”
“When this happens, Croatia is cut in half,” Horvath said. “The older roads are closed, typically, and of course, airplanes cannot fly. It’s very hard to go from the continental part to the coastal part.” Maritime transport is also closed to local fishermen in small boats, tourists using charter boats, and other small vessels.
“The thing is that bora is really abrupt,” Horvath said. “So you can have very calm weather with no wind, and then it can take as little as five minutes to go from zero to 40 meters per second (89 mph).”
Put the prosciutto out to dry
In Croatia, people have ways of spotting early signs of bora winds.
“A cloud cap (over the mountains) was always an early indicator that bora will come very soon, like (within five) or 10 minutes. So there was this kind of traditional warning for people in small boats,” said Horvath.
Also, because bora winds are very dry, “people dry clothes on the coast,” said Horvath. “And we also dry our prosciutto.”
Horvath says many people will hang prosciutto in their attics, open windows on both sides and leave them to dry very efficiently in the bora.
Bora damage in the US
Even landlocked states can see the occasional bora wind if conditions are right.
Back in April of 2018, the Front Range of northern Colorado experienced a significant bora windstorm.
“The high winds caused damage in some areas, power outages, blew over semi-trucks and quite a bit of dust,” the National Weather Service office in Boulder said. “The blowing dust was so bad in some areas that roads were closed due to very poor visibility.”
In April of 2022, winds across northern Colorado ranged from 70 mph to over 100 mph, which led to reports of broken tree limbs and shingles blown off buildings in both Adams County and Logan County, Colorado.
The Santa Ana winds in California are another example of a katabatic wind, but these vary slightly in terms of temperature and latitude. If that sudden wind is at a lower latitude, it may be warmer and drier, leading to wildfire concerns. That’s exactly what people fear when they hear, “Santa Ana winds are coming.”
With Santa Ana winds, Shriver explains, the air compresses and warms so much by the time it reaches lower elevations, that it becomes a hot, dry wind. Williwaw winds undergo the same compression and warming as they decrease in altitude and increase in pressure, but the source region is so cold it often remains a cool wind even near sea level.
Chinook winds are similar to Santa Ana’s in that they are both warm dry downslope winds, but unlike Chinook winds, which are initially very cold as they descend down the Rocky Mountains until they warm by compression, Santa Ana winds are usually already warm as they descend.
Because of that warmth, Chinook winds have also been called “snow eaters” for their ability to erase thick blankets of snow in a matter of hours.
And in Antarctica, there is evidence of foehn winds destabilizing ice shelves. When warm, dry air from foehn winds streams down an Antarctic mountain after cool, moist air has risen up on the other side, it can cause ice melt.
Bora, williwaws and Chinook: damaging winds you’ve probably never heard of | CNN