Foehn — The Alpine Wind

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“The Foehn wind is a real affineur of grapes . . .” from the diary of Weingut Bründlmayer

Evidence from wine regions everywhere suggests that if cool climate viticulture is to survive, then it must move north — or, in Switzerland’s case, up. Warming temperatures in formerly marginal regions such as Burgundy and Piedmont now require changes in viticulture and/or additions to the permitted roster of grapes just to keep up. Even with successful change, however, the freshness, delicacy, and intricate architecture we love in Pinot Noir and Nebbiolo may be harder to come by.

By contrast, Switzerland and parts of Germany are considered ideal for Pinot Noir now that the challenges of consistent ripening are a thing of the past. But while rising temperatures may benefit Switzerland and Germany in the near term, the question becomes: for how long? How long before the advantages of consistent ripening are outweighed by the unpredictability of climate chaos?

The most obvious and dramatic manifestations are already here. The series of devastating frosts in Europe and the violent hail storms that followed are evidence enough. So too are the monsoon rains, swollen rivers, and powerful floods that visited the Ahr and Mosel valleys during the annus horribilis of 2021. Sadly, there’s almost certainly more to come. Some of it will be dramatic — fires, drought, freak storms — but some of it will pass unnoticed, except by those closest to it.

In the category of “never-saw-it-coming,” I would include changes to the pattern and intensity of foehn winds in the vineyards of Switzerland.

Foehn, or Föhn, is the German word for “hairdryer.” [Ed. note: The word has far older origins and comes from a combination of Old High German and Latin terms meaning “spring wind” and the Latin fovere, meaning “to make warm.”] It’s hard to conjure a stronger image than that to describe the dry blast from these hot winds. They can occur throughout the year in Switzerland, but are most impactful, and essential when they extend the growing season. This is especially true for agricultural areas at elevation or when rain is a year-round occurrence.

A foehn event is initiated when a large air mass is drawn from an area of high to low pressure. When the flow of air is obstructed — by the Alps, in this case — it is forced to rise. Physics tells us that as it rises it expands and cools, and any moisture within it condenses and falls as rain or snow. When the air is at its driest, near the peak of its journey, it is primed for the reverse process to occur. As it crests and begins to descend on the leeward side, the forces of compression cause air temperature to rise. As it does, it gathers speed and strength, in a phenomenon known as the adiabatic process.

The strength of the foehn depends on the pressure difference between the high and the low and the elevation at which the air crests. It is either a deep foehn, when the pressure differential is great and the elevation is high, or a shallow foehn, when either or both of these conditions are less extreme.

During a deep foehn, temperatures can rise quickly, with correspondingly high winds. The infamous deep foehn of November 8, 1982 registered wind speeds of 246 kph with a range in temperature of -1ºC (30ºF) in Locarno and 25ºC (77ºF) in Zürich, only 200 kilometers north. It is the fifth most destructive windstorm in Swiss history and is considered by climatologists to be a once-in-a-century event.

A foehn of such intensity can melt a season’s snowpack overnight, creating avalanches and flooding at lower elevations. Forest fires, sparked by dry lightning, can be spread beyond control by fierce, super-heated winds. Dangerous shearing can threaten air travel, while transportation routes, especially mountainous ones, can be dangerous to navigate. Train derailments and cable car accidents are not unknown. There are even studies that link foehn winds to health emergencies, bouts of depression, and even suicide.

For these reasons, and others, forecasting has become increasingly important and, even though foehn winds have been extensively studied, they remain difficult to predict. Fortunately, shallow foehns are the rule in Swiss vineyards — for now.

Sometimes Good . . .

Because two of Switzerland’s most important wine regions — Valais and Bündner-Herrschaft — are located below major gaps in the Alps (the Simplon and San Bernardino passes, respectively), the foehn winds along these corridors are normally shallow and of short duration. They are precisely the type of winds that extend the growing season and make grape growing possible.

The 2021 vintage might have been saved in parts of Switzerland had
there been a normal foehn pattern. Instead, the foehn went missing. 

While winds in general can be both positive and negative for grapevines, foehn winds bring with them the complicating effects of high heat and low humidity. The foehn is, therefore, a powerfully “antiseptic” wind that discourages the growth of mold and mildew. It can limit the germination of fungal spores and even inhibit sporulation. A wet summer that threatens the health of the vines, or a wet fall that threatens the quality of the fruit, can be mitigated by a timely foehn. The mildew-plagued 2021 vintage might have been saved in parts of Switzerland had there been a normal foehn pattern. Instead, the foehn went missing. Vineyards in Thurgau, especially those farmed organically or biodynamically and those not planted with hybrids, were decimated.

Even though normal winds are inhospitable to vineyard pests, it’s the desiccating effects of dry foehn winds that make it especially difficult for insects to reproduce or for their larvae to develop. If well-timed, a foehn will do the work of several passes through the vineyard, eliminating the need for chemical sprays or decoctions. This is, once again, a boon for the farmer.

If well-timed, a foehn will do the work of several passes through the vineyard, eliminating the need for chemical sprays or decoctions.

When a foehn blows at or near harvest, sugar levels in the grapes can rise abruptly. The 2014 harvest in Bündner Herrschaft was saved by a late-season foehn that boosted Brix levels for Pinot Noir by an average of 15% — from 20.8º Brix before foehn, to 23.5º after. Meanwhile, average measurements from Thurgau, Zürich, and Schaffhausen, which were not affected by the same foehn, languished near 20º Brix.

Visiting a vineyard after a foehn episode can be an eye-opening experience. The appearance of wasted vines and wizened berries suggests disaster, but the grapevine has an amazing capacity to recover. If the foehn is short and there is sufficient moisture in the ground, the vine can return to normal overnight.

Sometimes Bad . . .

On the downside, an untimely or prolonged foehn at harvest can promote unwanted cooked fruit aromas and flavors that lack freshness. Growers in areas with little or no foehn exposure, such as in Schaffhausen, Aargau, and Neuchâtel, will sometimes feign dislike for the cooked fruit aroma in Pinot Noir from Bündner Herrschaft. Indeed, this can be a reliable tell when blind-tasting a range of Swiss Pinot Noir — a little bit is attractive, but too much becomes ponderous.

An untimely or prolonged foehn at harvest can promote unwanted aromas and flavors, which can be a reliable tell when blind tasting Swiss Pinot Noir.

When the foehn blows during the critical phases of the vine’s growth cycle, as it sometimes does, it can wreak havoc in the vineyards. Aside from possible damage to the vine canopy and trellising infrastructure, a pronounced foehn can initiate an early bud break and leave a vineyard vulnerable to late frost. This threat can dictate certain choices in the vineyard. For instance, late pruning is increasingly common as it can delay bud break by a week or two, even at the cost of lower yields.

The extreme low humidity that accompanies foehn winds can leave the delicate reproductive organs of a flowering vine incapable of pollination. Coulure is the likely result. This leaves early flowering Swiss varieties, such as Räuschling and Completer, particularly vulnerable. These two rare Swiss-German survivors were rendered nearly extinct because of unreliable yields. Climate chaos may threaten them again.

A foehn episode can shut a vine down by interrupting the process of transpiration. Heated winds cause the stomata in leaves to close and transpiration to stop. Unfortunately, the CO2 required for photosynthesis is blocked as well. Such disruptions can occur at any stage of development, but when they occur during critical phases of the vine’s growth, the consequences will invariably lead to lower yields.

The Future of the Foehn

For the moment, climate change has made grape growing in Switzerland an increasingly viable proposition. Especially when it comes to the production of quality wine. Ordinarily, this would be a good thing. But there is nothing ordinary about the climate extremes that come with rising temperatures. If the typical shallow foehn at the end of a successful growing season suddenly becomes a series of deep foehns, then there will be more to worry about than just yields and degrees Brix.

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