The zonda is a hot, extremely dry wind that descends the eastern slope of the Andes onto the Cuyo region of western Argentina, principally the provinces of San Juan, Mendoza, La Rioja and Catamarca. It is a foehn wind, and it is produced by the same mechanism as the chinook of North America: moist Pacific air is forced up the western flank of the Andes, drops its moisture as rain and snow on the Chilean side, then descends the eastern side as dry air, warming by compression at roughly 10 °C for every 1,000 metres of descent. Because the Andes here are the highest mountains outside Asia — Aconcagua, at 6,961 m, is only 100 km from Mendoza — the descent is enormous and the heating is correspondingly extreme. A zonda can raise the temperature by more than 20 °C in a few hours, push relative humidity below 10 per cent and occasionally to 2 or 3, and blow at 40 to 120 km/h with far higher gusts. It is a winter and spring phenomenon, most frequent between May and November, and it is the defining weather hazard of western Argentina.
A Foehn Off the Highest Mountains Outside Asia
The scale of the barrier is what separates the zonda from its relatives elsewhere. The Andes at this latitude carry a dense cluster of peaks above 6,000 m, and the range is continuous, so Pacific air has no way around it.
Air approaching from the west is lifted the full height of that barrier. It cools as it rises, saturates, and precipitates heavily on the Chilean slope — which is why central Chile is comparatively well watered while Cuyo, just over the ridge, is semi-desert. Having lost its moisture, the air descends the eastern side dry, warming at the full dry adiabatic rate the whole way down to the Cuyo plains at 600 to 900 m.
The rain shadow and the wind are therefore two faces of the same process. The aridity that forces Mendoza and San Juan to irrigate everything from Andean snowmelt is created by the same mechanism that periodically delivers a wind hot and dry enough to undo a season's work in an afternoon. The name comes from the Zonda valley and department in San Juan province, one of the routes by which the wind descends.
What a Zonda Day Actually Looks Like
The onset is abrupt and locally unmistakable. Residents describe a sequence that plays out over a few hours.
First the air stills and the sky above the cordillera takes on a particular hard clarity, often with a lens-shaped lenticular cloud sitting over the peaks — the visible top of the mountain wave, and the standard local warning sign. Then the wind arrives from the northwest, hot and dusty, and the temperature climbs steeply. A June afternoon at 12 °C can reach the low thirties. Humidity falls away to almost nothing. Dust raised from the dry basin floors cuts visibility, and the sky turns a flat yellow-brown.
Public life stops. Schools close, outdoor work halts, sporting fixtures are postponed, and the provincial authorities issue graded alerts. The wind typically blows for several hours to a day, and its departure is as sharp as its arrival — often with a cold front sweeping through behind it and the temperature dropping 15 °C or more within an hour.
Zonda Aloft and Zonda at the Surface
Argentine meteorology draws a distinction that outside accounts usually miss, and it explains why some forecast zondas never seem to arrive.
A zonda de altura — zonda aloft — is the situation where the descending warm dry air exists over the region but does not reach the ground, because a shallow layer of cooler, denser air is sitting in place beneath it and the descending flow cannot displace it. From below, the day looks bright and unusually clear, with lenticular clouds standing over the cordillera and perhaps a strange stillness, but the temperature stays ordinary. This is the common case.
A zonda superficial — surface zonda — is when the descending air does break through and reach the ground, usually because the pressure gradient is strong enough to scour the cold layer out or because the flow plunges hard enough down a particular valley. That is when the temperature jumps and the damage starts. The transition can happen within minutes, and it is frequently patchy: one town gets a full surface zonda while a neighbouring one twenty kilometres away, sheltered slightly differently, stays cool all day.
The forecasting problem is precisely this. The presence of foehn conditions over the Andes is straightforward to predict a day or two ahead; whether and where the air will reach the surface is much harder, and it is the question that determines whether schools close. Argentina's national weather service issues graded alerts for zonda events, and the uncertainty in that last step is why alerts sometimes cover a wide area for a wind that lands hard on only part of it.
Dust, Fire and Damage
The combination of heat, near-zero humidity and high wind produces a hazard profile closely comparable to California's Santa Ana conditions.
- Wildfire: fine fuels dry within hours, and any ignition spreads fast. Zonda events are behind most of Cuyo's serious rural fires, and provincial fire services stage resources on the forecast.
- Structural damage: roofs stripped, trees down, power lines brought down and outages across San Juan and Mendoza. Gusts are strong enough to overturn high-sided vehicles on the Andean highways.
- Dust storms: visibility on Ruta 40 and the mountain routes can fall to a few metres, and road closures are routine.
- Snowpack: a strong zonda strips and sublimates mountain snow, which matters enormously in a region where essentially all water for irrigation and urban supply comes from Andean snowmelt.
- Avalanche: rapid warming destabilises the snowpack on the high routes, including the Cristo Redentor pass carrying the main road and rail corridor to Chile.
The temperature records tell the story compactly. San Juan and Mendoza hold winter maxima that would be unremarkable in midsummer and are startling in June or July — readings above 30 °C in the depths of the Southern Hemisphere winter, every one of them a zonda day. The region's highest winter temperatures are, essentially by definition, produced by this wind, and it is one of the few places on Earth where the annual temperature record for a winter month is set by a wind rather than by sunshine.
The Complaints People Report
Argentine popular belief holds the zonda responsible for headaches, insomnia, irritability, anxiety and a general malaise, and the association is strong enough that it is treated as ordinary conversational fact in Cuyo. Local reporting during events routinely covers the mood of the population alongside the damage.
This is the same claim made about the Alpine foehn, where it has been studied longest, about the Santa Ana in California and about the chinook in Alberta, and it is genuinely unresolved everywhere. The proposed mechanisms have included atmospheric ion balance, infrasound from mountain waves and rapid pressure change, none of which has been established convincingly. The competing explanation is more mundane: several hours of heat, dust, noise, static, poor sleep, disrupted routine and closed schools would put most people in a bad mood without any exotic physiology.
What is not in doubt is the physiological load of the conditions themselves. Humidity in low single figures dehydrates people quickly, aggravates asthma and respiratory conditions, and irritates eyes and airways, and hospital presentations rise during events.
There is also a documented effect on air quality that has nothing to do with dust from the mountains. San Juan and Mendoza sit in basins, and the cold stable layer that a zonda has to displace is the same layer that traps urban pollution through the winter. When a surface zonda breaks through, it scours the basin out and air quality improves sharply for a day; when a zonda aloft sits over the top without reaching down, it strengthens the inversion and makes the trapped air worse. The wind can therefore either clear the city or seal it in, depending on whether it lands.
The Wind and the Vineyards
Cuyo produces the overwhelming majority of Argentina's wine, and Mendoza alone accounts for most of it, so the zonda is a wine-industry problem as much as a public safety one.
The timing is the difficulty. The zonda season peaks between August and November, which in the Southern Hemisphere covers budbreak and flowering — the vine's most vulnerable stages. A strong event then can desiccate young shoots, damage flowers, and cut the eventual crop substantially. Growers respond with windbreak plantings, careful site selection in sheltered positions, and irrigation timed against forecast events.
The same wind is not entirely hostile. Dry air suppresses fungal disease, and Mendoza's low humidity is one reason its vineyards need relatively little chemical intervention compared with wetter regions. The high-altitude sites that produce the region's best-regarded Malbec exist because the aridity that the Andes create is manageable with snowmelt irrigation — the rain shadow and the zonda are two consequences of the mountain range that also supplies the water. The physics underlying both is set out in the guide to foehn winds.