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Zonda wind is a regional term for the warm and dry föhn wind that often occurs on the eastern slope of the Andes, in Argentina.
Fohn winds are downsloping winds that become warm and very dry due to adiabatic compression because of the higher pressure at lower elevations. The compression also tends to accelerate the winds.
Separately, zonda is also used to describe a hot, humid north wind in the Pampas of South America.
1) A dry Föhn wind in the central Argentine
The Zonda is a dry wind (often carrying dust) that starts as polar maritime air, warmed by descent from the crest of the mountains, approximately 20,000 ft above sea level. The wind may exceed a velocity of 65 km/h (40 mph).
While this type of föhn wind may occur over most central parts of western Argentina, its effects are more impressive in La Rioja, San Juan, and northern Mendoza provinces, where the mountain barrier (the Andes) is higher, while to the north the Puna plateau dissipates these winds.
The westerly wind that produces the Zonda on the leeside of the Andes is moisture laden on the windward side having just come off the South Pacific. As the air is lifted over the mountains, it cools producing clouds and snow in the higher elevations. It is the main mechanism for snow precipitation at the high altitude chains. Therefore this wind is particularly important for this arid region, as it is connected to the buildup of the winter snow cover and accumulation over the scarce local glaciers. It is when the air descends on the leeside that the warm, dry Zonda wind develops.
According to studies (conducted over the period 1967–1976), the Zonda wind most commonly starts during the afternoon (between 12 and 6 PM), and tends to last between 1 and 12 hours, though it may present itself intermittently for as long as 2 or 3 days. It is countered usually by the entrance of cold air masses moving northwestward. In 90% of the cases, the phenomenon takes place between May and November.
Zonda winds are associated with mountain wave activity on the eastern side of the Andes.
2) A hot humid north wind in the Pampas
In a different situation, the term Zonda also is used to describe a hot humid north wind in the Pampas, in advance of a depression (low pressure area) moving eastwards, and preceding the pampero, a burst of cold polar air from the west, southwest or south on the pampas in the south of Brazil, Argentina, Uruguay, Paraguay and Bolivia. This wind is also called the sondo.