Monday, September 17, 2012

Macro-climates and their modifying agents

Climate, according to Dr. Tony Wolff (Lecturer and Viticulturist, Virginia Tech) and John D. Boyer, is the average course of weather in a region over an extended period as measured by temperature, precipitation, and wind speed, among other variables (Vineyard Site Selection, Virginia Cooperative Extension). Weather is itself defined as the state of the atmosphere at a specific point in time using the same variables as referenced in the climate definition above. The climate of a grape-growing region will determine, to a large extent -- and all things being equal -- both the grape varieties that can be grown and the styles of wine that can be produced.  That is not to say that these varieties cannot be grown outside of these environments; that is to say, however, that varietal typicity is compromised when these varieties are grown outside of their "zones."  In this post I will examine macro-climatic effects in greater detail and describe the agents which affect their impacts.

Macro-climate refers to climatic effects over large (hundreds to thousands of miles) geographic areas.  The table below shows a climate classification scheme based on the governing temperatures during the berry-growing season and examples of wine-growing regions that fall within the various climate bands.


Climate can also be described in terms of its "continentality", as shown in the table below.


Maritime climates are modified by proximate large bodies of water which heat up and cool down at a slower rate than does the adjoining land mass. This scientific fact results in the warming of winter winds as they blow over a warmer body of water and the warming of landside vineyards as the winds make landfall. This warming could act to extend the growing season and minimize the potential vine impact of winter low-temperature events. On the other side of the coin, warm spring air blowing in over the still-cold water will be cooled down and will retard the development of landside vineyards, minimizing their potential for damage from spring frosts.  This effect is not only limited to maritime areas, however, as regions that adjoin lakes and rivers will also be subject to similar effects. Franciacorta, for example, is moderated by winds blowing in off Lakes Iseo and Garde which protect the region from the autumnal and hibernial fogs that threaten from the Brescian Plains. Further, in the case of rivers, the water flow promotes air movement.  The downside associated with proximity to bodies of water is increased humidity levels and the risk of fungal diseases.

Offshore ocean currents can also have a moderating effect on a climate.  For example, the cool air blowing in from the Pacific Ocean mixes with the warmer air blowing in from the San Joaquin Valley creating an early morning fog in Napa's Oakville AVA.  This fog blows off by the middle of the day, allowing the grapes to gain the ripening benefit of the afternoon sun.  At the peak of the afternoon temperature, cooler air is once again funneled into the region from San Pablo Bay.  The Humboldt current off the coast of Chile serves much the same purpose for its regions, allowing grape growing in areas that would be otherwise too hot.

Mountains play a variety of roles in modifying a region's climate.  In the case of Alscae, the Vosges Mountains blocks the wine growing regions from the prevailing westerly winds but also provides a rain shadow effect which keeps most of the rainfall to the west side of the mountain and away from the vineyards. In the case of Oregon's Willamette Valley, climate is moderated by three openings in the Coast Range which provide gateways for the transit of cool air between the Pacific Ocean and the valley. The opening between Lincoln City on the coast and Salem in the valley is named Van Duzer Corridor. The remaining two (un-named) corridors run from Newport to Corvalis and Florence to Eugene, respectively.  Finally, temperature decreases by 33.08°F (0.6°C) every 328 feet (100 meters), thus allowing vineyards to be planted on mountain sides in areas where conditions would otherwise be uncooperative.

Forests can be the bane of vineyards in that they harbor birds but, as in the case of Bordeaux's Medoc, the forest to the west of the region serve as a barrier to the winds blowing in off the Bay of Biscay.

Vineyard selection and management practices can also work to blunt the effects of climate.  For example, by being on an appropriately sited slope, the vineyard can gain access to the sun's rays earlier and for longer periods of time thus aiding the ripening process in cooler climes.  A vineyard on a slope can also benefit from cold air moving downhill and being replaced by warmer air.  That zone of warmth can be hospitable for vine growth and berry ripening. 

Canopy management and trellising techniques could provide the berry with more or less access to the sun or protection from the elements as required.  For example, a significant challenge to Santorini viticulturists is the stiff wind that buffets the island during the growing season and could damage the berries if they were exposed to the elements. The solution that has been employed for eons is to (i) eschew vine density and (ii) train the vines such that they can afford protection to the otherwise vulnerable berries. Vine canes are intertwined and trained into a circle and the berries grow within this protective cordon. The circular structure can be positioned above ground or in a below-ground hollow where the top of the vine is parallel to the surface.

As it relates to the wine regions of the world, the ideal macro-climates for vitis vinifera are Mediterranean and marine west-coast climates, both of which are characterized by mild, wet winters and warm, dry summers. The mild winters promote long-term survivability of the vines (and increased quality of the juice as the vines age) and the wetness provides a reservoir of water that the vine roots can tap into during the grape maturation cycle. The warm, dry summers provide the heat and light that are the engines of vegetative and crop growth while keeping at bay the threat of rot and flavor dilution that would accompany summer/fall rains.  Grapes are, however, grown in a variety of areas which do not fit this profile, a situation made possible by a mix of natural events/edifices and human ingenuity.

©Wine -- Mise en abyme

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