The great wines of Burgundy, Alsace, Loire, Northern Rhone, Barolo, Mosel, Austria, Hungary and Slovenia use only one grape variety, while those of Bordeaux, Southern Rhone, Languedoc, Rioja, Ribera del Duero and Portugal are blended from many grape varieties. One can easily draw a line separating the blenders from the purists… but the line is not straight: neither geographic latitude nor Winkler index (GDD) correlate to this distinction. What if it all came down to how hazy the sky is?

Looking at a map of the great European wine areas – those with a not too recent history of superb winemaking – one could easily draw a curved line separating the generally more northern areas, emphasizing the purity and completeness of single varietal wines, from the southern areas, whose wines achieve comparable complexity by blending several grape varieties. The line is curved, because climate is influenced not only by latitude but also by winds, seas, microclimates, etc.. This paper results from a speculative but reasoned inquiry to see if I could identify a climate parameter that correlated well with the distinction between blending areas and single varietal areas.

There have been several interesting explanations for why certain areas blend grape varieties and others do not. Some have argued that it depends on the size of the average plots of land, small in Burgundy and Loire, large in Languedoc and Medoc: larger southern plots are easier to manage with a mix of early and late maturing varieties, extending harvests over a longer period and reducing peak labor demand. Others allege that it was the capitalist approach of Bordeaux that required the marketing flexibility of blending, in contrast to the Cistercian modesty and purity of chardonnay or pinot noir in Burgundy.

Let us begin with the following basic assumptions about the logic and history of the process of selecting grapes varieties in the world’s best winegrowing areas. This is essentially a summary of the little I have learned after decades of amateur observation and a humble passion for wine:

a)      The main function of the selection of grape varieties for a particular quality winegrowing area is to determine which grape varieties can enable a skilled winemaker to combine grape varietal character with the special complex characteristics that are imputable directly to the local soil and climate conditions (alias “terroir”). Thus the main role of grape selection is to find the best vehicle to convey terroir: what is important is the area of origin of these grapes, the way the plants are grown and pruned, their health, their ability to tap deep into the earth for water, and the experience of the winemaker who will accompany every stage of wine production. Terroir is not just about soil.

b)      Without this ability to express terroir, the resulting wines would be like soft drinks, one-dimensional and fairly homogeneous, with no interesting variations from year to year or from one producer to the next. Thus merely ensuring that the grape variety is suitable for the local soil and climate is not enough to create a quality wine growing area.

c)       The grape varieties that were successfully selected in high-quality winegrowing areas are so finely tuned to the local conditions that they amplify the minute terroir changes from various neighboring “climats” within the same area, thus creating many variations for each “cru”.

d)      Grapes selected per the above process for terroir expression also tend to amplify variations from year to year, with each vintage giving a different mix of terroir characteristics. Fully understanding a terroir requires also experiencing the lesser vintages, which are most revealing when it comes to the skills of the winemaker.

e)      Terroir results from a rare combination of circumstances; most wine appellation areas do not have any terroir potential. Conversely, there may be many yet undiscovered great terroir areas, especially in emerging wine countries.

f)       While the selected cultivars from the best terroirs are rarely truly local, they do result from a period of selection and adaptation, ranging from a few decades to several centuries, that justifies them being described as “traditional”, or even “autochthonous”.

g)      Within each quality winegrowing area, the selected “traditional” grape variety or varieties will produce the best results, with respect to terroir expression and aromatic complexity, in the cooler subzones of that wine area. It seems that grape plants, perhaps not unlike humans, perform best when pushed, though without being overly stressed. Thus Pinot Noir and Chardonnay are perfect in Burgundy, but in Languedoc they yield coarse uninteresting wines. Viognier is an extreme case, achieving excellence only in the tiny area of Condrieu, while practically everywhere else it yields heavy syrupy wines, even when blended.

h)      Each winemaker will use his skills to develop his very own interpretation of the local terroir. A good terroir will result in a wider variety of interpretations, depending on the choices made at every step, from grape growing to bottling.

i)        Terroir is therefore not at all a means of making all wines from a certain area similar (which is, paradoxically, what most appellation committees attempt to do: make them all “typical”), on the contrary, terroir generates wines that are very diverse due to minute variations in soil, climate, pruning, winemaking, aging, vintage, style, etc. and all the infinite combinations thereof, yet without straying too far from a common line that characterizes the area.

Given the above observations, one can group all high-quality winegrowing areas into two groups:

SINGLE GRAPE  – In the cooler high-quality winegrowing areas (generally, higher latitudes), a single grape variety tends to emerge as the best and indeed the only one adapted to producing full maturity and complexity. The most evident examples are Riesling in the Mosel, Chenin in the Loire, Pinot Noir and Chardonnay in Burgundy, Nebbiolo in Barolo. In a way, even single varietal wines originate from grapes that are not precisely identical, there can be many different clones of the same variety, and grapes can be subject to variations in climate and soil from one parcel of land to the next, thus one could argue that “blending” takes place even for varietal wines.

BLEND – In the warmer high-quality winegrowing areas (lower latitudes), the growing season is longer and several grape varieties can achieve excellence. The nobler varieties, usually the slower (late) maturing ones, yield interesting wines, but they usually benefit greatly from the contribution of the secondary grape varieties. In the classic Bordeaux blend, Cabernet Sauvignon starts off very tannic and bitter, almost undrinkable when young, yet with (very) long aging it opens up and develops wonderful aromas; blending with Merlot makes it smoother, rounder, reducing aging requirements to more reasonable levels. Grenache is high in alcohol and aromas, but adding Syrah and Mourvèdre gives it more body and aging potential, Carignan adds fresh acidity, and Cinsaut can make the resulting blend more harmonious. In warm climes, a blended wine is greater than the sum of its parts, while a single grape variety very rarely succeeds in completely expressing a warm area’s terroir. Bandol wines made from Mourvèdre alone are very rarely better than blended Bandol. Excellent blended wines achieve terroir expression and greatness comparable to excellent single varietal wines; they just go about it in a different way.

EXCEPTIONS

The clear separation of blending areas and single variety areas is especially true in areas with a long quality winemaking tradition. In areas where wine has only recently been oriented towards high quality, such as Veneto or even Tuscany, there are exceptions to this “rule”, one could almost say abnormalities or even errors.

In cooler hazy Valpolicella, wine is made from several grape varieties, and yet Corvina alone gives brilliant results (for example La Poja by Allegrini); I have not been able to determine what benefit Rondinella and Molinara bring to the mix, I suspect that their use is dictated more by habit than by real necessity: unlike Merlot in Bordeaux, these added grapes seem to have no clear function, other than “being there”.

In contrast, much further south, the great Brunello has being ideologically forced to become a single variety pure Sangiovese wine, though from its inception a century ago Brunello had always used the typical traditional Tuscan blend of Sangiovese with a small percentage of Ciliegiolo, Canaiolo and even white Trebbiano and Malvasia, much like white Viognier gets added to the great Côte Rôtie red wines. Even stranger, many producers, including top Brunello names, had been more or less overtly cheating by blending Sangiovese with, of all things, Cabernet Sauvignon, Merlot or Syrah. As absurd or wrong as these blends are, they do indicate that in such climates the dominant variety alone rarely yields a fully satisfactory expression of terroir, and a return to the traditional blends would be a better solution than stupidly adding international varieties.

And finally there is the apparent exception of Champagne wines, sometimes single varietal (Chardonnay Blanc de Blancs) yet more often blended. We will see below that the Champagne area exhibits a particular key climatic characteristic that is in fact similar to that of other far more southern blending areas.

WHY LATITUDE?

So far we have reasoned in terms of vague notions of warm and cool climates, these notions being roughly correlated with latitude. Yet latitude is not a good climate predictor: the latitude of Ottawa, where no great wine is likely to be ever produced, is similar to that of Barolo. Neither is net solar irradiation: Amarone and Taurasi have similar solar irradiation, despite a 4.5 degree difference in latitude, yet neither the wine nor the climate can be described as similar. Clearly there must be some other factor that is influencing viticulture.

A hazy concept: Linke Turbidity

Some years ago I developed a kind of intuition that the factor distinguishing wine blending areas from single varietal areas was whether the sky was clear or hazy. Areas with crisp clear blue skies seemed to me more likely to blend than areas where the sky is hazy, irrespective of summer temperatures or solar energy. Having recently worked on projects in renewable energy sector, I used a tool normally employed to determine the energy potential of a photovoltaic solar energy system, hosted on the website “PV Estimation Utility” [ by the JRC (EU project) http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php# ]. This tool provides very precise data on solar radiation and sky conditions anywhere in Europe, including a very interesting figure, the TL factor, named “Linke Turbidity Coefficient,” which indicates the level of attenuation of solar radiation through the atmosphere, and the resulting haziness.

The attenuation measured with the TL factor is mainly due to scattering by aerosols and absorption by various atmospheric constituents, such as ozone, water vapor, oxygen, carbon dioxide, etc.. Technically, for any location, TL is the number of clean and dry atmosphere “thicknesses” that would produce the same attenuation as the atmospheric conditions of that location. The TL factor varies from 1 (fully transparent) to 2 (cold air very clear, for example during a Mistral in Provence) to 3 (very clear warm air) to 4-6 (stagnant or hot moist air) to over 6 (polluted air).

All this will require more investigation by specialists, but I suspect that lower TL implies higher direct light content and/or a wider spectrum of light, and therefore a more efficient photosynthesis: direct bright white light can better penetrate palisade cells in a grape leaf and reach more chloroplasts. Because TL does not take into account condensing aerosols (clouds), it is a better indicator of the quality of direct light than the also available D/G figure (diffuse light compared to global), which is an overall measure of diffuse energy.

Below is a spreadsheet of the average Linke turbidity coefficient, during the months of April to September, for some of the areas that produce the best wines in Europe.

This table also shows the average solar radiation on a horizontal surface for these six months, in MJ/m2, but in fact this information is not very revealing, since each area uses a grape that is well adapted to the local radiation conditions (by definition adaptation means obtaining maturity from the available energy); however it is worth noting that Mosel Riesling makes wonderful wines with less than half of the radiation received by Grenache in Châteauneuf-du-Pape!

The table also shows the Winkler index (GDD or degree-days above 10C from April to September) calculated by me from the PVGIS data by the heat summation method developed at UC Davis.

 

Here is the table, in order of decreasing TL, and showing values averaged over April to September:

Wine (varietal/blend)	Grape varieties	Lat.	Horizontal Irradiance     Hh (MJ/m2)	D/G = Direct /Global	Linke Turbidity TL	Winkler Index GDD
Vipava	Rebula	45.85	112.0	0.43	5.00	1630.1
Alsace (Kientzheim)	Gewurztraminer	48.14	103.0	0.49	4.73	1209.6
Tokaji	Furmint	48.12	111.5	0.45	4.67	1396.5
Wachau (Dürnstein)	Grüner Veltliner	48.40	96.6	0.52	4.58	1194.3
Rheingau (Hochheim)	Riesling	50.01	103.7	0.50	4.53	1169.9
W Burgundy (Meursault)	Chardonnay	46.98	111.3	0.45	4.53	1240.5
Amarone (S. Giorgio)	Corvina,Rondinella, Molinara	45.52	124.2	0.39	4.52	1792.9
R Burgundy  (Chambolle M)	Pinot Noir	47.19	109.5	0.46	4.50	1219.1
Mosel (Wiltingen)	Riesling	49.66	99.8	0.52	4.42	1068.7
Barolo (La Morra)	Nebbiolo	44.64	126.4	0.37	4.40	1676.6
Friulano (Cormons)	Friulano	45.96	119.6	0.40	4.38	1703.8
Kent (Tenterden)	Bacchus	51.07	103.6	0.50	4.35	903.4
Loire (Montlouis)	Chenin	47.39	110.2	0.46	4.32	1231.3
Condrieu	Viognier	45.46	117.8	0.40	4.32	1461.4
Ribera Duero (Valbuena)	Tempranillo, Cab S	41.64	138.8	0.32	4.17	1452.4
Morellino di Scansano	Sangiovese +	42.67	140.8	0.31	4.12	1894.9
Sicilia (Vittoria)	Nero d’Avola, Frappato	36.95	151.7	0.28	4.10	2318.2
Rioja (Logroño)	Tempranillo, Garnacha	42.47	126.8	0.36	4.08	1528.9
Verdicchio (Matelica)	Verdicchio +	43.26	124.6	0.38	4.07	1600.4
Brunello	Sangiovese… +?	43.06	130.6	0.36	3.97	1545.3
Bordeaux (St Emilion)	Cabernet S, Merlot+	44.89	120.8	0.39	3.93	1494.8
Hermitage	Syrah +	45.07	126.2	0.44	3.88	1559.6
Champagne (Avize)	Pinot N, Chardonnay+	48.97	98.9	0.49	3.73	1096.5
Taurasi	Aglianico +	40.92	133.3	0.35	3.68	1980.8
Douro (Vila Nova FC)	Touriga Nacional, Touriga Franca +	41.08	142.0	0.29	3.65	1669.6
Bandol	Mourvèdre, Grenache, Cinsault+	43.18	143.0	0.29	3.53	1872.9
Languedoc (Aniane)	Cabernet S, Syrah, Mourvèdre +	43.68	128.8	0.34	3.38	1817.3
Châteauneuf du Pape	Grenache,Syrah, Mourvèdre +	44.06	138.1	0.31	3.18	1845.0
Rias Baixas (O Rosal)	Albarino, Loureiro, Caiño Blanco	41.94	131.9	0.32	2.98	1506.2

 

Conclusion

Some interesting “conclusions” can be drawn from the above table:

• all great single varietal wines come from hazier sky areas, with TL > 4.2
• all the great blended wines come from clear sky areas, with TL < 4.2
• for very clear sky areas (TL < 3.65) blends can include a large number of grape varieties; Châteauneuf-du-Pape has one of the lowest values of TL, and blends up to 13 grapes!
• the Champagne area, unlike any northern viticultural area, is characterized in April to September by clear skies comparable to Provence, and like all clear sky climates its wines are mainly blended (except of course for Blanc de Blancs).
• there are interesting borderline cases: where TL is between the above thresholds of 3.65 and 4.2, fewer grapes are blended, and generally there is a dominant main grape and a small percentage of a secondary grape (see Bordeaux, Ribera del Duero or Hermitage).

I should defer to the knowledge of agronomy experts like my great friend Dr. Andrea Pitacco, who was kind enough not to laugh at me when I showed him my “findings”, to obtain a real explanation for the distinction between blend and varietal viticultural areas. And I hope I did not rile up too many defenders of the status quo in Montalcino or Valpolicella.

My modest hope is that wine lovers will pay a little more attention to the issue of blending as something that is not merely accidental, but is dictated by local conditions. I find it astounding that the best winemakers in similar quality winemaking areas came, through long experience, to the same conclusions, with very few exceptions.