To the winemakers of the New World, and hence to Australia and New Zealand, climate is the most significant factor (outside their control) impinging on grape quality and wine style. For the winemakers of France, terroir (or soil) is equally important.

Indeed, if one looks at Bordeaux and Burgundy, France's two greatest wine districts, and then narrows down the microscope to their principal subregions, climatic variation has little relevance, and terroir becomes all-important in determining the character of the wines. For example, the Haut-Medoc is the same as that at Chateau Lafite Rothschild, 25 kilometres to the north, and abutting St Estephe. Similarly, if you consider the Cote de Nuits, the climate of Nuits-St Georges at the southern end is identical to that of Gevrey-Chambertin at the northern end.

It is true that spring frosts and summer hailstorms may hit one spot and miss another, and no less true that one chateau or grower may be more successful than his neighbour in one year but not the next. Even more so is it true that climatic swings from one vintage to the next are of crucial importance in shaping the quality (and to a lesser degree the character) of the wines of each vintage. There is a fundamental distinction between climate and weather, and by their very nature these swings or changes cannot usefully be individually recorded; one inevitably has to take long-term averages in ascribing temperature, rainfall, humidity, wind, frost and whatever other data one wishes to use in presenting an overall picture of the climate of a region. So it is understandable that the French tend to take climate for granted, and to look to the effect of terroir to explain and characterise their wines.

All of this in turn proceeds on patterns of classification and constraint which have been built up over many centuries, even if formal codification did not start until the middle of the nineteenth century and only gained legislative teeth in the twentieth century.

How different the position of the New World. There are effectively no constraints on which grape varieties you can plant, how you prune them or how you use and blend the wine you make from the grapes. Almost every one of the regions discussed in this book is of much larger scale and of more diverse topography than most of the regions of France: Coonawarra and Padthaway in South Australia are two exceptions on the Australian front, and (superficially) Marlborough might appear to fall into the same category in New Zealand.

If this were not enough, the New World's experience in matching terroir, climate and grape varieties is typically less than a century old, and frequently less than a decade old. As time goes by we may well see a greater degree of specialisation, although not to the degree found in France.

So with an impossibly complex matrix of grape variety, soil, aspect and topography within each Australasian region (and each subregion) we have had little option but to come back to climate as the most significant factor in determining wine character. But in doing so the experts in Australasia and the United States have encountered great difficulties in providing climatic indices which are on the one hand sufficiently succinct to be understood and of practical use, and which on the other hand are meaningful and reasonably accurate.

Nonetheless, most attention has focused on temperature as being the most important aspect of climate in determining wine style. In 1944 the distinguished American oenologists Amerine and Winkler introduced a classification system which traces its roots back to 1735, and then to the mid-nineteenth-century observation by de Candolle that there is little vegetative growth in the vine at temperatures below 10?C (50?F). Amerine and Winkler assumed a seven-month growing season (in Australasia, this means October to April) and calculated what is now called heat degree days or HDD by taking the difference between 50?F (or 10?C) and the mean temperature of the month, multiplying that difference by the number of days in the month, and then adding the resultant figures for each of the seven months. Having done that, they then divided California's wine regions into five, with 500-day degree increments, starting with Region I (the coldest) at less than 2500 HDD (expressed in degrees Fahrenheit).

The system has been refined, adapted and also roundly criticised, but remains the most widely used and understood system available, and in the regional summaries which appear throughout this book I have given the HDD figure. Nonetheless, it is interesting to look at the criticisms and refinements, and at the same time to recognise some decidedly curious statistical anomalies within the system.

The major refinement has come through the work of Dr John Gladstones, published in his Viticulture and Environment (Winetitles, Adelaide, 1992). This superb book, without parallel in any work published in English, explains and discusses all the factors which influence the way vines grow and grapes ripen.

I seriously contemplated abandoning the tables kindly provided by Dr Richard Smart, but the task of condensing Gladstones' masterpiece proved beyond me. In any event, his chief criticism of the Smart and Dry approach can be summarised. It is that it fails to adequately take into account temperature variability, and in particular fails to recognise the importance of the even accumulation of heat over the growing season (and, indeed, over each day in the season).

This, in turn, derives from the dual observations that the optimum temperature band for the ripening of grapes is between 20?C and 22?C, and for vine growth between 23?C and 25?C. The total span of 20?C to 25?C is narrow enough, that for grape ripening is even less. Quite obviously, there is no wine region in the world which precisely and regularly conforms to this ideal, but in John Gladstones' words: `A general conclusion can be drawn.

The narrower the range of variation about a given mean or average ripening temperature, the greater the grape flavour, aroma and pigmentation will be at a given time of ripening and sugar level. In cool climates the grapes will achieve full flavour ripeness even though sugar may remain low and acid high. In warm, sunny climates they will attain full flavour ripeness before sugar level and pH have become too high, and acid too low; or breakdown processes have started to pre-dominate in the berries. In all cases the colour, flavour and desirable aroma qualities in the wine that can only come from fully ripe grapes are enhanced relative to potential alcoholic strength.'

So, I have elected to use a composite table kindly provided by Dr Richard Smart which incorporates figures collated by him (and others assisting him) over many years. His primary indices (MJT and HDD) are measures of temperature; in the final analysis all commentators come back to this factor, simply using different numbers or expressions to measure the same thing. So in a sense it does not matter whether you look at HDD, MJT or MTWM; if mean numbers are high, you have a hot climate and an early vintage; if they are low, a cool climate and a late vintage.

On a broader view, there are climates within the Australasian wine growing regions which correspond to almost any Europeandistrict you care to name, and some well known French regions have been included for the sake of comparison. But no two districts, no two subregions, will ever precisely mirror the combined climate and terroir of each other. It is for this reason that books such as this are written and (hopefully) read.