If you had to judge a vintage from five measurements, these would be the ones. Each has a long pedigree in viticulture science, and each tells you something concrete about the wine.
1. Growing-season warmth
Heat accumulation from spring to harvest sets the ceiling on ripeness. Too little and the wines are lean and herbaceous; too much and they can turn jammy and low in acidity. Indices like the Huglin Index were designed precisely to place a region on this scale, and warming has been pushing Bordeaux up it for thirty years.
2. Harvest-window rainfall
Rain in the weeks before and during picking is the classic spoiler — it dilutes flavour and invites rot. A dry, settled harvest is one of the strongest fingerprints of a celebrated Bordeaux year.
3. Cool nights
Warm days paired with cool nights help grapes ripen sugar and tannin while keeping acidity and aroma. The Cool Night Index captures exactly this, and it is part of why river-cooled, well-exposed sites can shine in otherwise hot years.
4. Heat spikes
It is not just average warmth but extremes that matter. Spells above the mid-30s °C can shut vines down and scorch fruit. Counting these days separates a warm-but-balanced season from a stressed one.
5. Spring frost
A single cold night after budburst can wipe out a share of the crop, as Bordeaux learned painfully in 2017. Frost exposure is about volume as much as quality, and it is the kind of risk an estate wants flagged early.