In the run up to harvest, vignerons throughout France wait with bated breath and crossed fingers for their grapes to ripen perfectly.
When we talk of a great vintage, it often comes down to the weather before and during harvest. First of all, we don't want hail, which can strip the grapes off the bunches and damage whatever is left.
Rain is a mixed blessing. In 2016, France saw drought conditions in many wine regions which prevented the grapes from swelling to their proper size as well as preventing the vines from transpiring. When plants lack water, they close their stoma to prevent transpiration. This effectively slows down their rate of photosynthesis and respiration, which means that they don't ripen their fruit as quickly. So a hot dry year can lead, bizarrely, to unripe fruit.
On the other hand, too much rain can cause the grapes to swell too quickly, diluting the flavours and perhaps even splitting the skins, which, combined with the increased risk of botrytis from damp conditions, will lead to the bunches rotting on the vine.
Temperatures are important too as they determine the rate of ripening. Too cold and the fruit will produce tough green tannins and high acidity. Too warm and the acidity will fall too quickly, producing flabby wines with no freshness.
Most vignerons measure the sugar content of their grapes to determine harvest date. Every AOP and region has an upper and lower limit for potential alcohol, the percentage of alcohol the sugar will produce during ferment. Equipped with a refractometer, we can measure the specific density of the grape juice which is directly related to the sugar content.
To get a more precise picture of ripening, more exacting estates will measure the acidity of the juice using a pH meter and/or titrating with sodium hydroxide and phenolphthalein (for those who remember their school chemistry) until the juice changes colour.
As acidity falls and sugar rises, the grower tries to predict the perfect picking date, when the juice will have the best balance to make the best wine.
However, every grape, every bunch and every vine are slightly different. No vineyard is homogeneous and we want to pick the whole thing in one go. So the smart vigneron will devise a way to collect a representative sample of the whole vineyard. I do this by walking the rows in a fixed pattern, picking 5 grapes from a random bunch every 20m. I take two grapes from the top of the bunch, two from the middle and one from the bottom. When I have about 100 berries, I squeeze the juice out and take my measurements. I do this in every vineyard every three days. That takes a lot of time but I believe that a vineyard only has a three-day window where it can produce its most balanced fruit and I want to get it right.
To decide the perfect picking day, I need to look at the weather forecast to make sure there is no rain coming. Rain will cause the grapes to increase their water-content, diluting the juice and changing the acidity and sugar levels. I also don't want to pick in the rain because all that extra water on the grapes will dilute the juice too.
So after much gnashing of teeth and poring over figures, charts and weather forecasts, I choose the day and phone all my pickers. We start at dawn when the grapes are coolest. That way the ferment will not start too quickly or be at too high a temperature. It also means the workers will not suffer in the full heat of the afternoon sun and that we can have all the grapes safely in the winery before it's time for a cold beer and the traditional vigneron's lunch of a baguette and cheese, before we go out sampling and testing again to decide the next picking date and which vineyard to pick.