The climate of Bordeaux is maritime, with both the Atlantic and the Gironde estuary acting as moderating influences, protecting the vines from winter freezes and spring frost. Winters are short, springtime is usually damp and summers can reach high temperatures. Coastal pine forests protect the Médoc vineyards from harsh west and northwesterly winds, but rain is a worry, especially at harvest. The climate becomes more continental toward St-Émilion and Pomerol.
The wet spring season and humidity of the region can result in severe problems with mold and rot. While autumn humidity in the sweet wine districts of Graves is a cause for celebration due to the development of noble rot, the more malevolent grey rot can take hold in cooler years and destroy the fruit. Springtime frost can lead to coulureand millerandage, reducing the eventual yield, and the Bordeaux mixture – a mix of lime, copper sulfate, and water – is applied throughout the region to avoid fungal problems.
The Bordeaux climate, while less marginal than the Loire, Burgundy or Champagne, is nonetheless varied enough to produce wide vintage variation in the finished wines. Therefore, careful knowledge of the overall qualities of Bordeaux vintages is required for the sommelier to be a successful salesman of Bordeaux wines.