Spain's vineyards are characterised by their often extreme climatic conditions. Vines may be found growing in desert or volcanic regions or in landscapes blanketed by snow. They can grow successfully in places with very low annual rainfall, like the central and south-eastern dry regions, or in extremely damp areas like those of north-western Spain, influenced by Atlantic weather. In fact, in some areas the strength of the winemaking tradition is encouraged by the grapevine being the only plant capable of surviving and flourishing in extreme temperatures. Nonetheless, in general terms, vineyard development benefits from a warm and relatively dry environment with plenty of sunlight, long summers, and winters that are not too harsh.
Temperature is a key factor in regulating vines' basic functions such as respiration, transpiration and photosynthesis, which are activated by warmth. If temperatures are high, ripening speeds up, yielding sweet and sometimes fortified wines (licorosos). In the northern growing regions, or those at high altitude, where temperatures are lower, ripening is slower and more difficult, producing wines with a lower alcoholic content and marked acidity.
Moderate conditions play a vital role in the vine's physiological characteristics. In northern and north-western Spain, vines receive 2,000 hours of direct annual sunlight, and in the Gulf of Cadiz and certain south-eastern areas, more than 3,000 hours annually - one of the highest figures worldwide.
Rainfall is a key factor in the development of the vine, affecting not only the harvest's volume but also the fruit's quality. Winter rainfall improves the quality of the wine harvest while a modest shortfall of summer rainfall helps to produce abundant grapes, although not necessarily of the best quality. Very often the best vintages coincide with hot, dry summers.
Other weather features also affect Spanish vineyards. Frosts can damage the plant when it is producing new shoots during its growing cycle. On the other hand, vines can endure temperatures as low as -15°C if a cold spell strikes while they are dormant in winter. Wind can be a positive or a negative factor, depending on its speed, strength, persistence, and dampness: in general, moderate, slightly humid winds are beneficial. Mists and fogs always have a negative effect, especially during the growing season.
Finally, it should be noted that Spain possesses many vine-growing areas that benefit from special microclimates. The O Rosal, Priorato, Ribera del Duero and Sanlúcar de Barrameda regions are famous examples, but the list could be much longer. These microclimates, together with particular aspects of physical geography like the contours of the valleys and slopes, give each area special growing conditions and, hence, wines with a marked regional character.