Why is Diurnal Shift So Important?

         The Bordeaux wine region has a sizeable diurnal shift 

On all Wine-Knows’ tours to the world’s greatest wine regions the term diurnal shift comes up several times.  It is always mentioned during WineKnows’ opening seminar, and most winemakers discuss it at least once during the tasting of their wines.  This blog will discuss how this phenomenon is responsible for separating the top wine districts  from those producing simple table wines.

First, let’s review what the diurnal shift is.  Simply put, the diurnal shift is the difference between a vineyard’s daytime and nighttime temperature.  While the notion might be simple, however, this difference in temperatures dramatically effects the quality of a wine’s structure and complexity.

                           The Port region of Portugal also has significant day & night temps

A large diurnal difference helps grapes to ripen in a more balanced way, and therefore, to maintain the structure of the wine.  The warmer the day temperature, the better for sugar development.  The lower the temperature at night, the better it is for grapes to preserve their acid structure to balance these sugars.  Furthermore, a significant diurnal difference allows grapes to "rest" at night anf, thereby, to preserve their delicate aromas.

           Huge diurnal shifts appear in Argentina's premier wine district near the Andes 

World-class wine regions depend upon these large day-night temperature variations to make wines of deep complexity with great structure.   For example, the Napa Valley is <30 miles from the Pacific Ocean.  Daytime temps in the valley during summer can easily exceed 100 degrees, but the valley floor temps drop at nighttime due to the marine influence of cooling air from the ocean and nearby San Francisco Bay.  It’s not unusual to see a diurnal shift of 40 degrees.

The Central Valley of California (>150 miles further south), has similarly hot day temperatures during the summer grape ripening period.  In contrast to Napa, however, its nighttime temps remain warm as there are no large bodies of water to moderate the heat.   The Central Valley has little diurnal shift, hence, this is the main reason why its wines lack the complexity and structure of Napa.

                 World-class Rhone Valley wines are made in a climate with big diurnal shifts

Diurnal temperature swings are critical in making topnotch wines with great aromas and a balanced structure.   All of the world’s greatest wine regions have this day-night large temperature fluctuation.  Think of diurnal shift  as a winemaker’s dream.