n. As a biological rhythm in the simplest definition means daily. Diurnal can refer to a cycle specific to animals and plants – those that are active during the daytime versus nocturnal. In astronomy, diurnal motion refers to the motion of our stars around the Earth. From climatology, diurnal temperature variation is the cycle of daily temperature change.
The daily temperature variation is imperative to agriculture, especially in viticulture – the science of growing grapes. The diurnal shifts experienced are specific to a region, vineyard, and vintage. This pattern can put a fingerprint on any agricultural commodity that is grown during the entire year. The entire Santa Barbara County wine region has a distinct diurnal temperature fluctuation that renders warm days to near freezing conditions at night.
The beginning of the 2015 vintage well illustrates the extremities of diurnality. Throughout the end of March and early April temperature swings have sometimes been baffling, from record heats to freezing nights. For example, just after Easter of 2015, a record high of 90˚F hit followed that evening by a 5:00 AM frost, a near 60˚F shift in just one day. To viticulturist, this diurnal temperature change ensures that vines grow during the day and sleep at night; scientifically this translates to what is commonly called the “light” and “dark” reactions.
Photosynthesis, the engine behind making plants green and producing sugar upon consumption of CO2 and sunlight, is hence known as the “light” reaction. Acid respiration, the process of wine grapes slowly becoming less acidic, via various paths like glycolysis and the Krebs cycle, is more temperature dependent and thus can occur independent of light – hence we call it the “dark” reaction. The temperature influence regulates a wine grape’s acidity, the cycle shuts down in temperatures below 55˚F thus retaining more natural acidity ultimately in the wine.
Given mildly hot days during our peak part of the growing season coupled with cold nights from the diurnal temperature change, vines will produce wines that have terrific phenological ripeness with high acidity, both key components to make wines that are truly age worthy. Coupled with drought cycles, vintages like 2015 offer promise for an estate vineyard to render wines with complexity and acidity unparalleled. Fortunately the almost patented diurnal temperature shift of the Ballard Canyon American Viticultural Area (AVA), the home of the Larner Vineyard, ensures that every vintage is defined
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