Definition - What does Diurnal Variation mean?
Diurnal variation is a meteorological term used to describe the variations of temperature that occur throughout the day. Typically, diurnal variation keeps track of the highest temperature and the lowest temperature for a specific time period, usually per day. In the vineyard, keeping track of the diurnal variation, allows wine grape growers to monitor the ripening of the grapes and prepare for harvest.
WineFrog explains Diurnal Variation
Growing wine grapes requires that growers learn about all aspects of agriculture, climate and meteorology. Understanding how meteorology affects grapes allows growers to chose the right variety of grape for their region and ensures that the grapes will have the highest amount of sugar and acidity possible at harvest. The fluctuations in diurnal variation affect the ripening patterns of wine grapes. Warmth and heat during the day will boost production of sugar and raise brix levels - cooler temperatures (most often at night) allow for the grape to maintain acidity levels.