Luminosity - the Sheer Quality of Sunlight

Red wine grape clusters on the vine

Cleanest air in California
Altitude + clean air = solar maximum

5% - 10% higher UV levels than sea level
Thicker skins = phenolics, complexity

Lake County winegrapes receive excellent Ultraviolet radiation (U.V.) exposure. With as much as 10% more exposure than neighboring sea level valleys, increased U.V. triggers thicker skins, greater tannins, and intense wines with high phenolic content.

Peter Molnar, past Chair of the Lake County Winegrape Commission, talks about the effect of elevation and U. V. levels on winegrapes in this video.

Ultraviolet radiation is the electromagnetic radiation of the sun beyond the violet end of visible light, which our eyes cannot see. Developed by the National Weather Service and Environmental Protection Agency, the U.V. index scale measures the intensity of the sun’s ultraviolet radiation. Local factors including elevation, season of the year, and latitude affect the amount of U. V. light that reaches the ground.
At high elevations, the atmosphere thins and is less able to absorb U.V. radiation. With every 1000-foot increase in height, according to the National Institutes of Health, U.V. levels increase by about 4%. For Lake County vineyards planted between 1370 feet above sea level and as high as 2,600 feet above sea level, this means that UV exposure increases between 4% and 12% depending on location.
Dig Deeper - Solar Radiation
Incoming solar radiation (insolation) provides the energy necessary for grape growth and maturation. Throughout the growth stages of the grapevine, the amount of insolation is critical in maintaining the proper levels of photosynthesis. The most critical stages come during the development of the berries (yes, grapes are called berries) starting at bloom and continuing through the harvest. During bloom, high amounts of insolation result in effective plant tissue differentiation into flowers.
Low absolute insolation during the bloom stage can influence coulure or the failure to fully flower and set berries. The relationship between low amounts of insolation and coulure is not linear, nor predictable, but is more tied to cultivar characteristics. During the ripening of the berries, insolation mainly acts to control the amount of sugar in the grapes, and therefore, the wine’s potential alcohol content.
Controls on the amount of insolation include: 1) those that are inherent with Earth/Sun relationships, such as overall amount received by any point on the surface of the Earth, seasonal variations in the angle of incidence of the sun’s rays, and the day length, and 2) those that are controlled by variations at or near the Earth’s surface, such as cloud cover, the reflective nature of the surface of the soil, and the role topographic variations (slope, aspect, and obstructions) have on the relative amount of insolation received.

Grape growth chart by month

from "Climate Characteristics for Winegrape Production in Lake County, California", Dr. Greg Jones