Diurnal Temperature Variation – What and Why?

Diurnal temperature variation is most simply defined as the difference between daytime high and nighttime low temperatures – you know, what the weatherperson on TV reports each day.  OK, but what does that have to do with growing wine grapes?  In order to ripen grapes that make good wine, vines require sunlight, the fuel that drives photosynthesis to generate sugars in the grapes, and reach suitable ripeness levels.  Grape skins respond to heat from the sun, either remaining thin in lower temperature climates, or thickening to preserve grape moisture in warmer climates.  This is important since most of the tannin and color in (red) wine, and a good portion of the flavor, come from the skins.  This relates to why some grapes do better in cooler areas, while other varieties can prosper in hotter climes. 

Water from the soil, carrying nutrients and minerals that ultimately generate flavor and texture components in wine, is transported through the vine into the grapes.  Transport through the vines and accumulation of critical components in the grapes, along with sugar development and flavor integration, mostly occurs when the sun is NOT shining and vines cool off at night.  The greater the diurnal temperature variation, or the cooler it gets at night, the more the vine and grape clusters literally get to rest while accumulating and integrating sugars and flavors.  This can be loosely translated into the commonly used term, “hangtime.”  Greater DTV allows grapes more hangtime, and more hangtime allows grapes to fully ripen before soaring sugar levels literally force growers into a pre-mature harvest.  Climates that provide grapes more hangtime can produce riper fruit with more generous flavors, making the winemaker’s job much easier. 

Diurnal temperature variations are important for all of the world’s wine regions.  The primary factors that influence DTV are absorption of the sun’s heat during the day vs. radiation of heat from the earth at night.  Other factors that impact DTV include proximity to bodies of water (ocean, lakes, rivers, etc.), elevation, cloud cover (or not), humidity and length of days.  Below are listed some typical August harvest season hi/lo temps for various Texas growing areas. 

          Davis Mtns -        hi 90     lo 55             Ft Stockton -                 hi 100    lo 65-70

Texas vineyards get HOT during the day, but much of the state does not cool very much at night during grape ripening season.  In the TX Hill Country, the DTV is typically 20 degF.  The DTV is greater on the High Plains, 25-35 degF, and only in the high desert mountains does the DTV go over 35-40 degF.  Compare this to some California growing regions where daytime temperatures are 5-10 degF cooler than in Texas, and typical DTV’s can be 30-40 degF or more. 

          Santa Barbara -  hi 80-85     lo 50-60       Mendocino -        hi 80-90     lo 50-60

Comparison to the above DTV’s illustrates why Texas growers and vintners have to deal with more significant heat issues and generally less hangtime in the vineyard than their counterparts in California.

Now, let’s return to those original questions.  First, from the DTV’s shown above, the Texas High Plains appears to have a better DTV than most of the rest of the state.  The Davis Mountains have the greatest DTV, but limited road and water access make grape growing in that area a significant challenge.  Thus, the Texas High Plains, in particular the region around Brownfield in Terry County, has become the key grape growing area for Texas wine production.  As for pinot noir and chardonnay grapes, as well as many other cool-climate varietals, vineyard areas with preferred DTV’s of 30-50 degF are not common in Texas.  The hot daytime temperatures and limited nighttime cooling are simply not conducive to growing cooler climate grapes. 

Thus, Texas heat creates significant challenges to those brave souls who make the attempt to grow ANY grapes, but especially those that prefer cooler climates.  This has resulted in the proliferation of grapes in Texas vineyards that originated in warmer climates, the so-called Mediterranean grapes, like tempranillo, mourvèdre, viognier, roussanne and montepulciano.  We should all tip our Stetsons to the grape growers of Texas for stepping up to the hot climate challenge, and offer them encouragement to continue the efforts that will help make Texas a world-renowned wine region. 

Adapted from various sources, including Winemaker Magazine, Wine Spectator, Wine and Spirits and Food and Wine.  Particularly informative was a blogpost by Jane Nickles, “thebubblyprofessor”, that addressed the DTV subject, and from which I have chosen to borrow liberally for this segment.