Diurnal Temperature Change

During our two-week plus adventure in Napa Valley, we spoke an awful lot about the diurnal temperature changes, but we did so in a way that made it seem that it should be obvious what was going on and why it is important. But, that's not exactly the case. Today, we're going to peel back the onion a bit, but keep that onion far from the wine because I don't generally like wine with onion notes in it, and explain why diurnal temperature change matters and in doing so show how it makes Napa Valley one of the perfect growing areas in the world.

First, what is it? Diurnal temperature change is exactly what it sounds like or at least it does if we know what all the words mean. In a nutshell, how different are the daytime highs and the overnight lows?

Why do we care? Let's get a little bit geeky. You remember your junior high school/middle school life sciences class where you learned about things like photosynthesis? If you're as far removed as I am, it might be time to go to your personal wayback machine and see what you can recall.

So, here's what's going on. Warm or hot days with long-lasting brilliant sunshine result in faster development of sugars in the grapes through that photosynthesis process. But, then the cool nights slow down the aging process and put the grapes into temporary hibernation. 

Think of it as a workout for the grapes during which they are building up their bodies. Workout during the day using the sun and heat as stimulants and then a nice night's sleep to rest up for the next day. Or, put in more viticultural terms, the heat and sun drive up sugar levels and tend to reduce acidity (pH increases), but the cool nights slow or even temporarily stop sugar development while preserving the exisiting acidity (pH decreases). All of this produces an extremely rich, yet balanced wine.

In the coming days, we'll continue geeking out and understand why other elements of the terroir matter. What does elevation do? What does soil do?

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