The world of wine contains many surprises for the unwary. One rule of thumb is that as vineyard altitude rises, the average temperatures drop (0.6º C per 100m is the standard figure) and the resulting wine styles become fresher and more acidic. But Napa is different in this regard. As any wine student will know the primary cooling agent in Napa is the effect of morning fog rolling off the ocean. As a result, the coolest areas are at the bottom of the valley below the morning fog and temperatures rise as the effect of the fog reduces. Adam Lechmere (Decanter) and Rosemary Cakebread (of the eponymous winery in Napa) explained some of the other factors at play in a masterclass at the 2016 London Wine Fair.
Alongside the effect of fog, the further complication is what is known in meteorological circles as the ‘inversion layer’. Normally the air near the ground is warmer than the air above it, as we can experience as it gets warmer as we come down a mountain. This is due to heat from the sun being absorbed by the earth which then re-radiates it to the atmosphere directly above ground level. However, in certain conditions (near warm fronts and ocean upwellings) a warmer layer of air can be created around 15m above the ground. A study in New Zealand reported that the temperature in the inversion layer could be typically 3-5º C above that at vine level (1.3m), and occasionally 14º C higher! This is the phenomenon exploited by wind machines which basically mix up the warmer air from the inversion layer with the colder air beneath it, with the desired effect of keeping the vines above freezing point.
Frost protection apart, the inversion layer also leads to the paradoxical situation that in general sites on the hillside (generally 200-400m) in Napa can be warmer than those on the valley floor. Cooler air pushes hot air upwards and as a result, nighttime lows are higher on the hillside than on the valley floor, though day times highs are also lower. So there is less difference between day and night time temperatures on the hillsides.
And finally (I hear you saying), does all this make any difference to the wine in the glass? Generalisations are always dangerous as there are so many factors in play – topography, soil types, climate, aspect, row orientation, winemaker choices. But in general Cabernet Sauvignon wines from fruit grown on the valley floor are more red-fruited and floral with higher acidity due to both the effect of the fog and the inversion layer effect. By contrast, the wines from the hillsides are more black-fruited and, due to poorer soils and wind, have smaller berries which in turn are more tannic. Napa really does stand on its head!
In addition to the very useful summary on Wikipedia (‘inversion layer’), the best study of all this is the older but valuable New Zealand study: Practical Considerations for Reducing Frost Damage in Vineyards, 1999, M.C.T. Trought, G.S. Howell and N. Cherry