Yes, Virginia. There is a huge difference between light and heat

There is a rather poor way to measure the quality of a grapegrowing season. It is Growing Degree Days (GDD's), which in the US are calculated in three different ways. The most common is to calculate the average temp for a day [(max temp + min temp) divided by 2] and then subtract 50 degrees F. (50F is the baseline.) If the result is negative (which is the case today, Nov 18, where the high is not even 50F), then just give that day a zero. Then, you add up all such results, for each day of the year. For 2010 in the Portland area, this results in a total GDD for the year of about 1800 GDDs. In 2009 it was about 2200, or 22% more.

So why is this method a lousy proxy for sunshine? Because GDDs vary somewhat independently of sunshine. (What we need is a new way to measure and report actual sunshine units.) Here are some examples proving the shortcomings of GDDs:

1. Think of the US Northeast. It is often cloudy there in the summer, although it can get quite warm. And the nights are generally warm also, due to the high humidity. So the GDDs are pretty high (for a northern climate), but the actual sunshine on the grapes is low. There, the GDDs are misleadingly high, and only extreme Northern grapes tend to ripen there.

2. Think of West Texas. There, the sun is amazingly bright (because of the high altitudes--about 4000') and it shines on those high plains almost every day of the year. Yet, due to the high altitude and low humidity, the average temps, even in summer, fall pretty far at night, which drags down the day's average temp. So the GDDs there are misleadingly low, even though the sunshine is very strong and frequent and the afternoon temps are often in triple digits.

3. Now, consider Scandavia and Southern England. (Yes, wine grapes are being grown there now, just as they were back in Roman times.) There, the high latitude makes the sunshine very bright in summer, and the days are very long, so the actual sunshine totals are pretty high, although the GDDs, due to the cool temps, are very low.

I'm paraphrasing Bill Shoemaker, who is a grape breeder at U of Illinois: "Heat is like an accelerator; it regulates the rate of the plant's physiological engine. Light, on the other hand, is fuel [it is one of the inputs which the plant uses, via photosynthesis, to make sugar and cellulose from sunlight, water, and minerals]. Respiration is also key; it consumes the products of photosynthesis. Respiration is higher wherever the temp is high, so in Scandanavian nations, where temps are cool, respiration is lower and the plant retains more of its sugars. This (combined with the long sunny days in summer that far North) allows the plant to ripen its fruit fully, even though temps are cool and the growing season is shorter." The Portland area is similar.

This is a major reason why the same grape varieties will fare so much differently in different geographic settings. Then there are other important factors, such as soil type, drainage, color of soil and surface rocks (which affects heat absorption and reflection), wind, winter minimum temps, speed of onset of winter, etc. Altogether, it really does make "terroir" a critical component in winegrapegrowing. And it truly makes every wine, and even wine from each plant, uniquely different.