Anyone who has ventured into the coastal valleys of California in the past few weeks has probably noticed a little bit of extra hustle and bustle around the myriad of grape vineyards as the 2002 harvest gets under way. One of the most important tools a grape grower can employ for a successful harvest is an understanding of the weather. To do this, viticulturists keep an eye not only on the sky but also on their computers.

Historically, varieties of grapes have been planted based upon the local climate and microclimates of a region. On a larger scale, warm climates produce grapes with high sugar, low acidity, low color, low flavor and high yields. Conversely, cooler climes result in grapes with low sugar, high acidity, high color, high flavor and lower yields. This is why we find table grapes like the Thompson seedless in parts of the hot Central Valley and varieties like chardonnay and pinot noir in the cooler coastal valleys.

One way the proper climate is determined is by looking at the number of ``growing degree days'' for each region. A growing degree day (GDD) is a calculation of the amount of heat that is received and is similar to the heating and cooling degree days that are used in estimating heating and air-conditioning usage.

A GDD is determined by subtracting 50 degrees from the daily average temperature, which is the maximum temperature plus the minimum temperature divided by two. For example, if the maximum is 80 degrees and the minimum is 50 degrees, the average for the day would be 65 degrees and the number of GDDs for that day would be 15 (65-50=15).

The total number of GDDs are accumulated for the season, indicating the types of grapes that would be successful in a given region. In general, if a region has an annual GDD of less than 2,500, look for chardonnay, pinot noir and white riesling. If the GDD is from 2,500 to 3,000, you will find chenin blanc, cabernet sauvignon and gamay grapes, while in an area with an annual GDD of 3,000 to 3,500, you will find zinfandel varieties. When the GDD is above 3,500, the most common grape varieties are those of dessert wines, table and raisin grapes. A GDD above 4,000 is where you will find Thompson seedless.

In areas with hilly terrain, the number of GDDs is reduced by about 70 for each 100 feet of elevation. This is why you may see one variety on the lower slopes of a vineyard and a totally different one farther uphill. It is also why many vineyard operations have thermometers at several elevations to monitor the seasonal accumulation.

In addition to GDD, most grapes need a frost-free period of 165 to 180 days, and winter freezes cannot be too severe. Some viticulturists monitor the number of hours that the temperature is between 70 and 90 degrees by having networks of weather stations in their vineyards, to more accurately determine when to harvest.

In general, the perfect wine season is one that is preceded by a cool, wet winter followed by a lack of rain or frost as spring begins to warm. During the summer months, mild temperatures with no heat waves and no rain are ideal, with warm, dry days immediately before and during the harvest.

Q. You seem to be the local expert on La Niņa and expected rainfall. Does it look like this winter will probably be a La Niņa winter in the Bay Area with more than the usual rainfall? Loretta Matheson - Palo Alto

A. We are done with La Niņa, and a weak El Niņo has developed in the tropical Pacific and will persist through the winter. But unlike the strong El Niņos we had in 1982-1983 and 1997-1998, a weak event does not mean a bias toward either a wetter or drier than normal winter here in Northern and central California. The bottom line is that with a weak El Niņo, we typically see the usual mix of wet and dry weather, with everything from flooding to drought possible.

Q. I understand that wind chills are commonly calculated for atmospheric conditions when air temperatures are less than 50 degrees. Where can I find a chart that illustrates the cooling effect of wind (i.e., as a result of a wind chill factor) where the air temperature is in excess of 50 degrees? Bruce Filan - Victoria, B.C., Canada

A. The wind chill index (WCI) formula is designed with little impact from the wind if the temperature is above 50 degrees. Consequently, most WCI table do not show values above 50 degrees. For example, with a temperature of 60 degrees and a wind of 20 mph, the WCI is only 57. Go to
http://ggweather.com/windchill.htm  and there are links to an online calculator and an Excel spreadsheet that you can plug some temperature and wind numbers into.

Q. I like to go up to the top of a mountain and enjoy the view. Unfortunately, very clear days seem to be unpredictable. One formula I have tried is to look for a high over Oregon or Idaho, which should blow away the smog. This does not seem to really work. Can you suggest a better formula? Harry Pottol - Sunnyvale

A.  Probably the best weather pattern to watch for is when high pressure builds into Oregon and Northern California behind a cold front. The rain with the front will scour out some of the haze, and the northwest winds push what's left out of the area. But this usually doesn't happen with any regularity until we get into November. When we get offshore flow (higher pressures over Idaho and the northern Great Basin) during the warmer months of summer and early fall, there is also the formation of smog that gets trapped in the interior valleys and hinders visibility.