the WEATHER CORNER

BY JAN NULL
Special to the Mercury News

`Camelot' index precipitates mail

In the last Weather Corner, I explained my qualitative climate index for the hypothetical ideal climate, in which it was not too hot, not too cold, not too wet and not too dry. I called it the Camelot Climate Index, and it listed San Diego at the top, followed by a tie between San Jose and San Francisco. Here are some reader responses.

Q. Your Camelot Climate Index is quite interesting. I imagine that San Luis Obispo would rate pretty high, but it is probably too small to be included in the metro statistics. Do you have any information on SLO? Ian Jacobsen - Sunnyvale

A.  Unfortunately, the database I used to construct the CCI had only about 170 sites that had all the parameters. However, you are correct in assuming that San Luis Obispo would rank pretty high. I would estimate it would be at least as high as San Jose and close if not tied with San Diego at the top of the list.

Q.  King Arthur apparently failed to take account of wind, although in some areas it is the most dominant weather factor. Mariah, for example, ``makes the mountains sound like folks are up there dying.'' Waylon Jennings sings, ``The wind blows down the Texas plain. It makes some people go insane.'' And, of course, from Gordon Lightfoot: ``Superior . . . doesn't give up its dead when the gales of November come early.'' 

Despite the subjective and objective importance of wind, I can't find any information relating to average or seasonal wind conditions for given areas. My personal Camelot index would take into account such factors as average maximum wind velocity by month, number of days with wind velocity in excess of a certain speed, and so forth. Where is such information available? Joel Kahn - Half Moon Bay

A.  As much as I would have liked to include wind, the available data is very limited and would have reduced the sample even more. Plus, wind is a difficult parameter to include because you have to consider both wind speed and direction.  It also is very dependent on the time of day. For example, the average summertime winds around the Bay Area are strongly affected by the afternoon sea breeze. Consequently, a daily average would not be representative of much of anything. It's the same with wind direction. An average direction may miss the significant events. As far as data sources, there is very little on the Web. Some CD-ROMs are available from the National Climatic Data Center that have some detailed information, but they tend to be a little pricey.

Q.  Your CCI index is just not going to convince anybody that Eureka's weather is as good as Honolulu's. I understand your methodology but still . . .  Dinesh Desai - Los Altos

A.  Most people would agree with you. There are lots of ways that a climate index can be developed, and the one I used is biased by temperateness and mild weather. Consequently, Honolulu lost points because of the number of days hotter than 90 and its relatively high heat index.

Q.  During last week's testimony before the California Public Utilities Commission about the ongoing energy crisis, one of those interviewed said drought is part of the problem. I know that there has not been much rain lately, but is there really a drought? Tom Robinson - Redwood City

A.  I've also heard comments about ``how dry it is,'' ``aren't we behind on rainfall'' and the dreaded ``D'' word -- drought. Just like last winter, when significant rains did not begin until Jan. 10, I think it's way too early to be concerned.

It's important to understand that there is much more to a drought than just the lack of rainfall. The American Meteorological Society's ``Glossary of Meteorology'' defines drought as ``a period of abnormally dry weather sufficiently prolonged for the lack of water to cause serious hydrologic imbalance'' such as crop damage and water-supply shortage. The severity ``depends on the degree of moisture deficiency, the duration and, to a lesser extent, the size of the affected area. In general, the term should be reserved for periods of moisture deficiency that are relatively extensive in both space and time.''

For example, a water district with lots of water in the reservoirs and a high water table may not consider a period of low rainfall to be a drought. But to a rancher who depends on rain to grow grass for his cattle, a couple of months of below-normal rainfall may be a drought.

Q.  The Mercury News on Dec. 18 listed the lows for Truckee and Lake Tahoe as 10 and 27 degrees, respectively. First, into which 24-hour period do those lows fall? Second, how can there be a 17-degree difference when only 20 miles separates these two cities? Both are at approximately 7,000 feet, and there is a large body of water (Lake Tahoe) in the neighborhood to moderate temperatures. The cities are also linked by several valleys -- along highways 89 and 267 -- that cut through mountain barriers. Curtis Panasuk - San Carlos

A.  The Dec. 17 weather data shows a low of 27 degrees that morning at the South Lake Tahoe airport and 25 at the Truckee airport. However, the low was 10 degrees in Truckee on the previous morning, so I suspect that an error was made in data entry. In general, the similarity of temperatures at Truckee and Tahoe is because they both are in high mountain valleys relatively close to one another. But on calm nights when we get the coldest temperatures in the Sierra, there is little air
movement between the two valleys, and Lake Tahoe's effect on Truckee is negligible. 

Low temperature readings usually are for the midnight-to-midnight period and occur during the early morning hours. But if a cold front moves through with cooler air, the lows can occur later in the day.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also telephone and fax questions at (510) 657-2246 or e-mail them to (weathercorner@ ggweather.com).