Published Tuesday, March 27,  2001 in the San Jose Mercury News


Hot days in March not unusual

Special to the Mercury News

Between rolling blackouts last week, one of the comments heard most frequently was, ``Isn't this early in the year for it to be this hot?'' On March 19 and 20, the respective temperatures in San Jose hit 83 and 80, more than 13 degrees above the monthly average of 67.

It has cooled considerably since, but questions remain about how unusual the heat was and what we can expect in coming months. In many parts of California, higher temperatures mean air conditioners get switched on, which means power demand goes up.

Through the first three weeks of March, San Jose's average high has been about 2 degrees above the long-term average of 67 degrees. During that period, it is interesting to note that the daily high was above average on only 10 of the 21 days. The two days that reached 80 and 83 degrees were responsible for raising the average by about a degree.

These miniature warm spells are not unusual in March, with at least two days of 80 degrees or more occurring in about one of every two years. In March 1989, four days exceeded 90 degrees, with a March record of 95 on the 12th.

A single warm day is seldom a good indicator of what lies ahead. And as we have seen, an extended period of near normal temperatures can be significantly skewed by a couple of extreme days. The outlook for April from the National Weather Service's Climate Prediction Center is near normal temperatures.

Long-range projections for April, May and June indicate a slightly above average chance that those months will be warmer than normal. The probability is quite small, and the outlook does not quantify how much above normal is expected. Power watchers will have to watch the weather to see what late spring and early summer will bring.

Lacking any offshore weather observations, what should I make of conditions like those on a recent evening? At ground level, there was a brisk wind from the southeast (typical of the leading edge of a storm a few hours away), but aloft there were broken clouds on a brisk wind from the west (seemingly typical of a storm still a day away, or perhaps clearing after the previous storm). Kerry Abel - Mountain View

What you observe is not really very unusual. Southerly winds result from a low pressure center to the north as weather systems move into the area and air rushes to fill the area of lower pressure. These are enhanced by topography of the Bay Area, which tends to channel the winds. At the same time, the winds in the upper atmosphere are often from the west and southwest and are the storm track or pathway for the surface weather systems.

I need help to accurately set my new barometer. It seemed that I could see that 1013.2 millibars was my clue to set my barometer at 29.92 as a start. But the pressure was 1017 millibars on another Web site for atmospheric pressure reading. Would you be able to suggest how I convert or figure out what my atmospheric reading is and if given in units of millibars, where should set my barometer?  Richard Malone - via email

A barometer, an instrument used to measure atmospheric pressure, occasionally needs to be readjusted for accuracy. It also needs to be reset, as the pressure can vary with location or elevation.

Yours is a common question, and the issue of the correct units often compounds the problem. The current atmospheric pressure can be obtained for many Bay Area locations from .

Barometric pressure in English units is measured in inches of mercury, abbreviated as ``in Hg,'' while in the metric system the units are millibars. The average surface pressure is 29.92 in Hg, or 1013 millibars. To convert from inches to millibars, multiply by 33.865. Check out the conversion tables at or or use the weather calculator at

WANTED: Morgan Hill weather observer. The National Weather Service is looking for an official volunteer rainfall observer in Morgan Hill. For more information, contact the NWS at (831) 656-1725 or go to .

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service.  Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to Please indicate in your e-mail what city you live in.