Published Tuesday, September 23, 2003 in the San Jose Mercury News

Weather Corner

Diablo winds prompting recent hot, dry weather

By Jan Null
Special to the Mercury News

Why has it been so hot lately? What has helped keep the coast fog-free? Think "diablo.''

Several recent heat waves have been the result of offshore air currents sometimes known as "diablo winds.'' These are opposite in direction from the usual cool sea breeze that acts as the Bay Area's natural air conditioning. The meteorological differences for the two phenomena, however, are more than just the direction from which the wind blows.

The sea breeze results when air over the interior of the state warms and rises, allowing cool Pacific Ocean air to flow inland and fill the resulting area of low pressure. However, our warm offshore winds are a bit more complex. I have heard several erroneous reasons given for their warmth, including that they arise from warm air blowing in from the Central Valley or friction as they move down the mountains.

In reality, the Bay Area's offshore winds are warm and dry due to compressional heating, in which air flows from high elevations to the east and down to sea level. The air is compressed due to greater atmospheric pressure at sea level and becomes warm, just like the air in a bicycle pump warms as it is pumped. This process is called adiabatic warming and also causes the air to dry out significantly with relative humidities in the single digits.

This type of pattern is the same one that causes the warm, dry Santa Ana winds in Southern California and the chinook winds that warm the Plains east of the Rockies. Locally, they are sometimes referred to as diablo winds because they come from the general direction of Mount Diablo. Diablo, which means "devil'' in Spanish, is a good characterization for these hot dry winds.

Diablo winds are most common in the fall as the jet stream dips farther and farther south with alternating areas of high and low pressure affecting the state. This is also the time of year when wildlands are particularly dry, which, combined with hot dry winds, results in extremely high fire danger. It was just this sort of meteorological scenario that led to the Oakland Hills firestorm in October 1991.

Q Growing up in Georgia, I experienced my share of thunderstorms. The lightning always seemed to go from high to low in a vertical path. Recently I've noticed some photos of lightning that show it in a left to right or horizontal path. Can lightning indeed go horizontal and vertical? If vertical, does it always go from left to right? Does every lightning bolt always strike something? Brenda Anderson - Petaluma

A Lightning has been observed in every direction -- down, sideways and even up. Besides going from cloud-to-ground, lightning can be cloud-to-cloud, cloud-to-air and even cloud-toward-space. These spaceward lightning discharges have been observed by satellites and high flying aircraft and are quite colorful, called ``red sprites'' and ``blue jets.''

Q Recently there were media accounts of more than 10,000 people dying from the heat in Paris environs. Just how hot was it, and how was this possible? Lucy Baldwin - Palo Alto

A The temperatures were near 100 degrees in many locations, but the high death toll was the result of a combination of many factors, not all of them meteorological.

The persistence of the heat for more than a week and unusually high humidities probably exacerbated existing health conditions for many individuals. A report by the French government attributed many of the deaths to understaffing in the hospitals due to the summer holidays and to the large number of elderly patients that were taxing France's health care system.

Q We rent a house at Lake Tahoe that is at 7,200 feet. It has been my experience that it takes longer to boil water at that elevation. Since it takes longer for pure water to boil at high altitudes, does it take less time for pure water to freeze at higher altitudes? Sheri Wood - Santa Rosa

A The freezing point of water varies only slightly at higher elevations because the process of freezing is less dependent on atmospheric pressure than the boiling of water.

At higher altitudes, it takes longer to cook something because water boils at a lower temperature. This happens because the lower atmospheric pressure allows the bubbles of water vapor that occur when water boils to burst at a lower temperature. For example at 7,000 feet, the boiling point of water is only 93 degrees, so boiling a ``three-minute egg'' might actually take four or five minutes.

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 weathercorner@ggweather.com, or fill out a form online at http://ggweather.com/questions.htm. Please indicate in your e-mail what city you live in.