Published Tuesday, November 9, 1999, in the San Jose Mercury News


Special to the Mercury News

.  Why is the humidity higher on the East Coast than here in the Bay Area? Jeff Foster -  Cupertino

A. Believe it or not, the humidity in San Francisco is higher than in Miami, Atlanta, Birmingham or Washington, D.C., laying to rest the adage ``It's not the heat, it's the humidity.'' In reality, it's the combination of heat and humidity that most people find uncomfortable.

Why that much of the Southeast has heat and humidity has to do with large-scale surface wind patterns.   Off the Southeast is the semi-permanent Bermuda High, which has air flowing clockwise around it. Thus on the west side of this high, the air is moving northeastward, bringing warm and moist air from the Gulf of Mexico and the Gulf Stream.

California is on the east side of the semi-permanent Pacific High, so its clockwise flow brings cool, moist air to the region from the Northwest.

If you want to compare cities, check out the information at .

Q.  What other city has similar weather to San Francisco, and what factors contribute to this? This question came to mind when I woke up and it was clear blue skies and sunny. Yet the day before was cold and foggy. Mandy Forehan - San Francisco

A.  San Francisco's climate is unique because the city has water on three sides. In addition, the Mediterranean climate of the Bay Area is relatively rare, existing on less than 7 percent of Earth's land surface.

The primary characteristic that sets the Mediterranean climate apart from other midlatitude climates is its dry summers. Most other climate regions get a substantial
amount of their precipitation in summer.

Other areas with Mediterranean climates are around the Mediterranean Sea, the west coast of central Chile, the southwest coast of South Africa near Cape Town and the southwest coast of Australia near Perth.

Q.  I'm confused. Televised weather reports talk about hot weather in conjunction with high-pressure systems, and vice versa with cold weather and low pressure. I would think the increased energy from the sun would leave the equator with high pressure and the poles with low pressure. I sure notice an absence of wind when it's hot in San Jose. C. Zertuche - San Jose

A.  You are about the fifth person to ask about this in a week. The problem is actually with the mixing of various terms by those in the media, and their not delineating
whether they are talking about pressure systems near the surface or higher up in the atmosphere. The pressure patterns I spoke about in the last column related to
surface pressure and surface winds.

When the local forecaster talks about high pressure moving over us to give us hot weather, he is generally talking about a ridge of high pressure aloft. Associated
with high pressure aloft there is sinking air (subsidence), which warms by compression (just like the Santa Ana or Diablo winds).

Simultaneously, we may have a weak area of surface low pressure over us and the absence of our usual cooling sea breeze. All this adds up to a hot weather pattern.
Conversely, areas of low pressure aloft are associated with rising air, clouds and potentially stormy weather.

As you mention, the equator does receive the most energy from the sun. On the global scale this results in four `belts' of high and low pressure. Along the equator,
there is the equatorial low, which is created by the intense heating and rising air. At 30 degrees latitude or so there is a belt of high pressure. At 60 degrees latitude or
so, there is another belt of low pressure (subpolar low), and then there is high pressure over the poles.

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

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