Published Tuesday, November 23, 1999, in the San Jose Mercury News


Special to the Mercury News

Q.  What's the difference between the marine layer and fog?   Pam Nelson - San Mateo

A.   Great question, because sometimes they're the same, sometimes not. First, fog is just a cloud that touches Earth's surface. Most of the time this is a flat gray stratus cloud usually below 2,000 feet.

When this stratus forms along the coast and moves inland, it's often referred to as the ``marine layer.'' Where hills run into the stratus, or the stratus is so low that it touches the ground or water, it's called fog.

In addition to marine-layer-induced fog, in wintertime the valleys in and around the Bay Area sometimes experience ``tule fog.'' Technically, this is radiation fog, so named because it forms when Earth cools at night as it loses its heat to space. The cool Earth, in turn, cools the lowest layer of atmosphere, condensing out the atmosphere to form fog.

Q. I read with some interest your Nov. 9 column, especially regarding humidity and the Mediterranean climate. What a laugh! There must be some huge discrepancy between what the National Weather Service measures and what people actually feel. On any given day in Baltimore (nice middle latitude), in the summertime, the humidity exceeds 90 percent all day, and all night. Period. That's what it feels like, and that's what they always tell you on the radio and TV broadcasts. The Web page you referred us to lists nice, Bay Area-like humidities for Baltimore closer to 50 percent in the afternoons, when it is the worst. It also listed more believable figures over 80 percent for the mornings, but I've never heard of the afternoons being more comfy! Around here in San Jose, if the relative humidity even approaches 75 percent, we notice! What is going on with these numbers? Gary Newton - via email

A. I stand by the numbers, which show that the relative humidity is higher in San Francisco than in Miami and many other Southeast cities.  The problem stems from two issues. First, the hyperbole about 90 percent humidity is just not true. Even on a very hot and humid day in the Southeast, the highest humidity is typically only about 60 percent. Equally important is an understanding of the term ``relative humidity.'' Humidity is a function of the amount of moisture in the air and the temperature. It is called relative because it reflects the percentage of moisture needed for the air to be saturated, and warm air can hold more water vapor than cooler air. Air with a relative humidity of 60 percent and a temperature of 90 degrees holds three times more water vapor than when the relative humidity is 60 percent and the temperature is 60 degrees.

The final consideration is that of comfort. When air is warm and humid, the body has a harder time cooling itself, and we feel even more discomfort. This is why in some areas a heat index is used to reflect the effect of heat and humidity on humans.

Q.  My wife and I are having a Jan. 1 party at our home in San Mateo. We will plan outdoor activities if there is a small chance of rain. Are there Web sites that will project or predict weather on any specific date?  Wayne Deutscher -   San Mateo

A.  There is no way to accurately forecast weather on a specific day by more than about a week or 10 days. I guarantee that if there were, some commodity traders and their meteorologists would be richer than Bill Gates. One alternative is to look at the climatology or weather history for a particular day. In the case of rainfall on a specific day, the number of times that it has rained over a period of time can give you a percentage for that date. For Jan. 1, it is has rained 36 percent of the time in San Francisco and 31 percent of the time in San Jose in 100 years. With San Mateo between these, we could reasonably expect the chance of rain to be about 1 in 3. The probability for other dates in San Francisco and San Jose are at and --
which should be representative for most of the Bay Area.

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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