Published Tuesday, November 5, 2002 in the San Jose Mercury News

Weather Corner

Understanding a microclimate can be tricky

Special to the Mercury News

It's a phenomenon Bay Area meteorologists are frequently asked to explain -- the microclimate.

``What microclimate am I in?'' ``Are there really 20 microclimates in San Francisco?'' Microclimates are one of those things that everyone talks about, but to define them is like nailing jello to the wall.

The textbook definition of a microclimate is counter to the common perception of the term, which refers to a similar climate over a small area. Instead, the term refers to the vertical layer of the atmosphere from the surface to a height where the underlying surface no longer has an effect. Typically, this is something on the order of four times the height of surface vegetation or structures. Using this criterion, we can see where a microclimate could range from a few inches above a grassy surface to several hundred feet above a redwood forest.

But if we use the more conventional practice of defining climates in terms of a specific geographic area, we need to figure out how finely we want to slice the meteorological pie and even what we want the pie to be made of. For example, there is a different microclimate over a lawn than over a directly adjacent street or driveway. A neighborhood along a creek or river will be different than a nearby hill. And what meteorological parameters do we look at? Temperature? Rainfall? Growing season? How do we define a climate?

There are numerous methodologies for defining climates and microclimates, with most having their roots in the agricultural community. The Köppen climate system, one such methodology, was developed in the early 20th century. The system takes into account the average annual and monthly temperatures and precipitation of an area and spells out five major climatic types. Type A is ``tropical moist,'' B is ``dry,'' C is ``moist mid-latitude with mild winters,'' D is ``moist mid-latitude with severe winters'' and Type E is ``polar.'' Most of California is in the Type C category, which is defined on the basis of the coldest month's average temperature being below 64 degrees and above 27 degrees.

Each of these types is divided into subregions. The Type C subregions are Cfa, ``humid subtropical''; Cfb and Cfc, ``marine''; Cs, ``Mediterranean (dry summer)''; and Cw ``dry winter.'' Most of the Bay Area falls into one of these categories.

A different system developed by the United States Department of Agriculture uses a 20-zone plant ``hardiness'' scale which looks at only the average minimum temperatures for regions across the United States as the defining factor. For the full scales, see

A somewhat more robust classification scheme is the one developed at University of California-Berkeley, which has been adopted by Sunset Magazine as their ``western climate zones.'' The 24 zones for just the western United States take into account winter cold and summer heat, humidity, elevation and terrain, latitude, and varying degrees of continental and marine influence on local climate. See for the full listing of zones.

To find some of the particulars for the climates of various Bay Area cities, go to , or to check out the microclimate nearby, see

If you have a weather station on the Web and would like to be added to the WeatherCorner Weather Network, contact me at

Q.  Why do so many Bay Area weathercasters insist on calling every rain event a ``storm''? I'm embarrassed for them. It makes us seem like weenies. Go to other parts of the country or world. Now, they have storms. We do too, but not every time. Bob McCard - San Jose

A.  I couldn't agree more. I don't know if it's because there is no definitive definition of exactly what constitutes a ``storm'' or if it's simply media hype. For an event to be called a storm, there should be both significant precipitation and wind. Otherwise, I would refer to these minimal events as ``weak fronts'' or ``weak weather systems.''

In the American Meteorological Society's Glossary of Meteorology, a storm is defined as ``any disturbed state of the atmosphere, especially as affecting the Earth's surface, implying inclement and possibly destructive weather. From a local and special-interest viewpoint, a storm is a transient occurrence identified by its most destructive or spectacular aspect¿.''

In marine forecasting, ``storm'' does have a precise definition of wind greater than 55 miles per hour or with seas of 29 to 41 feet.

Q.  Could you explain why isobars -- lines that show pressure levels -- can never cross one another on a weather map? If weather map data represents surface conditions, what methodology exists to express the conditions throughout the height of the atmosphere? Is it not possible to have overlapping isobars due to pockets of unequal pressure in any given vertical cylinder of air? James Miner - Tracy

A.  First, a couple of basic concepts and definitions. Weather maps are only drawn on a horizontal plane. Thus, you would have separate maps for the conditions at the Earth's surface and at various layers in the atmospheres. The ``iso-'' lines that are drawn on weather maps show points that all share the same value. Isobars are lines of equal pressure, and isotherms are lines of equal temperature.

Isobars are a valuable tool for the meteorologist. They delineate areas of high and low pressure, and this tells us not only which way the wind will blow but also the location of weather fronts. Additionally the spacing between isobars on a weather map, called the ``pressure gradient,'' is indicative of the speed of the winds.

In the case of isobars, two lines of equal pressure can't cross because that would be saying that a single point had two separate pressures, which is a physical impossibility.

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, e-mail them to or fill out a form online at Please indicate in your e-mail what city you live in.