Published Tuesday, February 10, 2004 in the San Jose Mercury News

Wind vane crucial part of forecaster's tool box

By Jan Null

Bob Dylan sang, ``You don't need a weatherman to know which way the wind blows.'' But it does help to have a weather vane. More properly known as wind vanes and sometimes called a weathercock, they have dotted rooftops for more than two centuries as both meteorological instruments, advertising and ornamentation.

The wind vane is one of the most basic weather tools, with an asymmetrical design that causes it to point into the wind. This is accomplished because the part of the vane with the greatest surface offers more wind resistance and thus positions the vane to point in the direction from which the wind is blowing.

The earliest vanes date back to about 48 B.C., when a statue of the Greek god Triton adorned the Horologion (Tower of Winds) of Andronikos of Kyrrhos in Athens. The bronze figure revolved about the top of the octagonal building and would face into the wind.

In medieval times, small banners were used to tell the wind speed and direction. In Old English, these banners were ``fana''; in Middle English, ``fane.'' Over the centuries the term ``vane'' has evolved.

Wind and weather were key factors in the agrarian lifestyle, and wind vanes were common on rooftops as a practical tool. They were especially prominent on the steeples of churches, which were often the highest structures.

According to the reference staff of Orradre Library at Santa Clara University, it was early ninth-century Pope Paschal I who decreed that European churches be adorned with a rooster. This was to be a reminder to the faithful of the prophecy that the cock would not crow until Peter had denounced Jesus three times.

Immigrants to the American colonies adapted weather vanes to represent their new land as well as the old. New weather vane designs incorporated arrows and Indians as well as seaport symbols of lighthouses, ships and gulls.

Today weather vanes show an even greater diversity. Some are such beautiful pieces of art that they remain inside to be enjoyed and never feel a gust of wind.

I have put together a Web site with some of my pictures of wind vanes at

Q Does the wind speed increase when the sun rises? I remember, doing early morning chores, that it became windier after sunrise. Gary Hulbert - Morrice, Mich.

A In many places this is indeed the case. As the sun rises it begins to heat the Earth, but not uniformly. In areas that are heated, the air rises and adjacent air flows in to replace it.

On different scales this concept of differential heating is what causes winds and weather worldwide. Our afternoon sea breeze is a byproduct of the interior valleys warming more than the cooler coast. And globally, the difference in the amount of heat received at the equator vs. the poles is what drives large-scale weather systems.

Q What is the technical difference between ``mostly cloudy'' and ``partly sunny''? I am interested in how they relate to the aviation terms ``clear,'' ``few,'' ``scattered,'' ``broken'' and ``overcast.'' Walter Windus - San Jose

A There are no hard-and-fast definitions used by the media. One person's ``partly cloudy'' may be another's ``partly sunny.'' However, the National Weather Service defines the sky as clear or sunny when there is 6 percent or less cloud cover. From 7 to 31 percent coverage, the terms ``mostly sunny'' or ``mostly clear'' are prescribed, and from 32 to 69 percent the NWS uses ``partly sunny'' or ``partly cloudy'' synonymously. If there is 70 to 94 percent cloud cover, they say the sky is ``mostly cloudy,'' while greater than 94 percent garners the term ``cloudy.''

In aviation terminology, the amount of sky cover is defined by the number of eighths or octas that are covered. If less that one octa is covered, then it's ``clear''; for one to two octas, the clouds are said to be ``few''; for three to four octas there are ``scattered clouds''; for five to seven, ``broken clouds''; and for eight octas it is ``overcast.''

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