Wind vane crucial part of forecaster's tool
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
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
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
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
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.''