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Question: I enjoyed your site and learned a great deal about the history of weathervanes. But I would still like to know how changes in the wind suggested certain weather shifts. Can you tell me how weathervanes were used to predict the weather?
Answer: Your question is a good one and the answer depends to some degree on where the weathervane is located. That is, it has better predictive success in midlatitudes that the tropics.
My answer assumes that the weathervane is properly located and exposed. For example, one located on an apartment balcony usually cannot given the true nature of the wind affecting an area, because certain directions are blocked by the building. (Though if you watch one enough, it can give strong clues.)
Weathervanes, as well as properly located windchimes and windsocks, align with the wind and respond to changes in both the locally controlled wind regime and larger scale wind systems such as around high and low pressure systems.
Let's look briefly at the local circulations first. Two of the more prominent are land-sea (or lake) breezes and valley winds.
In a sea-breeze situation, at some time during the day -- usually mid-morning, the excess heating of the land causes the wind to blow inland. This wind is accompanied by the cooler air present over the water, which causes the temperature to drop. The initial change in temperature may be quite distinct or take a little while to develop. Therefore, if I see the wind is flowing off the water, I can predict a period of cooler temperatures for the rest of the day.
A valley breeze, which flows along the axis of a valley, might be an indication of a particular local weather situation. For example, after a calm spell early in the evening followed by pick-up in wind speed flowing down the valley slope, one might expect much cooler temperatures during the night and perhaps frost in the spring and autumn.
One the larger scale, since we know wind flows in a fairly regular pattern around high and low pressure systems, we can use the wind direction -- with or without other observations such as pressure change or visible cloud types -- to give an indication of the type of weather to expect over the next hours or even days.
For example, in most mid-latitude regions away from large water bodies, a shift in wind to south or southwest indicates the warm sector of a low pressure/frontal system is present over an area. This would give the likelihood of warmer temperatures. Some time later, a shift from southwest to northwest will accompany a cold front's passage. The north and northwest winds usually presage cold, or at least cooler, and often drier weather. However, in the early cold season along the southern shores of the Great Lakes, strong winds form the north and northwest can signal the likelihood for lake effect snowfalls and squalls.
In New England, particularly along the coast, a freshening northeast wind generally precedes an approaching storm system, thus the term Nor'easter.
When we become weatherwise about our home region, knowledge of weather patterns gained through experience can help us foretell quite a bit about the current and impending weather situation. There is usually a good degree of regularity in the movement of weather systems that make a knowledge of wind direction a good weather forecasting tool. Farmers and mariners are among the best at reading subtle changes for what is in store from such simple indicators, and this ability is enhanced when combined with cloud observations or pressure change.
A book could be written about all the possible combinations of weather and wind direction that may occur, even the shift in the monsoon climates of south Asia comes with major shifts in the prevailing wind direction. Alan Watts' books are examples of practical field guides to weather prediction from wind direction and other simple observations (e.g., The Weather Handbook).
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