Weathervanes have adorned the roofs of homes, barns and churches, both huge and humble, for almost two millennia. Their basic function is to indicate wind direction, from which, it is theorised, you can determine incoming weather.
A traditional weathervane comes in two sections. The lower-half of a weathervane is fixed and this is the section where the four points of the compass are aligned to their correct positions. The upper section is the part of the vane that rotates and thus indicates wind direction. This section can be as ornate as you wish, and the decorations often reflect the building the weathervane is adorning - images can include Old Father Time, Mother Nature, Crucifixes, Crescents etc. A weathercock is also a weathervane, it just has an effigy of a rooster as its decoration. The only rule governing the upper section is that 'the greatest amount of mass is on one side of the spindle' or, it has to have unequal areas either side of the spindle.
So How Do they Work?
The inequality of mass mentioned above causes resistance for the oncoming wind and hence forces the section with greater mass to the back and forces the pointer (the lighter end) to face the wind. For example, if the weathervane is pointing towards 'S', it means that the wind is coming from the south.
Where the Wind Blows
Related BBC Link