Weather Eye is on Lake Constance still, or on the Bodensee as the Germans like to call it. This time it is Meersburg, another half-timbered medieval jewel in a necklace of such towns stretching in a half-circle around the northern Bavarian shore. This is wine-growing country, and the same benign climate and sunny southern aspect that makes it ideal for this purpose has led to its being also a favourite place for holidays and for retirement.
These pleasant conditions are not entirely unconnected with the presence of the lake itself. Water acts like a storage heater, responding only very slowly to changes in the temperature of its surroundings. A good-sized area of deep water like Lake Constance, therefore, tends to moderate the local climate, protecting it from the extremes to which it might otherwise be subject.
On a hot summer day the air over deep water remains relatively cold, and so a gentle cooling "lake breeze", like a miniature sea breeze, may develop to waft lightly onto the adjacent land. Air that moves shorewards in this way is generally rich in moisture, and enhances any clouds tending to form some distance inland from the shoreline.
The shore-bound air, moreover, must be replaced by air sinking down towards the lake from higher levels in the atmosphere, so lakes tend to be areas of what meteorologists call subsidence - regions of descending air. And consequently, since clouds are associated with rising rather than sinking air, the skies above a large expanse of water like Lake Constance tend to remain relatively cloud-free compared to the surrounding countryside.
This effect is particularly noticeable in spring and early summer, when any rise in temperature experienced by the water lags far behind that of the adjacent land.
In autumn and winter, however, these processes tend to be reversed: cloudiness tends to be greater over and near a water surface than it is elsewhere. In a similar way, cold nights produce the opposite effect to a warm sunny day: a light current of cool air can be felt drifting out onto the surface of the lake to replace any relatively warmer air over the water itself that has succumbed to a tendency to float upwards.
Most importantly, however, for those whose livelihood depends on it, is the fact the moderating influence of the nearby water make the vines in the locality much less subject to sharp winter frosts than those in vineyards equally sunny, and at the same latitude, but otherwise less fortunately endowed.