This chapter is mainly concerned with climatic developments with a characteristic duration of a few centuries. They seem to be introduced by abrupt occurrences of short runs of remarkably similar 'bad years', or years which appear anomalous and out of character with the experience of people living at the time. Such occurrences are, of course, not always followed by any noteworthy, longer-lasting change of climate, but they may be a danger signal. This presentation on the subject was written for a volume dedicated to the memory of the great Swedish meteorologist, Professor Tor Bergeron, one of the original members of Vilhelm Bjerknes' Bergen (Norway) School, which in 1917 introduced the concept of fronts and frontal analysis to modern meteorology. Bergeron himself was distinguished for his pioneering work on three-dimensional weather analysis (1928) and for his own most elegant analysis of synoptic weather maps. He was, naturally, intolerant of frontal patterns drawn on weather maps by meteorologists in some countries involving stiff and angular shapes which could not possibly develop in a free-flowing fluid such as the atmosphere. The results of such analysis include lack of logic in the tracing of the fronts' progress, fronts in wrong positions, and, as result, wrong forecasts of their arrival time.