If the average citizen's surroundings defined the national climate, then the United States grew markedly warmer and drier in the postwar decades. Migration continued to carry the center of population west and began pulling it southward as well. The growth of what came to be called the Sunbelt at the "Snowbelt's" expense passed a landmark in the early 1960s when California replaced New York as the most populous state. Another landmark was established in the early 1990s when Texas moved ahead of New York. In popular discussion, it was taken for granted that finding a change of climate was one of the motives for relocating as well as one of the results. It was not until 1954, though, that an American social scientist first seriously considered the possibility. The twentieth-century flow of Americans to the West Coast, the geographer Edward L. Ullman observed in that year, had no precedent in world history. It could not be explained by the theories of settlement that had worked well in the past, for a substantial share of it represented something entirely new, "the first large-scale in-migration to be drawn by the lure of a pleasant climate." If it was the first of its kind, it was unlikely to be the last. For a set of changes in American society, Ullman suggested, had transformed the economic role of climate. The key changes included a growth in the numbers of pensioned retirees; an increase in trade and service employment, much more "footloose" than agriculture or manufacturing was; developments in technology making manufacturing itself more footloose; and a great increase in mobility brought about by the automobile and the highway. All in one way or another had weakened the bonds of place and made Americans far freer than before to choose where to live. Whatever qualities made life in any spot particularly pleasant thus attracted migration more than in the past. Ullman grouped such qualities together as "amenities." They ranged from mountains to beaches to cultural attractions, but climate appeared to be the most important, not least because it was key to the enjoyment of many of the rest. Ullman did not suppose that all Americans desired the same climate. For most people, in this as in other respects, "where one was born and lives is the best place in the world, no matter how forsaken a hole it may appear to an outsider."
On the coupling of vector fields to the Gauss-Bonnet invariant
