The now well-known twentieth-century warming of climates seems to have reached its main climax in the first half of the century, around the 1930s and 1940s, a sort of climatic 'optimum', at least in the northern hemisphere. The great prevalence of relatively simple, zonal westerly, wind patterns over middle latitudes, with which its development was associated, culminated earlier, in the first three decades of the century, particularly the 1920s in the northern hemisphere, though the North Atlantic westerlies returned to a later, somewhat lower, peak again around 1950. This is the commonest individual wind pattern that can be recognized in this part of the world, probably at all times since the last ice age, but its frequency undergoes significant variations. An apparently corresponding climax of the prevailing westerlies over the middle latitudes of the southern hemisphere is shown by the indices used by Lamb and Johnson (1959,1961,1966) around 1900-25. It is the overall average temperature level at anyone time, and the temperature difference between tropics and polar regions, that is fundamental to the condition of the climate. But the strength and patterns of the global wind circulation may provide the readiest index of it.