Most of the United States experiences four distinct seasons. “Traditionally, winter is the three coldest months, summer is three warmest, and what’s left over is spring and fall,” Crimmins says. Seasons aren’t the same everywhere, however. Large swaths of Asia divide their year in two: the dry season and the rainy, or monsoon, season. (Even in Arizona, where Crimmins recently moved from four-season Michigan, the dry and rainy season is one of the more useful paradigms.) Some ecologists describe climate as having a six-part cycle: prevernal, vernal, estival, serotinal, autumnal, and hibernal. Parts of West Africa have a season all their own—harmattan—defined not by temperature, but by dry air and dusty winds.