We were taught in school that astronomy determines climate, and the four seasons. Let’s take the astronomy first. From our earthbound view the sun seems to move north and south across the sky over the year. The extreme points are reached on the days of the solstices: the longest day and the longest night. The climate for a featureless earth follows from this, alternate heating and cooling of the atmosphere above the earth would produce two extremes. Culture determines the seaons: how would you want to divide the parts between the extremes of hot and cold? Into two, or four, or twelve? That would determine how many seasons you have. The simple geography of mid-latitude Eurasia and a large part of Northern America gave rise to the cultural artifact of the four seasons.
But there’s more to the climate than this simple and ancient model. The heating by the sun causes convection in the atmosphere. The rotation of the earth then breaks these convection cells into several northern and southern pieces. Approximately around the equator, these convection zones come together and create the monsoon. So, any continent that lies just north or south of an open equatorial ocean has monsoons. In our present geological epoch that is mainly Asia and Australia.
Unlike the rest of these two continents, India is also cut off from the polar circulation by the high east-west barrier of the Himalayas. This means that its climate is again particularly simple: the seasonal heating and cooling by the sun, and its interaction with the monsoon can be summarized into the six seasons. Essentially, the seasons of the mid-latitudes get interrupted by a season of rainfall. Summer is duplicated into grishma and sharad, and interrupted by varsha.
The rest of the world can be more, or less, complicated. The Aztec and Mayan civilizations counted two seasons. At the other end of the scale, Korean tradition counts as many as 24 seasons, Japan counts 12 in its classic poetry. The counting of seasons is a wonderful convergence of physical conditions and culture.
Photos from top to bottom: (1) Winter in the Thar desert of India (2) High summer in Greenland (3) Late spring in Germany (4) Early winter in Korea, the season of Ipdong.