Every morning I take a look at the satellite photos on the website of the Indian Meteorology Department. Normally, at this time of the year, my intention is to track the progress of the monsoon. The first sign of this is that the band of clouds along the equator (called the intertropical convergence zone) starts moving into the continental landmass of Asia. I marvel at the tools that are now at our disposal. Fifty years ago, we were entranced by a few photos of the the blue marble: the earth seen from space. Now, there is a torrent of such images. In about the same number of years the internet has expanded to be able to bring these images into our homes the moment we want it.
But the news is not always welcome. This is the week of Cyclone Amphan. The enormous extent of these cyclones is amazing: they are as big as continents. Amphan has currently gained enough energy to be called an extremely severe cyclonic storm (typhoon, if you are in the Western Pacific, and Category 3 storm if you are in the Atlantic or Australia). With the seas warming up, extreme weather of this kind is now an annual affair. Fortunately, disaster preparedness and response has improved over the last twenty years, to the point that last year the casualties were in the single digits. The human misery and economic cost remains severe, though. And I wonder how any physical distancing can be maintained when people are crammed into cyclone shelters for a couple of days. Bengal and Bangladesh are in for a bad time.
By afternoon the cyclone has gained enough energy to become a super cyclonic storm (typhoon, if you are in the Western Pacific, and Category 4 storm if you are in the Atlantic or Australia).
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.