The Monsoons

The reader will find here an account of how the genesis of the monsoon was traced with constant level balloons during the Monsoon Experiment of 1979: how weather satellites are used to locate cyclonic storms over the Indian ocean, and, finally, how the climate of the Rajasthan desert may be made more friendly. This book, which has now been updated for the fourth time, raises quite a few topical concerns related to atmosphere too and should be Of interest to all who would like to know more about the monsoon, which is so much a part of our national life and heritage.

Dr P K Das was the Director General of the National Meteorological Service of the Government of India. He is the author of several research publications concerned with the Indian monsoon. He is a recipient of the third IJMG Award, a prize given biennially to the author of the best paper published in the Indian Journal of Meteorology and Geophysics (now named Mausam). Dr Das was closely associated with many international scientific societies and organizations dealing with meteorology. He was the first Indian to be invited to deliver the prestigious IMO Lecture on monsoons by the World Meteorological Organization (WMO) at its Filth Congress in Geneva in 1983.

After retiring from the Meteorological Department in India, Dr Das was a Senior UNDP Expert in the Department of Meteorology at the University of Nairobi, Kenya. After his return to India, he was Professor of Meteorology at the Indian Institute of Technology, New Delhi, and later a CSIR Emeritus Scientist attached to the Department of Ocean Development, New Delhi.

I am indebted to the National Book Trust, India, for inviting me to bring out another edition of The Monsoons. The book was first published in 1968 and has now completed more than three decades of its existence. The generous response of my readers to this book has been most encouraging.

As mentioned earlier, this is an introductory text. It was meant for interested students and those who would like to know more about this fascinating phenomenon. I have tried to make it as simple as possible so as to bring out the global and the regional aspects of the monsoons. There is a revival of interest on monsoons today because it is one of the most important seasonal effects of the world. Some day it may become possible for us to know if there are similar monsoons in other planets of the solar system which will make it more universal.

The present text has been enlarged and revised at a number of places. I have tried to remove some of the blemishes of the earlier edition, but the framework remains the same as before. There is a more detailed version of what we are beginning to learn from space meteorology, especially under the aegis of the India Space Research Programme. The chapter on Long Range Forecasts of Monsoon Rain has been revised and enlarged to include more recent developments.

I realize that opinions differ on many facets of the monsoons, especially on long range prediction techniques have tried to present an integrated picture but there will be certainly more exciting developments in the years to come, because it is not yet clear to what extent are seasonal rains predictable.

I have added some material to indicate the impact of changes in climate on the monsoon towards the end of the chapter on monsoon climatology. In this context, I have also indicated that a few more experiments the now in progress to investigate the impact of changes in the Bay of Bengal and the Arabian Sea or the summer monsoon. But, as the full results have not yet been published, this has been only briefly mentioned.

I would like to thank the Director General of the Meteorological Department of India for allowing me to use some of the Satellite pictures taken by his department.

I also wish to thank Dr. S. K. Guha, an eminent seismologist, and Mr. S. K. Ghose, who was a U. N. Expert in Brazil for many years, for many helpful discussions while preparing this book.

Historical Background
The word Monsoon owes its origin to an Arabic word meaning ‘season’. It was used by seamen, several centuries ago, to describe a system of alternating winds over the Arabian Sea. These winds appear to blow from the northeast for about six months and from the southwest for another six months.

One of the oldest literary works of the Aryans is the Rgveda. It contains over a thousand hymns and songs sung in adoration of the deities. There are many references in this work to the rivers, the mountains, the sea and the desert in the vicinity of north-west India, where the earliest Aryans settled on coming to India. Scholars of history have placed the Rg-Veda between 1200 and 500 BC; but some place it between 4000 and 6000 BC.

Although the word Monsoon was coined much later, M. V. Unakar, an Indian scholar, drew our attention over sixty years ago to many verses in the Rg-Veda which praise Parjanya, the God of rain and the creator of plants and living creatures. The following verse, reproduced from Mr Unakar’s treatise, provides a description of what could be the monsoon rains in north-west India:

Thou hast poured down the rain-flood, now with hold it.
Thou host made desert places fit for travel,
Thou hast made herbs to grow for our enjoyment,
Yes! thou hast won praise from all living creatures.
They who lay quiet for a year, the frogs,
Have lifted up their voice, the voice which Parjanya inspired;
Soon the rain time in the year returneth.
The last few lines are interesting because they refer to te return of a ‘rain-time’, that is, the rainy season of the year. The fact that there was a period when it rained heavily every year was observed by the early Aryan set- tiers.

In later periods there occur descriptions of the monsoon in the early history of countries which had a maritime interest over the Arabian Sea and the neighbouring parts of the Indian Ocean. The arrival of the Greeks in India could be traced to Alexander (355-323 BC). A Greek historian of this period noted how the winds near the Indus delta reversed their direction between summer and winter, and there was heavy rain over the plains of India in summer. There occurs a description of the monsoon in the Periplus of the Erythraean Sea written about AD 60 by an unknown Greek sailor. A periplus is a navigator’s pilot- book, and Erythraean Sea was the name given by the ancient Greeks to the Red Sea, the Persian Gulf and the Arabian Sea.

Around AD 400 Pa Hsien, a Buddhist scholar, visited India from China. Shortly after his visit he wrote a ‘Record of the Buddhist Kingdoms’, in which there was a reference to the winter monsoon encountered during a voyage along the east coast of India. Perhaps the most beautiful description of monsoon clouds appear in a Sanskrit classic, Meghdoot, by Kalidas around the fourth century. In this book Kalidas described the arrival of the monsoon over Ujjain (in Madhya Pradesh) on the first day of Asadha (15 June); this is a surprisingly accurate estimate of the date on which the monsoon rains arrive over this part of India.

Near about the tenth century, Al Masu’di, an Arab explorer from Baghdad, wrote a book entitled The Meadows of Gold and Mines of Gems, in which there was an account of the reversal of ocean currents over the north Indian Ocean. The book has one of the earliest descriptions of ocean currents. The monsoon currents of the north Indian Ocean were described as a sequence of ‘ebb’ and ‘flow’, meaning a piling up of water in summer and an outflow in winter.

Another explorer from Portugal, Pedro de Mascarenhas, made important discoveries in the sixteenth century. He discovered the islands off Mauritius, especially Reunion. They are often referred to as the Mascarenes Islands. The origin of the summer monsoon over India can be traced to a large centre of high pressure over Madagascar. This is now named the Mascarenes High.

These historical events tell us how the monsoon rains influenced the lives of men for nearly two thousand years over our sub-continent.

Monsoonal Changes
While there is lack of agreement on a precise definition of the monsoon, the term is used to connote a seasonal wind, which blows with consistency and regularity during a part of the year, and which is absent or blows from another direction for the rest of the year. Such seasonal changes of wind are primarily the result of differences in the quantity of heat received from the sun by different parts of the earth.

There is a striking difference in the response of the continents and the oceans to seasonal changes in solar energy. As a consequence of its chemical composition and the structure of soil, the conduction of heat into the earth is a slow process. In summer, for instance, only a shallow layer of a few centimetres of soil are heated by the energy received at the ground Most of the solar energy received by the land is used up in heating the air rather than the earth’s surface. On the other hand, solar energy is able to penetrate to much greater depths in the oceans because of [he stirring which goes on under the action of the wind.