 where an alternative name for this series of lectures could be monsoon for bird watches. It gives me immense pleasure to give these set of lectures on a phenomenon I have studied for several years at the centre for atmospheric and oceanic sciences at the Indian Institute of Science. Over a billion people residing in the monsoonal regions of the world associate the monsoon with joy and love. However, every year they await the forthcoming monsoon season with some trepidation as a deficit monsoon can have a disastrous impact on the well-being of the people, the flora and the fauna. Scientists of different use as well as lay people are therefore curious about the monsoon and its vagaries. In this lecture series on the monsoon and its variability, I have tried to elucidate the important facets of this fascinating phenomenon, what we understand about the mechanisms leading to its variation in space and time, adaptation to monsoon variability and finally, problems and prospects of monsoon prediction. I have adopted an approach which should make the lectures comprehensible to anybody with a desire to know the nature of this phenomena. I have assumed no prior knowledge of metrology and defined the jargon as and when required. I have attempted to elucidate the physical processes involved generally without using equations. At the same time, I have dealt with each topic in some depth and included recent advances in the field. This should make it useful also for students of atmospheric and oceanic sciences particularly those interested in the tropical atmosphere and oceans. Monsoon is one of the most challenging problems in atmospheric science today. It has a rich history with great minds having contributed to our understanding of the Indian monsoon from 1880s onwards. I have talked about how ideas have evolved, discussed the present understanding of the monsoon and its variability and also brought out the scientific challenges that remain in modeling and prediction. I have considered quantitative assessments of the impact of the monsoon on agriculture and economy. Given the large socioeconomic impacts I have also discussed at some length the adaptation of farming strategies to the variability of the monsoon. I hope that by the end of the lecture series at least a few amongst you come under the spell of this fascinating complex system and take up the challenge of understanding, modeling and predicting the monsoon and its variability. Thank you. Namaskar and good afternoon. I am Sulochana Gadgil. My passion for over 4 decades has been the monsoon. I have been working on the monsoon from 1973 in the Indian Institute of Science. And so it is a great pleasure for me to give this course under NPTEL on the monsoon and its variability. Now in this set of lectures I shall share with you what we know about the nature of the monsoon and its variability from observations and discuss the present understanding of the underlying mechanism. Now systematic observations of the monsoon of the atmosphere over India actually of temperature, pressure, rainfall etc are available for over a century. With the analysis of the data of the first few years a very lucid account of the nature of rainfall and its variability over the Indian region by Blanford was published in 1886. Of course since then our knowledge about the weather and climate of India has increased continuously with the efforts of many monsoon metrologists. Now with the advent of meteorological satellites in the mid 60s there was a quantum jump in our understanding of the monsoon, the nature of the system responsible for the monsoon and of the links of the Indian monsoon to events over the surrounding seas meaning the Bay of Bengal, Arabian Sea, Equatorial Indian Ocean as well as other parts of tropical belt like the Pacific. Now observations over the tropical oceans and the atmosphere above the ocean with special ocean based platforms called buoys established in the 1990s particularly over the equatorial Pacific have further added to our knowledge of the monsoon and its teleconnection. Thus much of our continuously growing knowledge of the monsoon has been gained empirically. Such an in depth knowledge of the nature of the system is essential for modeling and hence predicting the monsoon. Now the important role of empirical knowledge in advances in atmospheric science was pointed out by Ian Lawrence a genius one of the most brilliant meteorologists who is perhaps better known to you all as the father of predictability and chaos. Well he remarks a full century has passed since Albert Einstein announced his famous theory of relativity and among the previously unobserved and unsuspected phenomena demanded by this theory was the bending of light rays that passed the sun. Now subsequent careful measurements confirmed that the theory was correct in this respect. So the prediction of the theory was borne out by observation subsequent to its publication. Now meteorology is an example of a science where this sort of a story cannot be told when for example sounding balloons first reveal the presence of the stratosphere. You know that in we are used to living in an atmosphere in which temperature decreases with height. This is why hill stations are cool and sea level stations are rather warm. So when a sounding balloons first reveal the presence of a stratosphere in which temperature increases with height. So the presence of a stratosphere also a century or so ago no theoretical framework had anticipated an extensive layer of the atmosphere in which the temperature did not decrease with elevation. Another example of new phenomena being discovered in the atmosphere which were not anticipated by theory at the time is more recently the earliest report of ozone depletion during the Antarctic spring known as the ozone hole. Now were attributed to instrumental error and the very first observations were thrown off. No existing theory could account for them. So this is a remark made by Lorenz in a forward to a very interesting book called Empirical Methods in Short Term Climate Prediction. We will allude to this later in the course of our lectures. Now let me give you an overview of the lecture series in what way I hope to cover the topic. I begin with what the monsoon means to the life in a monsoonal region such as ours and why I consider the study of monsoon the most enjoyable and worthwhile of our scientific endeavours. Then I elucidate the important facets of the monsoon and its variability because that is what we are trying to understand and eventually predict. There are great demands of on prediction of the monsoon. So we should know what are the facets of the monsoon that we should try and understand and predict. Now after describing what we know from observations as the important facets of the monsoon and its variability, I will discuss the background necessary to understand the physics of the monsoon and the proposed theories for mechanisms governing the variability. So I will develop some discussion on the background that we need and the background I do not assume too much of a background from the students. So the background will involve the nature of our atmosphere, important concepts and results from fluid dynamics because you must remember that the atmosphere is a fluid after all and atmospheric circulation is flow of this fluid as well as geophysical fluid dynamics. Now why geophysical fluid dynamics? Because the atmosphere is on a rotating earth and we measure winds or currents in the ocean relative to this rotating earth. So geophysical fluid dynamics concepts also are important. Now I have attempted to define the terms used and explain all the basic concepts without introducing too many equations. So I do hope that some naturalists and biologists who are also interested in the monsoon will come along at least for the first part of this course. Now after developing the background necessary then I consider our understanding of the nature of the system responsible for the monsoon and the mechanisms involved in the important facets of the variability of the monsoon. Now I shall initially focus on the Indian monsoon because I worked a great deal on the Indian monsoon but extend the discussion also to the other monsoonal reasons of the world. Finally I would discuss another topic of great interest and great possibilities for application to society namely the relationship between monsoon variability and agriculture and also economy. What is the kind of information about monsoon variability which would be useful for say enhancing our agricultural production? What are the kind of predictions which would be useful and would contribute to the decision making of farmers to enhance the agricultural production and what is the impact assessment quantitatively of what is the impact of the monsoon on our agriculture and an important measure of the economy the GDP. So with this overview you will get a feel for how I am going to go about discussing the monsoon. Now let us begin with lecture 1. In lecture 1 the kind of topics I want to cover is first of all I will define what we mean by the monsoon then try and share with you the response of nature around us and the humans to the seasonal cycle because those of us who live in the monsoonal regions of the world have a very strong link with this monsoon system. Then I talk about the important phenomena of onset and retreat of the monsoon that is the seasonal variation and then talk to you about the climate. In other words what is the mean rainfall in the major seasons of the year. So they would be the pre-monsoon season, the summer monsoon season, the post-monsoon season and the winter season. So this is the outline. So let us begin with what is the monsoon. Now the term monsoon is derived from the word mausam in Arabic which is also the word in Hindi for season and this term was first used by Arab sailors for the large variation in the direction of winds which season observed over the Arabian Sea. So to come from where the Arabs live to India in one season the winds were very favourable right. They had a component which was going from the west to the east but in another season the winds would change direction and would be against them. So this is something that the Arab sailors observed and called this seasonality of the direction of winds mausam or the monsoon. Now for many of us living in the monsoonal regions of the world wind really is not that important. It does not have much bearing on our life. But what we care about is the variation of rainfall with season. So the market variation of rainfall with season is a far greater interest to us than the seasonal variation of the wind which was of interest to the Arab sailors. Now I consider first the observations of the rainfall and its variability. The other facets of the atmosphere such as pressure etc will be discussed in subsequent lectures. Now why study the monsoon? Why is it of such interest? First and foremost for us scientists to get excited about it it must be on the cutting edge of science and indeed understanding and predicting the monsoon and its variability is one of the most challenging problems in atmospheric science today. So it is very exciting for us to work on this problem. Also as I mentioned before the monsoon governs the pulse of life for millions of humans as well as the rich flora and fauna living on the Indians subcontinent and also the other monsoonal regions of the world. So this is the second reason then understanding and predicting the variability of the monsoon Indian monsoon is very important because it has a very large impact on agriculture and economy of the country. In fact in the colonial era a British are supposed to have remarked that the Indian economy was described as a gamble on the monsoon rains. So dependent was our economy on the monsoon rains. Now it was felt that with the kind of development we have had after independence from colonial rule. One would have expected and many people predicted that we would not be so sensitive to the vagaries of the monsoon. However, even after more than 6 decades of development the monsoon continues to have a large impact on our economy. Now this point was driven home by the opening remark of the finance minister Pranab Mukherjee in his presentation of the budget for 2011-12 in our Indian parliament the Lokasabha. He began this budget speech saying I seek the blessings of Lord Indra to bestow on us timely and bountiful monsoons you see. So thus the importance and remember that he was the finance minister then thus the importance of identifying and adopting strategies which are appropriate for the monsoon variability we experience cannot be overemphasized it is an extremely important problem. Now having talked about the monsoon and seasonality let us see how the rainfall varies over a region. So first we are going to consider the mean or what is called climatology by metrologist. This is average over many many years typically 50 or more years. And furthermore we are also going to average over spatial scales. So we are going to look at rainfall average over the entire Indian region to begin with. And we are going to see how monthly rainfall average over the entire country varies from month to month. Now what you see here is plotted the variation of all India rainfall from January to December as I mentioned this is average rainfall. This is not the rainfall of any specific year but is it is the average rainfall over many many years. So you can see that there is hardly any rain January February March and hardly any rain in December. So December to March hardly any rain April it has picked up a little may it is still higher and June to September are the major rainy months as far as all India average rainfall is concerned. When it decreases in October but is still significant decreases further in November and becomes very small from December to March. So so over the Indian region there is hardly any rainfall between December to April and most of the rainfall occurs as you saw during the 4 months June July August and September. Now this June to September period this season is called the summer monsoon season. Now in fact in India the word monsoon in common parlance refers to the system that visits us every year and gives us rain. So monsoon to us is the rain giving system or that comes and visits us every year and the commencement of the rainy season is associated with the onset of the monsoon or the first arrival of the monsoon on the Indian region. Now end of the rainy season coincides with the retreat of the monsoon from our region. Now for those of us who have who live in this country know very well that in Indian poetry and films the monsoon is associated with abundant joy and love. Life of plants animals as well as human beings on the Indian subcontinent is tuned to the seasonal cycle which we call rutu chakra rutu as you know in Sanskrit means season chakra is the cycle. So this rutu chakra of the rainfall associated with the annual visit of the monsoon is what most of our plants and animals birds and bees are tuned to. So let us just have a glimpse of how nature around us responds to the seasonal cycle of the monsoon. Now the new year according to the Indian calendar is around the spring equinox right 21st of March which heralds the beginning of the hot dry summer preceding the monsoon. Now at the height of this long dry season where it is so less what gives us so less is many trees burst forth into bloom as the seeds must be ripe and ready to sprout with the arrival of the rains. The most beautiful of these trees is the flame of the forest which you see in the next in this slide and this again has inspired many a poet to write beautiful poems. Another spectacular tree that blooms in the hot dry season before the monsoon is the Indian laburnum cashia fistula and in fact there is an interesting story about this tree people in Kerala believe that the onset of the monsoon in Kerala the date of onset of the monsoon in Kerala is linked to the date when this tree first blooms before the monsoon season. Of course in this hot season we have coils which are cocoons singing in mango trees in fruit and they lend magic to Vasantharitu or spring that just follows the spring equinox. Now as the dry season drags on the legs begin to dry and fish become very easy picking for the pelicans. Now when the monsoon sets in their chicks fly away and the pelicans close shop. As the rains approach nomadic shepherds head back towards their monsoon settlements in the semi-arid tracks of the peninsula obviously they want to go into regions where there is much less rain so they start moving towards the semi-arid tracks of the peninsula. Now this is a very famous bird called Chataka and the legend goes that Chataka which is actually a pied crested cacoo drinks only rain water no other water would do. So Chataka's intense pining for rain is often compared with the pining by humans for their loud ones again a source of lot of poetry in Indian literature. Now at the end of the hot dry season or what marks the end of the hot dry season certainly in some parts of India is the burst of the monsoon and you see here these black clouds cumulonimbus we will meet them again when we talk of clouds occurring over coast of Kerala which herald the burst of the monsoon. As soon as the burst of the monsoon occurs the mother earth changes you immediately see these flowers around many ephemerals flower with the first rains these are two of them then many medicinal herbs like shatavari which is asparagas sprout soon after the first rains termites take to wing the with the first rains mating and establishing new colonies in fact as soon as we hear the thunder and rain appear very often termites invade our houses also because they come out in such large numbers at this time and with these large numbers of termites on wing skillful flyers like drongo hawk this bounty of nature on wing enjoying a feast of a lifetime of course during the monsoon the spectacular dance of the peacock is what we all enjoy seeing the it characterizes the monsoon season peafowl breed at the height of the monsoon feeding on the reptiles that thrive during the rains now as rivers and streams swell with the monsoon rains many fish swim up the current to spawn in pools near the river origin and you see one beautiful one here now these are nests built by bio weaver birds now bio weaver birds take advantage of the abundance of insects that feed on the monsoon vegetation to raise their chick painted stalks breed at the height of the monsoon feeding on snails that thrive during the rains humans also have festivals which are connected with the monsoon and the one that attracts a lot of attention during the monsoon is the famous dahi hindi festival this takes place in goku lastami day and the one in Mumbai is very very famous it involves a pyramid of children human pyramid trying to break a earthen pot full of yogurt by eventually reach which is hung way above where they can reach so this is the celebration ritual celebration of the rains called dahi hindi now teak a valued timber tree of India flowers at the height of the rain these are enchanting flowering meadows on hill plateaus which are a grand spectacle as monsoon progresses and as the fury of rains abades fisher folk who had given up fishing during the monsoon season celebrate the resumption of fishing in the sea on on a full moon day the naradi purnima and this is called the coconut day and they have they are going out to with coconuts to celebrate this as the monsoon season draws to a close these gorgeous beasts of the forest the elephants begin to head back to the rain forest of the western guards which they had left for the deciduous forest just before the onset of the monsoon now as rains abate nomadic cattle keepers of kutch which is on the west coast to the north of the west coast move towards the forested tracks of central India seeding of grasses marks the end of the monsoon you see them all in seed there and you know in India we have typically two harvest season the kharif season the sowing occurs with the onset of the monsoon and harvest occur at the end of the monsoon now kharif harvest following the summer monsoon are celebrated by Deepavali again another very fast famous festival in India called the festival of lamps with fireworks seen here on top of the beautiful golden temple of amritsar now many birds of prey breed in the dry season that follows the monsoon and you can see them here in this picture taking advantage of the abundance of rats and mice that have patterned on the grass seeds the final retreat of the monsoon from the Indian region that is from the southeastern parts of the Indian peninsula is celebrated with another very colorful festival called pongal in south India makarasankranti in the north and what you see here is people very colorful clothes wearing very colorful clothes and decked and dancing to the tune of the music then comes holy again it is a festival of colors and it is celebrated towards the end of the Indian calendar here and in fact it is celebrated in the interim when there is some leisure following the rabbi harvest as I said there are two harvest rabbi is planted soon after the monsoon is over and in fact the crops grow on stored water in the soil after rabbi harvest and before they have to start preparing for the kharif season there is some leisure and it is in this leisure that holy the festival of colors is celebrated soon after celebrating holy shepherds move up the Himalayan slopes to high altitude pastures or boogyals so you see life for humans and other creatures plants animals bees birds actually is tuned to the seasonal cycle which is what we call the monsoon now so far we have seen how this life on the subcontinent revolves around the mean seasonal cycle the seasonal cycle itself is not the same over different parts of the country we have to realize that because the only picture we have seen is the all India average but the average does not reflect their experience of different parts of the country so in fact there is a great deal of spatial variation in the rainfall from place to place I must say one of the great advantages we have in studying the monsoon is the rich data sets that we have in fact data on observed rainfall and its variability are available at a large number of stations with modern rain gauges for the Indian region right from 1876 onwards. Now these have been extensively analyzed I said to begin with with Blanford and then by many meteorologists after that and actually some kind of analyzed data namely monthly data for averages over the entire Indian region or what we call meteorological subdivisions to which again I will come to later in the lectures are available actually to anybody who is interested from the website of the Indian Institute of Tropical Metrology at Pune. Now in addition to these rainfall data which are averaged over met subdivisions or over the entire country a graded data set has been prepared recently in fact it was published only in 2006 by Rajiv and others from India Met Department and these data are available from the website of the India Met Department. So, now let us see what are the major facets of the monsoon or the rainy season. Now the rainy season commences rather dramatically with the burst of the monsoon or what we call the onset of the monsoon and the first onset of the monsoon over the Indian region first occurs over Kerala. So, rainy season for the Indian region commences with the monsoon onset over Kerala which is recognized as a rapid substantial and sustained increase in rainfall. Now you can see that in the picture on your screen very clearly you see initially what is done here is this is what we call a composite picture. So, you know the onset date varies from year to year but what is done is every year the date on which the MOK or the monsoon onset over Kerala actually occurred is taken as 0 and then you count days from that days after that in terms of positive numbers here days before that in terms of negative numbers here. So, what you see here is the rainfall before the onset and rainfall after the onset this is obtained by averaging over all the years. And the reason we see such a sharp peak is that we have not plotted calendar dates here with calendar dates the you would not really see how abrupt the changes. But remember that in spite of the fact that it is an average over 80 years 1901 to 1980 in spite of the fact that there is its average over such a long period you see such a spectacular sharp change. So, almost a step function change in the rainfall because we have adjusted the time and dates such that all the onset dates fall on 0 this is what we call a composite. So, we have this spectacular onset over Kerala and the mean date is around 1st June. Now, in the onset phase of the monsoon which lasts for about 45 days the monsoon moves northward you see the monsoon moves northward in the onset phase. So, having come to Kerala it starts its journey northward in the onset phase and moves northward and northwestward to eventually reach cover most of the country by 1st July. But this is not the only track of the monsoon monsoon also onset of monsoon over Andaman also occurs early and in fact in late May and from there again it starts moving here. So, eventually around 1st June headway of Bengal also has had its onset and so then the monsoon moves from east to west towards Rajasthan and also from south to north towards the central parts of India. So, this is the onset phase of the monsoon and you can see that typically this onset phase takes about a month by early July. In fact, the almost the entire country has come under this way of the monsoon. So, this is the onset phase of the monsoon which takes typically 45 days because if you go to the edge here the Thar desert is somewhere here if you go to the edge here this is around 15 July and this is 1st June. But much of the country is already covered by 1st of July. So, this is how the monsoon comes this is the Agamana or the onset of the monsoon the retreat of the monsoon commences in September and it in fact retraces its steps in a way. So, 1st September the retreat begins in the northwestern part then it starts going slowly eastward by 1st October it has gone beyond 80 degrees east or so and it has also moved south. So, it has gone south of Mumbai and it has come halfway across the central plains by 1st October. So, a large portion of India is no longer under this way of the monsoon by the end of September. Now, by mid October actually most of the region north of 15 degrees north is out of this way of the monsoon the monsoon is restricted to the southern peninsula to the peninsula region south of about 15 degrees north. Now, it keeps retreating from there and eventually completely retreats from our region only by about 15th of December. So, this is the retreat phase of the monsoon and again so what have we seen then that the entire country comes under the sway of the monsoon at the end of the onset phase around the end of June or beginning of July. The monsoon begins its retreat from the northwest region around 1st of September. So, July and August are therefore considered peak monsoon months and by the 1st half of October the monsoon has gone south of 15 degrees north. Now, as we had seen earlier you know if we consider the country as a whole most of the rainfall comes during June to September this is called the summer monsoon season and has been the focus of most of the studies on the monsoon. Let us look at what does the rainfall look like over the Indian region during June to September. So, we are now looking at the average summer monsoon rainfall average over June to September at every place and what you see here is the rainfall pattern and you can see this tick tells you how much rain has occurred and please note that the units are centimeters. So, this very dark green the darkest green here in the northeast and also part of western Ghats and west coast actually correspond to more than 200 centimeters of rain in 4 months. So, it rains copiously in some parts then you can see that below 25 centimeters in 4 months we have not even shown any colors. Then this is 25 to 50 centimeters and there is actually a major rain belt that you see here in what we call the monsoon zone. Now, that is marked by red here so the monsoon zone actually has a major has the major rain belt of the summer monsoon and what is happening you can see is that the rain is maximum near the headway near the east coast and as we go along the monsoon zone towards the northwest the rainfall decreases from its maximum here to its minimum here. So, this is the large rain belt which is the major rainy system of our summer monsoon. In addition to that lot of rain also occurs heavy rain over west coast of the peninsula and the western Ghats and the northeast region. This is the spatial pattern of the mean summer monsoon rainfall please keep in the back of your mind that I am not talking of the rainfall of any year this is the mean picture or what reflects the climate of the region. Now, we have seen that the mean rainfall during the summer monsoon is characterized by the major rain belt over the monsoon zone which we saw with the rainfall decreasing northwestward from the maximum over the eastern part and in addition to this major rain belt very large scale rain belt heavy rainfall also occurs along the west coast of the peninsula and over the northeastern region. So, these are the typical 3 rainy regions of the southwest monsoon. Now, this monsoon zone is very important because in a way that is the seat of the monsoon during the peak monsoon months and what we have plotted here is again still the mean over many many years and this is the rainfall and this is the date. So, this will be the mean rainfall for 16th of May and red corresponds to the all India rainfall and blue corresponds to monsoon zone. So, what you see is that the variation of both is very very similar until the onset phase ends. In fact, the monsoon zone rainfall is smaller than all India rainfall because you must remember that monsoon has not made it yet to hold sway over the entire country. So, in the onset phase it is less, but you can see that the variation is still very very similar then during July it comes closer to the all India August it is very very close and then again once the retreat begins they part ways because Indian rainfall has contributions from other regions whereas, monsoon zone rainfall is decreasing. So, you can see here that the pattern of variation seasonal variation now this is much on a much smaller scale than month this is on a weekly scale is very similar for the monsoon zone rainfall and all India rainfall or the Indian region average rainfall. Now, there are many special features of the Indian region and one amongst them is our somewhat spectacular topography. Topography is mountain ranges and so on and so forth topo features if you wish. So, one important topography here is the western guards part of which is also called Sayadeh, but it contains also many other mountain ranges. So, these western guards play a very important role and the other is the spectacular Tibetan plateau which actually extends through almost half the troposphere and it is a huge massive plateau which certainly has an impact on the regional climate and the monsoon. Now, it is believed that topography of the western guards and the topography you see here lot of hill ranges and so on of the northeast contribute to the rainfall over those regions. So, this is the scenario as far as the average rainfall during the summer monsoon period is concerned. Now, although the focus of most of the studies of the Indian monsoon has been on the Indian summer monsoon. The summer monsoon is not the major rainy season for other for all the for the entire region it is not the major rainy season. In fact, let us see how the rainy season varies from place to place over India. So, what you see here is again mean monthly rainfall like we saw in the first slide, but this time it is not averaged over any special region. It is the mean monthly rainfall at different stations different stations that represent different parts of the country. So, now, let us begin here with the west coast we already saw that west coast receives a lot of rainfall and this is Mangalore on the west coast and the mean rainfall is shown here mean monthly rainfall and you see the scale is such that the maximum here is 120 centimeters in a month mind you. So, this is 120 centimeters in a month and what you see here is that there is hardly any rain up to March or even in April there is not much rain in May it picks up somewhat, but June, July are the very high rainfall months August it decreases on the west coast, but is substantive and September also it gets rain October also there is rain and November it starts peatering out by December there is not much rain. So, this is the situation this is about 13 degrees north this is on the west coast. Now, in the same latitude belt let us consider two other stations one is Mangalore from where actually this course is being recorded this is a Bangalore where the Indian Institute of Science is situated and let us see the monthly pattern for Bangalore now. Now, for Bangalore again you see January, February, March and December there is hardly any rain it picks up by April and is quite substantive in May decreases somewhat in June as the monsoon progresses northward of Bangalore. Again July, August, September it keeps increasing it is rather high in September and October and then decreases in November. So, it seems to get rain like Mangalore does in June, July as well as in the earlier part of the season the pre monsoon rain is also quite good here it gets rain in June, July like Mangalore continues to get rain in the latter part of the summer monsoon season as well and then gets quite a bit of rain in October and November. So, this is during the summer monsoon it gets rain it also gets pre monsoon rain it also gets rain in October November. Now, let us go to the east coast same latitude 13 degrees north and this is of course, the ancient city of Chennai. Now, Chennai as we went from Mangalore sorry as we went from Mangalore to Bangalore in fact, what happened was that the contribution of the October November rainfall increased. Now, as we go to Chennai in fact, it is October November rainfall that dominates June, July, August, September there is hardly any rain no pre monsoon rain and October November there is substantive rain which continues into December. So, you see there is considerable variation in pattern of rainfall over the peninsula, but by and large the contribution of October November rainfall is there for all the stations. Now, let us go to the southern tip of India this is in Tamil Nadu Palyam Kottay and this a bit like Chennai gets rain mainly in October November continues to get rain in December, but gets much more rain during the winter months as well as the pre monsoon and gets almost no rain at all from June to September much less than Chennai actually. Now, we should note that the rainfall involved is different here the maximum on the scale for Bangalore is only 20 centimeter same for Palyam Kottay and, but for Chennai it is higher. So, Chennai is getting more in these 2 months. So, so much now is the story for peninsula and I kind of dwelt on it a little bit because it is different in that it does not get rainfall only from the summer monsoon. Now, when we go to these stations all these stations get rainfall maximum rainfall during the summer monsoon on the east coast you get rainfall again June, July, August are the major months this is again Amravati in central India again here July gets more than August and September, but all 4 months get rain then this is in the north and again all 4 months get rain and then we go finally to Jaisalmer and Jaisalmer where June rain is not much because monsoon has not made it yet and July and August get enough rain and by September it starts retreating. So, there is considerable variation from place to place in the rainfall patterns. So, the pattern of variation of mean monthly rainfall is not the same across the Indian region we have seen this over the monsoons on most of the rainfall occurs during the summer monsoon and this season has also been called the southwest monsoon. If you remember your geography lessons in school we were taught that this was the southwest monsoon season. On the other hand south eastern peninsula and southern trip of peninsula most of the rainfall occurs during October to December which we call the post monsoon season, but which has also been called the northeast monsoon. Now, I will come back to these nomenclature. Now, I we have to note that the west coast of southern peninsula gets rain in the summer monsoon as well as in the post monsoon and that the summer monsoon season is also called the southwest monsoon season as I mentioned earlier and the post monsoon season is also called the northeast monsoon season as we were taught. Now, when I discuss the system responsible for the monsoon rain I will show that the name southwest and northeast for the seasons are misleading and inappropriate, but we will save that for the time when I talk about the system responsible for the monsoon. Now, the differences in the rainy season across the country are manifested as the differences in the spatial patterns for the different seasons obviously. So, let us now quickly see what are the what is the rainfall pattern like for the pre monsoon season which is March to May. So, March to May you can see that most of the rainfall occurs in the peninsula south of 15 degrees north this is where we could also see some pre monsoon showers in places like Bangalore and so on and so forth it also extends on the east coast. So, now there is also rain occurring at higher latitudes here, but this rain is not associated with tropical systems rather northern parts of India come under the mid latitude system in these months and so this is associated with mid latitude system. Now, this is the summer monsoon pattern which we have seen already and then comes the October to December pattern this is very similar to the pre monsoon pattern again you have rain primarily over the peninsula and over the east coast here and then we come to the winter which is January to February and this rainfall has nothing to do with tropics and you can see it is restricted to the northern parts which come under the sway of mid latitude systems or higher latitude systems. So, this is something else this is not monsoon rain this is some other rain. Now, so far I have talked of only what we can call the climate or what is the mean pattern mean temporal pattern we talked of average monthly rainfall both at stations as well as all India level as well as mean spatial pattern associated with each of the seasons the pre monsoon the post monsoon. So, what is the mean rainfall expected over different parts of India for during this season that is to say averaged over the season. Now, the whole point is that in no year does the Indian average rainfall look like the all India average mean rainfall that we have seen every year differs in all facets and so every year will be different and that is what makes the variability of the monsoon and it is the variability of the monsoon from climate which we are interested. So, the next focus is going to be on the nature of variability of the important facets of the monsoon as revealed by analysis of analysis of conventional as well as satellite data. Now, as far as this lecture is concerned then let us just review very briefly what we have learnt. We have learnt that this drama the monsoon the drama that occurs year after year with very similar events taking place but with considerable variability in the timing and distribution of these events. So, that it makes fun to study the monsoon variability, but the same events take place year after year. So, the first event is of course, in the drama is monsoon onset over Kerala and you saw how spectacularly the monsoon changed the rainfall increased and so it was called a sudden increase and this was sustained until the end of the season. So, it is then sustained after that you know it does not go back to what it was before the onset of the season. Then you recall we looked at what are the mean onset dates of the monsoon over different parts of the country mean dates of retreat of the monsoon from different parts of the country. We saw that the onset phase is about 45 days beginning with around 1st June with monsoon onset over Kerala. Then the monsoon progresses northward as we go and also westward at the same time that the monsoon onset occurs over Kerala. In fact even before that onset of monsoon occurs over the under months and then it progresses in this direction with the head way already getting the onset by about 1st of June and from there it progresses westward. So, you have the northward advance and the westward advance during the onset phase and the entire onset phase if you start from here and go right at the end takes about 45 days. Then we have also seen that the retreat of the monsoon is reversing the steps and by it starts in 1st of September and by the end of the summer monsoon season a large part of India has been left by the monsoon. Only this is covered by the monsoon by 15th of October most of the entire monsoon zone is out of the sway of the monsoon and it is only peninsula south of 15 and part of the east coast that has rain and eventually after giving rain in the so called post monsoon season the monsoon retreats from our countryside around December. So, this is what we have seen this is the summer monsoon rainfall which is really been the focus of most of the studies and which is something we understand most about and which I will also focus on. So, in the next lecture then what we will do is look at the variability of the monsoon by analysis of conventional and satellite data what are the important facets what are the things that we would like to understand and predict what are the facets that it is important to predict and that we would like to understand model and predict. Thank you.