 வணக்கம் சொல்லேன் உங்கள்வி respect the topic the subject of atmospheric science so in yesterday's class we talked about the mass of the atmosphere which turns out to be some 5.1 1 in the 10 to the power of 18 kilogram and then we saw that the means the mean the globally average surface pressure is 1 enter 10 to the power of5 or 1.013% of 5% and so on, and then we looked at the chemical composition which is largely dominated by diatomic molecules, nitrogen is heavy, some 78% by volume, oxygen is some 21% are gone and then other gases, water vapour can vary as low as 10 parts per million to 5% depending on whether it is a desert region or it is a tropical region and so on. So, the composition which we studied in which we discussed in yesterday's class is on a dry basis, a dry air basis, so it is not, H2O is not factored in into the calculations, okay. Then we saw that using the chemical composition, we figured out that the molecular weight of air is about 28.92, 28.97, whatever, it is very close to 29, so it is closer to nitrogen than oxygen, okay, so 29, 28.92 or 97 kg per kg mole, alright. From that we figured out how to convert this volumetric or molar analysis into a mass-based or a gravimetric analysis and we figured out that oxygen is 23% by weight, so the 21% has become 23% basically because oxygen is 32, has a molecular weight of 32 whereas nitrogen has a molecular weight of only 28. Out of all these gases, we mark certain gases with an orange pencil, so we call them as greenhouse gases because they are capable of interfering with the electromagnetic radiation. What electromagnetic radiation, electromagnetic radiation can be incoming radiation or outgoing radiation. The incoming radiation is largely from the sun, okay, and the sun is at a temperature of 5800 Kelvin, so later on we will study in atmospheric radiation that the Wien's displacement law, the lambda max into T, the wavelength at which the maximum intensity of radiation occurs for a black body which is at a temperature of T is given by 2898 micrometer Kelvin. So this 2898 can be approximated to be 2900, it so happens that the photosphere of the sun, the outer surface of the sun can be assumed to be a black body at 5800 Kelvin, so you get a magic number, lambda max is half a micrometer, it's beautiful because 0.4 to 0.7 micrometer is the visible part of the radiation, so that is why sun's radiation is very, very important and we are always trying to get this daylight lighting, all our tube light everything, we are trying to mimic the solar radiation, but of course this is not at 6000 Kelvin and so on, right. So the incandescent bulb will be at a temperature of 2900 Kelvin, therefore sometimes they say it reduces more heat than light, okay, still it will illuminate the incandescent bulb, tungsten bulb is at 2900 Kelvin, but if it is 2900 Kelvin, the lambda max is about 1 micrometer which is just outside the visible part of the spectrum, you are already getting into the infrared part of the spectrum. Don't worry about all these terminologies, we will flesh out all these terminologies as we enter into the atmospheric radiation chapter. So the incoming radiation is half micrometer, by the same token the surface of the earth is assumed to be at a temperature of 300 Kelvin, assuming this to be 3000, so the lambda max will be 10 micrometer, that is the radiation coming out of the earth, right. So these gas molecules have got peculiar properties where they absorb, they can allow radiation to pass through in one part of the spectrum, they don't allow radiation to pass through the other part of the spectrum and this is very important because the giving and taking is from bodies at different temperatures, therefore it leads from imbalances and that is why we call them as greenhouse gases and they cause some warming and how much warming they will cause if carbon dioxide increases by so on so on all this, that will be the subject of climate dynamics, climate science, climate change, climate dynamics which will be the last chapter of our course, alright. So this is a brief overview of what we did in the last two classes, so continuing with the vertical structure we figured out that p equal to, okay, so the pressure varies exponentially with height where h is the scale height of the atmosphere, so yesterday we saw, we worked out in a problem that h turns out to be approximately 8 kilometers, so the h is called the scale height of the e folding depth, okay, so if z equal to 8 kilometers, p equal to p0 e to the power of minus 1, if z equal to 16 kilometers, p equal to p0 e to the power of minus 2, if it is about 30 or 40 kilometers, e to the power of minus 4 or 5 is already gone, 1 by 2.7, 10 to the power of 4 will be a high value, so it rapidly decreases, the pressure rapidly decreases with height, so there are two ways of plotting it, so this by the way is called the Wien's displacement law, we said p was a height, right, z is p0, but this is an acceptable plot for the usual kind of plot in engineering, but in atmospheric science there is a peculiar way of plotting, what is that peculiar way, pressure will be in the x-axis, z will be in the y-axis, because it is analogous to how it will be when you actually see, right, so from z equal to 0 to some height, so we will interchange the coordinate and try to plot it, this is going to be useful, let us not disturb this, I will come back to this, so the main part of today's lecture is that the standard at moisture, so let us rework on this, so this is p and this is z, okay, but is Newton per meter square is a very small quantity, it is too small for us, okay and all this you will get very huge numbers here, so we work with some of the units for pressure, okay, what is one hecto, hecto is 100, 100 hPa, okay, hectopascal, therefore one atmosphere, how many hectopascals, 1,000 correct, so you can rework this plot and say this hecto and start with 1,000, okay, so then you can whatever 800, 600 whatever, it is not to scale, do not try to prove me wrong, we are just trying to look at the qualitative aspects of this, this fellow will be in kilometers, alright and there is something which we use in, there is one more unit which we use in atmospheric science which is called the milli bar, okay, so one hectopascal, one mb, okay, this is milli bar, okay, so one atmosphere is equal to 1,000 milli bar, is it correct, I hope everything falls in place, okay, so these are some, just like in mechanical engineering we use mPa, mega pascal bar, okay, so 10 bar equal to 1 mega pascal because pascal is a very small quantity, it is like paisa, there is no value, rupee itself has no value, so the paisa is absolutely, okay, so this pascal 1 Newton per meter square is a very small quantity, okay, for our application, for physics people it may be very high value, if they are doing something exotic, alright, now can I clear the board, I will not do anything to this, we have to get back to that, so continuing with this vertical structure, the rho and p, the rho and pressure p, rho is the density and the pressure p, the variation with z is similar, okay, so to a large, to a large extent, rho and p are not functions of x and y, if you still go ahead and use a Cartesian coordinates, x, y, z for the earth system, so rho p are not a function of x, y, which means it is possible for us to have rho is a function of z, p is a function of z only, okay, so when we talk about the density and the pressure where we just call it as rho and allow for variations in rho and p only as a function of z, then it is intrinsically implicit, it is intrinsically implicit that you are carrying out a horizontal averaging and we are also averaging over time, so it is average over a large time, it is spatially averaged over a large x y, so this means whenever I am saying the pressure is varying with z, the density is varying with z, all the other integrations are carried out and it is a mean value with respect to the other coordinates, okay, horizontally and spatially, what is that horizontal and spatial average, what I am trying to get at? A horizontal and spatial average is possible to define, which means we can think of a standard atmosphere which varies only with z and this standard atmosphere can be used in Africa, Asia, America, whatever, so that is the concept of standard atmosphere which I have given on the right side, okay, so I have a power point, this thing also, so take a few minutes and then please copy down this, so this is the standard atmosphere, so the x axis is the temperature in Kelvin and the y axis is basically the height in kilometers, so we are interested only in about 0 to 100 kilometers as far as atmospheric science is concerned and the temperature range is basically 160 to 300 Kelvin, okay, if you want you can take either of this, whichever is easier for you to copy, okay, now if you see it has got something like a z profile, much more involved than a z, now let us spend a few minutes trying to understand this vertical structure of the atmosphere, okay, now starting from 300 Kelvin at height z equal to 0, the temperature linearly decreases up to a height of 10 or 12 kilometers, okay, so the place where this activity takes place is called the troposphere, after a few minutes we will say why we will try to articulate why it is decreasing, why sometimes it is not increasing and so on, okay, the first 10 kilometers what happens is the temperature decreases with height, okay, so if you go to hill station it is colder basically because the hill station is still in the troposphere, okay, now as you reach some 10 kilometers height, okay, the temperature becomes insensitive to height for a few kilometers, maybe 8 to 10 kilometers, so that region where this region where the temperature is insensitive to height is a pause or a gap, okay or insensitivity with respect to height this is called the tropopause, so this is troposphere, okay, after crossing the tropopause the temperature again increases and it exhibits two slopes, okay, we will articulate why it is exhibiting two slopes, then it reaches again a local maxima at a height of about, height of about 50 kilometers, okay, so this region is called the stratosphere, don't get misled by the terminology stratosphere, no, no, it is actually here, yes, correct, yeah, the stratosphere means the all the guesses are stratified, there is no mixing because cloud activity is not there in the stratosphere, so since it is stratified this the concentration of the gases and the distribution everything will remain same for a considerable length of time, so if the aircrafts are flying in the stratosphere region and they are emitting this carbon dioxide and all this it will take a long time, they can be cleansed, it will take a long time for them to be cleansed, there should be a big convective cloud which goes all the way up to them, up to that height and remove all that, otherwise the cleaning up process is easier in that troposphere where there is violent activity, yesterday evening also we got rain, that is low convection, a lot of heat and then water because the sea is close by, water gets evaporated and then when it starts going up then it reaches the zero degrees, it condenses and if there is a wind from the ocean, from the bay of Bengal, from Marina beach it comes into first VR in Adair, so first we will get, maybe the western fringes may not get that much, this is typical tropical convection, so if you, if some gases are emitted in the troposphere and all that then there is a possible for cycling for them to get flushed out, but in the stratosphere, so this is very dangerous if some things are left in the stratosphere, so if you have some radioactive emission, if you take some nuclear explosion and these radioactive materials are pushed up to the stratosphere, it will remain there for years, so that problem is there, now that is the stratosphere as far as this thing is concerned up to 50 kilometers, then there is again a stratopause where the temperature does not change, then the temperature decreases again and then up to 80 kilometers, so this is called the mesosphere, then again it becomes invariant with respect to height for some region, this is called the mesopause, after this it will keep continuously, after this it will keep continuously increasing with height, then your regular fund-off it is getting closer to sun and all those things will apply, it will not apply in the first, silly question will be as you go up the sun's radiation will be less, why is it cooler in OT or Darjeeling compared to Chennai, silly question right, but it is, why is it a silly question, because it is not completely determined only by the radiation, okay, if you take a distance between sun and earth to be something, OT and this thing will be only 3 kilometers difference, if you take the radiation one by r square it will it won't appear matter at all, so what determines the temperature will be the balance of winds, moist clouds and all this, at least in the first order of first tens of few tens of kilometers, that fund-off you are closer to sun, all that will apply as you keep going up, okay, so the dynamics controls the weather, the dynamics controls the temperature distribution to a large extent, radiation will also control, but radiation plus dynamics will control the distribution in the atmosphere, so this is basically the vertical structure. Now, we will have to dig a little deeper into this, if you consider shall we go to the other side, okay, so I want to discuss something about this, so if it is approximately if it is z is greater than 105 kilometers, mean path, the mean path between molecules is greater than one meter, if the mean path between the molecules is greater than one meter, then what does it mean, individual molecules are sufficiently mobile, okay, when individual molecules are sufficiently mobile, each gas will behave as though it existed alone, this homogeneous mixture of nitrogen, carbon dioxide, all that which you study, which we discussed in yesterday's class will not be applicable after this 105 kilometers, okay, gases behave as if they existed, okay, molecules are mobile, each gas behaves, okay, so this 105 kilometer above 105 kilometer, 105 kilometer is called the turbopause, okay, what are the things are evident from here, any comments, what are the other things you can, so if each molecule, each gas species behaves as if it is alone, what will happen after 100 kilometers, the lighter species has a chance to go up, it is quite possible that the hydrogen may completely escape the earth's atmosphere, it is possible, so that is called the hydrogen cycle, hydrogen is the lightest, so the hydrogen may just, the hydrogen may just escape, some hydrogen may just escape out of the earth's atmosphere, it is possible, then if these gases are present for example, in this region, you have H, H2 helium, they are all very light gases, molecular weight of 1, okay, in its atomic form, hydrogen is atomic form, molecular form and helium, okay, but what is happening is, there is a strong irradiation which is coming, because of which there are two processes which are taking place, photo-ionization and photo-dissociation, okay, so this will decide the fate of the, so this will result in actually this photo-ionization, photo-dissociation also increase the energy content that is contributing to the increase in temperature in these regions which we call as the thermosphere, okay. Now, let us look at the physical processes which are taking place here, what about mixing, the first 10 kilometers it is strong mixing or weak mixing, strong mixing, very good, then in the stratosphere poor mixing, correct, okay, now, why is the temperature decreasing, so they actually basically there is a radiation cooling also, there is an absorption of radiation, there is an emission of radiation, the emission is much more than what is absorbing, so generally in troposphere, there is a radiation cooling, then here there is ozone absorption, O3 absorption of ultraviolet radiation that is happening in the stratosphere, okay, and radiation cooling is also taking place, there is a competition between these two, the absorption overcomes this radiation cooling, therefore the temperature increases in the stratosphere, okay, this activity, this absorption because ozone is not so much present in the troposphere, the heavy absorption of ultraviolet radiation by O3 molecules is not a big activity in the troposphere, so the temperature increases, but even in these cases there can be regions where temperature locally increases instead of decreasing, such things are called local temperature inversions, okay, suddenly it may go like this, okay, within the boundary layer this may cause some, so this is called the ramdas effect and ramdas layer, ramdas effect and so on, some people have researched this, particularly during night time some temperature inversion takes place or in early morning, dew and these things, there are some conditions which are created, so this, what is temperature inversion means, if generally in that layer temperature is decreasing with height, locally suddenly it increases with height somewhere, then it is an inversion or it is supposed to increase with height, suddenly it decreases, it is an inversion, okay, this temperature inversion is common in the troposphere in desert regions in, okay, so this temperature inversion is also some people say is responsible for the reverse swing and other things in cricket, which is available on certain, which can be effectively used only under certain conditions it works, right, you have studied that, so that if you want you can go to science directly look at the pages, I do not know whether it is extensively research, but there is a particular amount of moisture, dew and temperature conditions such that there is an inversion, then if the bowler does the right kind of thing, you can get some surprise swing, okay, which is called the reverse swing, okay, so if you are interested in all this, there is a separate subject of study called sports aerodynamics, how a golf ball, okay, how the dimple in the golf ball increases or decreases, so there is a separate field of study called sports aerodynamics, okay, you can specialize in that if you want later on, okay, now what about this region, okay, so there is a decreased solar, there is a here, sorry, here there is a decreased solar heating, somebody should raise sir, you are, you are concentrating yourself sir, how can we decrease solar heating, I am going closer to the sun, oh baba, decrease solar heating because gases are not present, the solar heating absorption and cooling is because of these mischievous fellows, who are mischievous fellows, carbon dioxide, ozone, whatever, all these, okay, as you go up, those fellows are reduced, the concentration of this is reduced therefore, but incoming radiation is there, okay, decreased solar radiation and increased cooling, okay, so we cannot, if all this logic does not apply it to the only thing is basically, if you do a measurement using a balloon, you will get this and second, if you do the radiated transverse simulations on your computer, you will get this, therefore this profile is indeed correct, now we have to have, now to be at peace with ourselves, we need to have a logical explanation of why this happens, so this is an explanation, there could be better explanation that is called theory, okay, there could be better explanation, but we know that, if you do the radiated transfer model, if you decrease the concentration and all this, you can get, you can simulate a different profile, which actually confirms that our hypothesis that there is increased cooling, there is a decreased cooling, there is O3 and all that is actually correct, so by systematic studies, we can show that, alright, so this is a very important, this is very, very important, okay, so this, for every, you studied, temperature decreased by 1 degree centigrade, 165 meters, is it, of this order, right, this is actually called gamma, okay, so this gamma is basically dT by dz, this is called as the lapse rate of the atmosphere, of the troposphere, okay, I have not drawn properly, it is okay, you correct it, it is a standard atmosphere, there you may get 9 or 10 or we will sort out all these things in a little later, okay, so this is about, yeah, now what I have done is 6.67, how much do you get per meter, point? 80 Kelvin, what happened? no, let us start with, 80 Kelvin per 10 kilometer, so you want it a 70, right, so I should have started at 290, it is okay, you can adjust a little bit, this is only, I told, it is only approximate, right, you correct it now, post facto, anyway it is good that you are thinking, what he is saying is he is getting 80 Kelvin per, but the mistake he made was, the mistake he made was, he took it as 1 kilometer, correct, this is 10 kilometer, yes or no, so let it get helped, 80 K, divided by 10, so it is 8, so I am off by, you can cut some marks for me, I am off by, I am off by some 1 or whatever, alright, fine, now this troposphere is a very important fellow because we found out that 5 and a half kilometers, in 5 and a half kilometers, how much of the mass of the earth is, how much of the mass of the atmosphere is present? Ha, now tell, now problem number 5, problem number 5, determine the mass of the atmosphere or determine the fraction of the mass of the atmosphere in the troposphere, determine the fraction of the, so the proxy for that would be the pressure, alright, and g you are assuming constant, you can agonize it by taking g is equal to g0 to 1 by r squared plus z squared, where you are reaching up to the center of the earth and all that g calculations you can do, you know 9.82 and maybe 9.805, pack all that, so problem number 5, what is the fraction of the mass of the atmosphere that is contained in the troposphere, how much is it, using this P by P0, isn't it, then what do you do, you calculate the P and then the M or what is it, are you going to work out like this or how will you find out, what is M, M equal to integral, rho dz, okay, then 0 to 10, okay, fine, so fraction is, this is real fun because I didn't come prepared, I just shot out that question, I hope we will get the answer, okay, rho dz divided by 0 to infinity, very good, rho we can substitute P equal to P by RT or how do you, how do you want to proceed, volunteers, anybody got this, how do you calculate the mass, but T will be a problem, know that lapse rate, we will pack the lapse rate or whatever, how do we calculate the mass in the first class, okay, so this will require ideal gas equation, right, have you done it already, please do it, I can erase this now, so at 10 kilometers you want to find the pressure, okay, very good, P0 you are taking, 1000, okay, oh that guy started, P to the power of minus, how much is it, yeah please, 0.286, 1 to 10 power 5, 0.286, no, no, tell me in Pascal, 28650, how do you proceed, is not as easy as you think or is it easy, then you just want P by P0, yeah, okay, so I already told you that rho and P follow a similar variation, so P by P0 is, okay, so is contained the first 10 kilometers, correct, correct me if I am wrong, is it okay, on the troposphere height you will also vary from sometimes it is 10 kilometers, sometimes it is 12 kilometers and so on, so it is fine, so generally the thumb rule is tropospheric air accounts for 80%, tropospheric air accounts for 80% of the total mass of the atmosphere, okay, so the cloud processes play a major rule in the troposphere, okay, precipitation cloud processes and all this play a major role in the troposphere, the cloud processes play a limited role in the stratosphere, that is why if you dump something in the stratosphere it is going to take a long time for it to cleanse, okay, heating by O3 molecules, okay, heating by O3 molecules is the highest around 40 to 50 kilometers of the earth's atmosphere, so the tombs, the total ozone mapping spectrometer is designed in such a way, the sensor is designed in such a way that will try to measure the ozone which is, whose absorption is speaking around 40 to 50 kilometers, that we can, so we can, so this is basically atmospheric remote sensing, so you can do that, okay, then finally as h increases the temperature keeps on increasing that is called the thermosphere, okay, so in this thermosphere as z increases t increases, so this is, alright, so this is the vertical structure of the atmosphere, the vertical structure of the atmosphere is preserved throughout the globe, okay, but at any given level in the atmosphere, at any given level in the atmosphere the temperature varies with latitude, please don't forget that, at any given height the temperature varies with latitude, that is why northern hemisphere Europe is colder than the same 500 meters height, the temperature will be much lower in Europe compared to Chennai for example, okay, so the structure remains the same, but within a particular z, at a particular z the temperature varies with latitude, I will write it if you want, the t is a function of the latitude, now if you take what is called as a climatological mean, what is a climatological mean, you send a balloon, get the temperature versus height in that place, Monday, Tuesday, Wednesday, January, February, up to January to December, January to December, January to December, you do it for many years, then you take an average that is called a climatological mean, you send the same balloon in Chennai, Nungambakkam or Meenambakkam, in Chennai, the average over many years is called the climatological mean, okay, so if you look at the climatological mean data at, pretty obvious right, North Pole it will be very okay, so I think whatever I want to cover in today's class is done, so this is very important, they used to call it as the US standard atmosphere for whatever reason, I think we can call it as Indian standard atmosphere also, why should we call it as US standard atmosphere, so this is the standard atmosphere whose structure, whose shape will be the same regardless of the location you are taking, but some slight variations will be there, so if you average it over several years, so you get what is called the climatological mean, so that climatological mean, so you say at a given z, temperature will vary with 1 by latitude and species like nitrogen, argon, carbon dioxide, they are well mixed, they are well distributed throughout the atmosphere, therefore if you take carbon dioxide concentration in one place, it is expected to be the same everywhere, okay, the lighter species are to be found at the higher levels and these fellows have a chance to do all this process like photo ionization, photo dissociation apart from the increased temperature in the thermosphere, there is also possibility for them to escape the earth's atmosphere, so this is a very important component in the study of atmospheric science, the study of the temperature profile, we did so many things, you should know when a pilot is taking his aircraft, you should know what is the outside temperature, if he does not have atmospheric science knowledge, he cannot adjust his AC because when the flight is flying at 12 kilometers, then outside temperature is minus 52 degrees centigrade, so what he does is he heats the cabin such that people are comfortable between 22 and 25 degrees centigrade, so he should have a knowledge of what is the pressure outside, that pressure if it is exposed then there would not be any oxygen people will die, so he has to increase the pressure, he has to pressurize the cabin, he has to air condition the cabin and all this and because of this pressure differences, so there are stresses which are occurring, therefore it should be structured, even the glass, it should be structurally stable such that this difference in pressure and all this it can manage, otherwise it will lead to windshield cracking and all that which you are looking, which you are reading in papers nowadays, all right, so we will stop here, in tomorrow's class I want to complete this introduction lecture where we look at winds and precipitation, what are the various types of winds and the precipitation patterns, with that this first chapter will be over, then we will go to the second chapter where we look at the various components of the earth's climate system including the mantle, the earth's crust, the ice content in the earth and then we leave the atmosphere alone because after this chapter is over, the remainder of the course is only on atmosphere, okay.