WEBVTT 00:01.367 --> 00:05.997 RONALD SMITH: Well, we were talking about mixing in the 00:06.000 --> 00:08.700 atmosphere. 00:08.700 --> 00:13.370 And we did two types of analysis. 00:13.367 --> 00:16.797 First of all, we assumed that there was a turbulence in the 00:16.800 --> 00:21.970 atmosphere that would mix gases or particles into a 00:21.967 --> 00:24.327 larger and larger volume. 00:24.333 --> 00:26.103 And we did three problems like that. 00:26.100 --> 00:35.030 One was adding a material to a confined valley. 00:35.033 --> 00:39.733 The second was adding material to the atmosphere in an 00:39.733 --> 00:42.233 unconfined situation. 00:42.233 --> 00:46.373 Where the turbulence would act to spread that material out 00:46.367 --> 00:49.067 progressively with time. 00:49.067 --> 00:53.397 So the longer the time had elapsed, the larger would be 00:53.400 --> 00:56.970 the volume into which the material had mixed. 00:56.967 --> 01:01.627 And the more dilute it would have become. 01:01.633 --> 01:06.273 The third problem we did was adding material at a steady 01:06.267 --> 01:14.227 rate into an atmosphere that has both turbulence and wind. 01:14.233 --> 01:18.333 In that case, the added material forms a kind of a 01:18.333 --> 01:23.803 plume downwind of the source point, spreading out as it 01:23.800 --> 01:27.170 goes horizontally and vertically, and becoming more 01:27.167 --> 01:29.327 dilute for that reason. 01:29.333 --> 01:31.903 It spreads out into more and more volume as 01:31.900 --> 01:33.030 it moves down wind. 01:33.033 --> 01:37.503 So we did problems like those three last time. 01:37.500 --> 01:42.470 And then I went on to start to talk about the role of the 01:42.467 --> 01:45.467 temperature gradient in mixing. 01:45.467 --> 01:49.527 It turns out that the temperature profile in the 01:49.533 --> 01:53.073 earth's atmosphere plays a very important role in how 01:53.067 --> 01:56.827 material mixes into the atmosphere. 01:56.833 --> 02:02.933 And that arises because under certain circumstances, a 02:02.933 --> 02:11.133 parcel of polluted air will be able to rise and spread into a 02:11.133 --> 02:13.133 large volume aloft. 02:13.133 --> 02:17.433 In other situations, that parcel if it tries to rise, 02:17.433 --> 02:21.703 may fall right back down to where it started and be unable 02:21.700 --> 02:22.900 to mix vertically. 02:22.900 --> 02:25.970 And this has to do with the vertical temperature gradient. 02:25.967 --> 02:29.227 So I'm going to pick up on that theme today. 02:29.233 --> 02:31.833 But if there are any questions on it at this 02:31.833 --> 02:36.473 point just call out. 02:36.467 --> 02:36.867 All right. 02:36.867 --> 02:42.627 So this is the diagram that I had on the board a little bit 02:42.633 --> 02:43.133 differently. 02:43.133 --> 02:48.903 But I was defining atmospheric stability as the resistance to 02:48.900 --> 02:50.170 vertical motion. 02:53.767 --> 02:57.397 And this is a plot of temperature on the x-axis 02:57.400 --> 03:00.330 versus height on the y-axis. 03:00.333 --> 03:05.303 And we put on there a reference line, which this 03:05.300 --> 03:08.000 author has written DALR. 03:08.000 --> 03:11.130 That's the dry adiabatic lapse rate. 03:11.133 --> 03:13.203 I call it just the adiabatic lapse rate. 03:13.200 --> 03:17.030 But it's good to call it the dry adiabatic lapse rate 03:17.033 --> 03:21.833 because inside a cloud, that takes on a different slope. 03:21.833 --> 03:24.133 So this is one when you're outside a cloud, there's no 03:24.133 --> 03:25.673 water vapor condensing. 03:25.667 --> 03:28.927 So we'll call it the dry adiabatic lapse rate. 03:28.933 --> 03:34.373 And all these other curves are possible, actual atmospheric 03:34.367 --> 03:36.067 lapse rates. 03:36.067 --> 03:38.267 On Tuesday, maybe the temperature 03:38.267 --> 03:40.667 decreases aloft like that. 03:40.667 --> 03:42.397 On Wednesday, maybe it's like that. 03:42.400 --> 03:43.730 The next day, maybe it's like that. 03:43.733 --> 03:44.903 It might even be like that. 03:44.900 --> 03:47.700 In other words, from day to day from hour to hour, you're 03:47.700 --> 03:52.900 going to have different actual lapse rates. 03:52.900 --> 03:56.430 And in each case, we're going to want to compare that with 03:56.433 --> 04:01.173 this reference curve, the dry adiabatic lapse rate. 04:01.167 --> 04:07.327 And as I discussed last time, all of these that are less 04:07.333 --> 04:10.903 horizontal, that is more vertical or more to the right, 04:10.900 --> 04:13.200 than the dry adiabatic lapse rate, are going 04:13.200 --> 04:14.400 to be called stable. 04:14.400 --> 04:20.130 Because if you lift a parcel in the air, it's going to be 04:20.133 --> 04:23.903 cooling at a rate of 9.8 degrees per kilometer. 04:23.900 --> 04:26.530 It's going to end up-- 04:26.533 --> 04:29.433 if I lift it from there to there, it's new temperature is 04:29.433 --> 04:30.473 going to be here. 04:30.467 --> 04:34.597 And that's going to be in all cases colder than the 04:34.600 --> 04:38.330 environment into which it's been lifted. 04:38.333 --> 04:41.003 If it's colder, it's going to want to sink back down to 04:41.000 --> 04:42.230 where it came from. 04:42.233 --> 04:44.773 So we call that stable. 04:44.767 --> 04:48.627 The unstable case is where you have a very strong decrease in 04:48.633 --> 04:52.033 temperature with height. 04:52.033 --> 04:56.173 Then if you lift a parcel along the dry adiabatic lapse 04:56.167 --> 05:00.697 rate, it's actually getting warmer than it's environment. 05:00.700 --> 05:03.330 So when it's been lifted a little bit, it finds itself 05:03.333 --> 05:05.703 warmer than it's local environment. 05:05.700 --> 05:06.930 That means it's buoyant. 05:06.933 --> 05:09.203 And it's going to rise further. 05:09.200 --> 05:12.400 That's the unstable situation. 05:12.400 --> 05:17.970 So you'll want to practice sketching these diagrams up to 05:17.967 --> 05:20.327 be sure that you always get the right answer about what 05:20.333 --> 05:22.303 the parcel is going to do. 05:22.300 --> 05:26.400 You might want to practice lifting a parcel as well as 05:26.400 --> 05:28.630 pushing a parcel downwards. 05:28.633 --> 05:33.073 And in all cases, you should be able to recover this basic 05:33.067 --> 05:37.667 idea that the atmosphere is either stable or unstable 05:37.667 --> 05:42.197 depending on its lapse rate compared with the reference 05:42.200 --> 05:45.970 value of the adiabatic lapse rate. 05:45.967 --> 05:48.827 Questions on that? 05:48.833 --> 05:49.303 Yes. 05:49.300 --> 05:52.530 STUDENT: Is the dry adiabatic lapse rate always 9.8? 05:52.533 --> 05:55.673 PROFESSOR: Yes, in the earth's atmosphere, it's always 9.8 It 05:55.667 --> 06:01.827 turns out to be the ratio of the surface gravity, 9.81, to 06:01.833 --> 06:04.473 the heat capacity of constant pressure for 06:04.467 --> 06:06.527 air, which is 1,004. 06:06.533 --> 06:09.633 So as long as we're talking about planet earth, the 06:09.633 --> 06:12.973 adiabatic lapse rate is always 9.8, the dry 06:12.967 --> 06:13.727 adiabatic lapse rate. 06:13.733 --> 06:16.633 Now if there is moisture present, then 06:16.633 --> 06:17.503 it's a different value. 06:17.500 --> 06:19.670 STUDENT: And that's in what units? 06:19.667 --> 06:23.297 PROFESSOR: That would be 9.8 would be in degrees Celsius 06:23.300 --> 06:24.530 per kilometer. 06:29.000 --> 06:29.330 OK. 06:29.333 --> 06:34.533 Now I want to apply this to some situations. 06:34.533 --> 06:36.073 And let me darken this a little bit 06:36.067 --> 06:37.297 so you can see that. 06:42.767 --> 06:47.727 I want to walk through a typical diurnal cycle, a day 06:47.733 --> 06:48.803 night cycle. 06:48.800 --> 06:52.170 And today would be a perfect example of this, clear sky 06:52.167 --> 06:55.067 overnight, clear sky during the day. 06:55.067 --> 06:58.627 It's going to work very much like I described 06:58.633 --> 06:59.773 in these four panels. 06:59.767 --> 07:06.867 So at 6:00 AM this morning, you have built up a nocturnal 07:06.867 --> 07:08.627 boundary layer. 07:08.633 --> 07:14.373 This is temperature versus height plotted here with the 07:14.367 --> 07:15.767 purple curve. 07:15.767 --> 07:20.027 Notice that there's a positive lapse rate in the lower part 07:20.033 --> 07:22.903 of the atmosphere in the early morning. 07:22.900 --> 07:26.200 The ground has been cooling off all during the night by 07:26.200 --> 07:29.730 radiating infrared radiation to space. 07:29.733 --> 07:34.633 That's drawn heat out of the lowest few meters of the 07:34.633 --> 07:35.733 atmosphere. 07:35.733 --> 07:39.933 And has produced that positive lapse rate. 07:39.933 --> 07:42.103 You can call that an inversion. 07:42.100 --> 07:45.870 In fact, I'm going to call it a surface level inversion. 07:45.867 --> 07:49.667 It's an inversion located right at just above the 07:49.667 --> 07:51.167 earth's surface. 07:51.167 --> 07:55.367 And that typically happens overnight on a clear night 07:55.367 --> 07:59.367 when the radiation can leave, can escape the earth's 07:59.367 --> 08:02.197 atmosphere without hitting clouds. 08:02.200 --> 08:04.470 So remember where that fit on the diagram. 08:07.067 --> 08:09.867 Just to go back. 08:09.867 --> 08:12.527 That's this. 08:12.533 --> 08:15.903 So that's a very stable atmosphere. 08:15.900 --> 08:18.570 And turbulence will be suppressed. 08:18.567 --> 08:21.267 There will be very little turbulence. 08:21.267 --> 08:23.327 Very little vertical motion. 08:23.333 --> 08:29.903 And the air will just lie there quietly in layers. 08:29.900 --> 08:33.600 The wind, which has continued to blow aloft all during the 08:33.600 --> 08:36.500 day and night, will be calm at the surface. 08:36.500 --> 08:40.330 Because without the turbulence, there's no way to 08:40.333 --> 08:45.133 mix that momentum down to the surface of the earth. 08:45.133 --> 08:48.373 Remember the wind aloft doesn't feel the 08:48.367 --> 08:50.827 diurnal cycle very much. 08:50.833 --> 08:52.703 So it blows day and night typically. 08:52.700 --> 08:55.900 That depends more on the local weather systems and so on. 08:55.900 --> 09:00.770 But the winds often go calm at night at the very surface 09:00.767 --> 09:03.267 because of this lack of turbulence to mix 09:03.267 --> 09:04.767 the momentum down. 09:04.767 --> 09:07.697 You must have noticed this if you're a camper 09:07.700 --> 09:10.130 or an outdoors person. 09:10.133 --> 09:13.403 You will have noticed that very often, wind decreases to 09:13.400 --> 09:14.800 calm at night. 09:14.800 --> 09:17.130 And then picks up the next morning. 09:17.133 --> 09:20.573 I noticed it myself this morning, for example, sitting 09:20.567 --> 09:25.667 at home 7:00 in the morning, looked out, none of the leaves 09:25.667 --> 09:27.527 were moving. 09:27.533 --> 09:31.333 And then around 8:00 or 8:30, suddenly the leaves begin to 09:31.333 --> 09:32.603 move finally. 09:32.600 --> 09:36.370 Because the earth had heated up the atmosphere, convection 09:36.367 --> 09:41.427 began, and you start to get the wind being brought down. 09:41.433 --> 09:44.103 And the wind starts to move at the earth's surface. 09:44.100 --> 09:45.670 So let's go to that next stage then. 09:45.667 --> 09:51.367 So 11:00 AM, the sun is heating the earth. 09:51.367 --> 09:54.467 If the earth is getting warm then it's putting heat into 09:54.467 --> 09:56.697 the lowest few meters of the atmosphere. 09:56.700 --> 09:59.930 And that's going to bring that lapse right around this way, 09:59.933 --> 10:03.603 hotter below and cooler above. 10:03.600 --> 10:07.970 And that's going to fit into one of these categories, 10:07.967 --> 10:10.467 probably that one or that one. 10:10.467 --> 10:15.067 If it goes unstable, convection is going to begin. 10:15.067 --> 10:16.697 And that's what they've drawn here. 10:19.833 --> 10:24.433 The heat being provided by the surface of the earth produces 10:24.433 --> 10:28.633 a strong negative lapse rate, pushes the atmosphere into an 10:28.633 --> 10:32.473 unstable state, and convection begins. 10:32.467 --> 10:33.967 The air begins to turn over. 10:33.967 --> 10:37.667 When it does that, it brings some of that general wind down 10:37.667 --> 10:39.827 to the earth's surface. 10:39.833 --> 10:42.273 That trend continues. 10:42.267 --> 10:46.727 By 3:00 in the afternoon, that convection layer has grown to 10:46.733 --> 10:49.803 a kilometer or a kilometer and 1/2 deep. 10:49.800 --> 10:54.100 Very often there are cumulus clouds with their flat bases 10:54.100 --> 10:56.670 right at the bottom of that-- 10:56.667 --> 11:00.897 or right at the top of that convective layer. 11:00.900 --> 11:04.430 So this afternoon, if you look out, and you see cumulus 11:04.433 --> 11:07.373 clouds, you'll know that this has happened. 11:07.367 --> 11:10.797 And that cloud base is right at the top of the convection 11:10.800 --> 11:11.570 boundary layer. 11:11.567 --> 11:15.897 And then when we get to the evening again, 11:15.900 --> 11:19.300 the sunlight stops. 11:19.300 --> 11:23.200 The earth's surface cools with infrared radiation. 11:23.200 --> 11:26.200 And you start to build up that surface level 11:26.200 --> 11:28.230 inversion once again. 11:28.233 --> 11:34.003 So the cycle just repeats day and night, day and night. 11:34.000 --> 11:36.370 Any questions on this? 11:36.367 --> 11:36.867 Yes. 11:36.867 --> 11:38.127 STUDENT: I'm still a little confused about why the 11:38.133 --> 11:39.403 inversion one is the stable atmosphere? 11:43.833 --> 11:47.573 PROFESSOR: The question is why is the inversion so stable? 11:47.567 --> 11:49.327 Well, think of it this way. 11:49.333 --> 11:53.403 When you lift a parcel, it gets colder with height. 11:53.400 --> 11:58.230 And yet if you have a background lapse rate like 11:58.233 --> 12:00.673 this, the environment is actually 12:00.667 --> 12:03.267 getting warmer with height. 12:03.267 --> 12:06.267 So you're getting both things working 12:06.267 --> 12:08.597 in the stable direction. 12:08.600 --> 12:11.630 The parcel is getting colder because it's expanding and 12:11.633 --> 12:14.433 cooling adiabatically. 12:14.433 --> 12:17.673 Yet, you're pushing it into an environment which is 12:17.667 --> 12:20.667 progressively warmer and warmer. 12:20.667 --> 12:23.797 So almost certainly, that parcel is going to find itself 12:23.800 --> 12:25.830 colder than its new environment. 12:25.833 --> 12:29.203 And it's going to want to sink back to where it came from. 12:29.200 --> 12:34.630 So the reason why we say very stable is because both the 12:34.633 --> 12:38.003 adiabatic expansion and the movement into a new 12:38.000 --> 12:43.000 environment both work to make the parcel want to return to 12:43.000 --> 12:44.230 where it came from. 12:50.367 --> 12:52.467 Any questions about the diurnal cycle? 12:57.500 --> 12:57.970 Yes. 12:57.967 --> 12:58.767 STUDENT: Is it just the Earth's surface temparature 12:58.767 --> 13:00.097 that drives it? 13:04.833 --> 13:05.133 PROFESSOR: Yes. 13:05.133 --> 13:06.073 That's right. 13:06.067 --> 13:10.067 So remember the sun's radiation is coming through. 13:10.067 --> 13:11.767 The lower part of the atmosphere is generally 13:11.767 --> 13:12.527 transparent. 13:12.533 --> 13:15.473 And so the sun's radiation is coming through and hitting the 13:15.467 --> 13:19.827 earth's surface, warming it up during the day. 13:19.833 --> 13:22.873 And that heat is then transferred immediately from 13:22.867 --> 13:25.927 the earth's surface to the first meter or so of the 13:25.933 --> 13:26.873 atmosphere. 13:26.867 --> 13:30.267 And that's what causes that lapse rate to tip over. 13:30.267 --> 13:32.097 Because you're adding heat from the bottom. 13:32.100 --> 13:35.600 Here, you're subtracting heat from the bottom, giving you 13:35.600 --> 13:37.870 this inversion. 13:37.867 --> 13:40.767 So you have the earth's surface playing a very 13:40.767 --> 13:41.797 important role in this. 13:41.800 --> 13:43.700 That's a good point. 13:48.467 --> 13:51.467 Now you could also get what's called an elevated inversion. 13:54.867 --> 13:57.567 And that's what's pictured here. 13:57.567 --> 14:01.267 Whenever you have slow subsiding air up through the 14:01.267 --> 14:05.627 troposphere, as you would, for example, as you do today, 14:05.633 --> 14:09.233 whenever you have high pressure and clear skies, you 14:09.233 --> 14:12.633 can be pretty certain that there is very slow subsidence. 14:12.633 --> 14:17.603 The air is slowly descending through the troposphere. 14:17.600 --> 14:21.170 Well, that air cannot descend through the earth's surface 14:21.167 --> 14:22.697 because the earth's surface is solid. 14:22.700 --> 14:25.930 So it has to spread out. 14:25.933 --> 14:29.503 And you get warming here but not warming at the surface. 14:29.500 --> 14:34.530 So you end up in this subsidence situation creating 14:34.533 --> 14:36.803 an elevated inversion. 14:36.800 --> 14:39.730 If I plotted the temperature, it would probably look like 14:39.733 --> 14:45.333 decreasing, then increasing, and decreasing again in an 14:45.333 --> 14:47.273 elevated inversion. 14:47.267 --> 14:50.827 An inversion of this type too is very stable. 14:50.833 --> 14:53.403 It's almost impossible to mix air across it. 14:53.400 --> 14:56.730 So any pollution put in beneath will be trapped, 14:56.733 --> 15:00.533 almost as if it were a rigid lid. 15:00.533 --> 15:01.133 But it's not. 15:01.133 --> 15:02.603 It's air above and air below. 15:02.600 --> 15:06.130 But it's warm air above. 15:06.133 --> 15:09.333 And therefore parcels that try to rise up through here find 15:09.333 --> 15:11.433 themselves too cold, too dense. 15:11.433 --> 15:12.703 And they sink back down. 15:12.700 --> 15:15.570 And they can't mix that pollutant up into the free 15:15.567 --> 15:18.397 atmosphere. 15:18.400 --> 15:23.400 Most of the pollution episodes are connected with these 15:23.400 --> 15:26.870 elevated inversions. 15:26.867 --> 15:31.697 For example, some of the most famous polluted cities in the 15:31.700 --> 15:35.170 world: Los Angeles; Santiago, Chile; Mexico 15:35.167 --> 15:37.527 City; Beijing in China. 15:37.533 --> 15:39.733 You see pictures of them here. 15:39.733 --> 15:43.003 In each case, in this case, you can see the inversion. 15:43.000 --> 15:45.370 Here you can see it as well. 15:45.367 --> 15:48.167 There is, an elevated inversion trapping that 15:48.167 --> 15:52.567 pollutant, preventing it from being diluted up into the rest 15:52.567 --> 15:55.367 of the free atmosphere. 15:55.367 --> 16:01.167 So inversions go together with air pollution episodes. 16:01.167 --> 16:04.427 They are very closely linked. 16:04.433 --> 16:07.273 If the inversion was not there, this pollution could 16:07.267 --> 16:09.197 just quickly mix up into the free atmosphere. 16:09.200 --> 16:12.900 It would be much more completely diluted. 16:12.900 --> 16:16.000 And you wouldn't have the concentration of sulfur 16:16.000 --> 16:19.570 dioxide, and NO, and ozone, and so on that you have in 16:19.567 --> 16:22.097 these polluted episodes. 16:25.067 --> 16:30.427 I want to give you three examples of rather famous gas 16:30.433 --> 16:34.403 releases that kind of illustrate some of the 16:34.400 --> 16:37.330 physical principles I'm discussing today. 16:37.333 --> 16:46.733 The first two are going to be brief emissions of toxic gases 16:46.733 --> 16:50.273 that just happen to take place at night. 16:50.267 --> 16:51.567 Now this was a problem. 16:51.567 --> 16:54.967 If the same thing had happened during the day, it would not 16:54.967 --> 16:56.067 have been so much of a problem. 16:56.067 --> 17:01.067 But at might remember, you've got this stable, nocturnal 17:01.067 --> 17:04.827 surface level inversion that prevents vertical mixing. 17:04.833 --> 17:08.673 And therefore, if you emit something at night, it's not 17:08.667 --> 17:09.567 going to be diluted. 17:09.567 --> 17:13.367 It's going to stay in high concentrations close to the 17:13.367 --> 17:14.627 earth's surface. 17:14.633 --> 17:18.203 So unfortunately, these first two cases, Lake Nyos and 17:18.200 --> 17:23.130 Bhopal, were both night time releases. 17:23.133 --> 17:24.403 Here's the Lake Nyos one. 17:24.400 --> 17:26.270 It's a lake in Africa. 17:26.267 --> 17:31.227 It's a deep, volcanic lake that over a period of months 17:31.233 --> 17:36.033 or years, because the water had been circulating through 17:36.033 --> 17:40.903 volcanic rocks beneath, had built up a high dissolved load 17:40.900 --> 17:42.830 of carbon dioxide. 17:42.833 --> 17:46.433 It was dissolved in the liquid just like it is when you buy a 17:46.433 --> 17:47.573 can of Coke. 17:47.567 --> 17:51.927 Most of the CO2 is dissolved in the liquid. 17:51.933 --> 17:55.133 And then for some reason, and no one to this day knows 17:55.133 --> 18:01.573 exactly why it happened or why it happened at that point, 18:01.567 --> 18:07.227 that CO2 one night came out of solution to form bubbles, rose 18:07.233 --> 18:10.173 to the surface of the lake, and then spread 18:10.167 --> 18:12.597 out across the landscape. 18:12.600 --> 18:16.530 Remember CO2 has the molecular weight of 44. 18:16.533 --> 18:18.033 Air has 29. 18:18.033 --> 18:20.203 So it's a denser gas than air. 18:20.200 --> 18:23.870 That's going to make it want to hug the surface as well. 18:23.867 --> 18:25.367 And because it was a night, there was a 18:25.367 --> 18:27.297 surface level inversion. 18:27.300 --> 18:30.100 That prevented any vertical mixing. 18:30.100 --> 18:33.970 So the CO2 stayed in a concentrated form and just 18:33.967 --> 18:38.627 spread laterally away from the lake killing thousands of 18:38.633 --> 18:43.173 cattle but also hundreds of people as well as they slept. 18:43.167 --> 18:47.427 This layer of CO2 just slid over them and then basically 18:47.433 --> 18:48.303 suffocated them. 18:48.300 --> 18:53.130 Because you can't breathe carbon dioxide. 18:53.133 --> 18:57.033 I want to make the point again that this would not have 18:57.033 --> 19:00.173 happened if the emission had occurred during the day. 19:00.167 --> 19:04.427 There would have been enough mixing, enough convection to 19:04.433 --> 19:06.803 dilute that carbon dioxide. 19:06.800 --> 19:10.130 And you wouldn't have had those deaths. 19:10.133 --> 19:17.603 The Bhopal incident in India, some 25 ago, was a Union 19:17.600 --> 19:24.330 Carbide plant making what's called MIC, methyl isocyanate. 19:24.333 --> 19:27.873 It sounds pretty ugly, and it is. 19:27.867 --> 19:31.327 There were some storage tanks that leaked. 19:31.333 --> 19:37.303 And the normal systems for capturing that leak were not 19:37.300 --> 19:38.670 functioning properly. 19:38.667 --> 19:42.197 And it happened to then get released into the lower 19:42.200 --> 19:46.430 atmosphere unfortunately at night. 19:46.433 --> 19:50.603 So rather than getting mixed and convectively diluted, it 19:50.600 --> 19:55.230 spread out as a layer to the surrounding houses and 19:55.233 --> 19:59.073 hundreds of people were killed as they were overtaken by this 19:59.067 --> 20:02.627 layer of methyl isocyanate. 20:02.633 --> 20:07.073 And that was a big deal at the time, front page Time 20:07.067 --> 20:09.227 magazine and so on. 20:09.233 --> 20:12.673 I was in India a few months after that and 20:12.667 --> 20:13.727 took a lot of abuse. 20:13.733 --> 20:21.673 Because this incident started an intensive ten years of 20:21.667 --> 20:25.527 anti-Americanism in India because of this incident. 20:25.533 --> 20:27.273 And it's still remembered today. 20:27.267 --> 20:30.627 If you talk to people from India and mention Bhopal, they 20:30.633 --> 20:35.803 will talk about how the evil US chemical corporations 20:35.800 --> 20:38.970 caused this terrible accident in their country. 20:38.967 --> 20:40.327 And they have a good point. 20:40.333 --> 20:41.833 But there was some luck involved too, 20:41.833 --> 20:42.803 bad luck that is. 20:42.800 --> 20:47.200 Because the release happened to occur at night rather than 20:47.200 --> 20:50.970 during the day 20:50.967 --> 20:55.967 So Chernobyl, a different kind of situation; it was the 20:55.967 --> 21:00.327 meltdown of a nuclear power plant. 21:00.333 --> 21:02.733 It lasted for several days. 21:02.733 --> 21:06.333 So it's not a question of day versus night. 21:06.333 --> 21:10.973 In this case, the warm radioactive gases released 21:10.967 --> 21:15.297 from that plant did lift upwards into the atmosphere 21:15.300 --> 21:18.030 and then the wind carried it to great distances. 21:18.033 --> 21:21.473 So maybe the drawback, perhaps the drawback of having a 21:21.467 --> 21:24.997 daytime release is it gets diluted. 21:25.000 --> 21:27.830 But it also gets up in the atmosphere where the winds can 21:27.833 --> 21:29.333 carry it to a greater distance. 21:29.333 --> 21:33.533 And if the substance is diluted to a safe level by 21:33.533 --> 21:35.533 that mixing process that's OK. 21:35.533 --> 21:38.273 But in this case, there was so much radioactive material that 21:38.267 --> 21:42.397 even though it was spread over large parts of eastern Europe 21:42.400 --> 21:46.530 and northern Europe, it still caused damaging levels of 21:46.533 --> 21:50.833 radioactivity spread by the winds. 21:50.833 --> 21:55.203 This was mostly radioactive iodine that then settles on 21:55.200 --> 21:57.870 the grasses and people get it into their system. 21:57.867 --> 22:02.597 And they end up with radioactive induced diseases. 22:02.600 --> 22:04.630 Same thing happened in this country during the nuclear 22:04.633 --> 22:08.833 testing episode back in the 50's and early 60's when we 22:08.833 --> 22:11.803 were doing, believe it or not, aerial 22:11.800 --> 22:16.630 nuclear tests in Nevada. 22:16.633 --> 22:19.303 Where in mid latitudes, the winds are normally west to 22:19.300 --> 22:20.900 east in these latitudes. 22:20.900 --> 22:23.830 So this shows some of the radioactive regions that were 22:23.833 --> 22:29.703 found coming from aerial explosions and the wind 22:29.700 --> 22:35.270 carried that radioactive material from west to east. 22:35.267 --> 22:36.527 Any questions on this? 22:39.433 --> 22:42.433 The point here is just to talk about what happens when you 22:42.433 --> 22:44.773 put substances into the atmosphere. 22:44.767 --> 22:46.327 How does it get mixed around? 22:46.333 --> 22:47.933 What conditions control that? 22:52.600 --> 22:57.400 Forest fires are a good example. 22:57.400 --> 22:59.200 You're burning biomass. 22:59.200 --> 23:01.930 You're producing smoke. 23:01.933 --> 23:06.103 And depending on the atmospheric conditions, that 23:06.100 --> 23:08.900 may hug the surface and move downwind 23:08.900 --> 23:09.770 like it's shown there. 23:09.767 --> 23:15.267 Or it might rise to a higher altitude and then move across 23:15.267 --> 23:18.027 through the atmosphere without polluting 23:18.033 --> 23:19.173 the very lowest levels. 23:19.167 --> 23:21.497 Eventually it'll fall back down to the surface. 23:21.500 --> 23:24.730 But if it's hot enough material, it will rise and get 23:24.733 --> 23:29.103 carried away some kilometers above the earth's surface. 23:29.100 --> 23:29.730 Those are the two 23:29.733 --> 23:33.033 possibilities with forest fires. 23:33.033 --> 23:34.473 Smokestacks are the same way. 23:34.467 --> 23:40.727 Now a smokestack is designed to get this smoke released at 23:40.733 --> 23:45.003 a point that's above any surface level inversion. 23:45.000 --> 23:47.300 So then it'll be carried quickly into the free 23:47.300 --> 23:50.700 atmosphere and become rapidly diluted. 23:50.700 --> 23:55.970 So that protects the local environment from the negative 23:55.967 --> 23:57.667 effects of the smoke and the air pollution. 23:57.667 --> 24:00.567 But it does, of course, mean that now the material will be 24:00.567 --> 24:04.967 carried to greater distances because you've put it up into 24:04.967 --> 24:06.927 a level where the winds are stronger. 24:06.933 --> 24:09.973 And it will be carried off downwind. 24:09.967 --> 24:14.467 As in our little problem we did last time with a point 24:14.467 --> 24:16.867 source and the wind carrying it away. 24:22.200 --> 24:26.530 During the first Gulf War, the Kuwaiti oil fires. 24:26.533 --> 24:29.573 You see them here burning off patches of 24:29.567 --> 24:31.027 oil, producing smoke. 24:31.033 --> 24:32.173 They're rising. 24:32.167 --> 24:35.127 Typically they're so hot that they'll rise a couple of 24:35.133 --> 24:37.033 kilometers up in the atmosphere. 24:37.033 --> 24:38.203 And then they'll level out. 24:38.200 --> 24:42.730 And they'll move horizontally under the action of the wind. 24:42.733 --> 24:45.933 In just a moment, I'll talk about how you can estimate how 24:45.933 --> 24:51.673 far these things rise before they start to spread downwind. 24:51.667 --> 24:57.767 Here's an aerial photo of these black, sooty, smoky 24:57.767 --> 25:00.427 plumes from the oil fires being 25:00.433 --> 25:01.673 carried away by the wind. 25:05.000 --> 25:08.170 So the most recent, interesting example we've had 25:08.167 --> 25:10.927 of this are these wonderful Iceland volcanoes. 25:10.933 --> 25:14.033 I say wonderful because while they caused a lot of damage, 25:14.033 --> 25:15.833 they were amazing to see. 25:15.833 --> 25:19.003 The pictures were really, quite remarkable. 25:19.000 --> 25:20.900 Here's an example. 25:20.900 --> 25:22.530 You can't make out the mountain very well. 25:22.533 --> 25:25.233 But there's a volcanic plume coming out of it. 25:25.233 --> 25:27.703 It's rising to a certain level and then 25:27.700 --> 25:31.030 spreading out downwind. 25:31.033 --> 25:34.633 And I'd like you to think for a minute about exactly why the 25:34.633 --> 25:37.733 plume looks like that? 25:37.733 --> 25:42.273 So I pulled off a balloon sounding for Iceland. 25:42.267 --> 25:45.027 It wasn't at the moment of that eruption. 25:45.033 --> 25:46.333 It was just from a couple days ago. 25:46.333 --> 25:49.673 But I think the story is the same. 25:49.667 --> 25:52.167 Here's temperature on this axis versus 25:52.167 --> 25:53.267 height on that axis. 25:53.267 --> 25:57.197 And the temperature is the black curve here. 25:57.200 --> 26:01.070 And the blue curves are that reference line we've been 26:01.067 --> 26:03.497 using, the dry adiabatic-- 26:03.500 --> 26:08.070 sorry green curves are the dry adiabatic lapse rate. 26:08.067 --> 26:15.727 So if I put in an air parcel at 40 degrees Celsius. 26:15.733 --> 26:18.403 The air temperature at the ground is only on that day 26:18.400 --> 26:22.230 about plus four Celsius. 26:22.233 --> 26:27.033 That air parcel is going to be hotter and therefore buoyant. 26:27.033 --> 26:28.633 It's going to rise. 26:28.633 --> 26:32.273 As it rises, it's going to cool adiabatically. 26:32.267 --> 26:36.667 And eventually, it will come to the same temperature as the 26:36.667 --> 26:38.497 environment. 26:38.500 --> 26:41.430 At that point, it will have the same density and 26:41.433 --> 26:42.803 temperature as the environment. 26:42.800 --> 26:44.270 It'll stop rising. 26:44.267 --> 26:47.167 And it'll go flat and just move downwind. 26:47.167 --> 26:53.497 So the point is that a hot, buoyant plume of air will 26:53.500 --> 27:00.700 rise, cooling as it rises, until its temperature matches 27:00.700 --> 27:02.600 that of the environment. 27:02.600 --> 27:05.530 And then it has found it's appropriate level, there's no 27:05.533 --> 27:07.473 reason for it to rise further. 27:07.467 --> 27:09.997 It'll then spread downwind. 27:10.000 --> 27:14.030 Eventually, the particles in that plume will begin to fall 27:14.033 --> 27:15.433 out gravitationally. 27:15.433 --> 27:18.933 But that could take weeks or even months to get that 27:18.933 --> 27:19.933 material out. 27:19.933 --> 27:23.403 So when you see a diagram, you see a picture like this, just 27:23.400 --> 27:26.300 be aware that if that air were even hotter 27:26.300 --> 27:27.530 it would rise higher. 27:27.533 --> 27:30.773 If it were cooler, it'll rise less high and spread out at 27:30.767 --> 27:31.597 this level. 27:31.600 --> 27:35.700 That elevation is being controlled by the properties 27:35.700 --> 27:38.670 of the air that's being emitted. 27:42.100 --> 27:43.330 Questions on that? 27:45.467 --> 27:45.927 Yes. 27:45.933 --> 27:47.533 STUDENT: What if there was wind right at the bottom-- 27:47.533 --> 27:47.573 right above the volcano? 27:47.567 --> 27:48.827 Would it go that--? 27:53.867 --> 27:55.297 PROFESSOR: The question is what if there 27:55.300 --> 27:55.970 is wind at the surface? 27:55.967 --> 27:58.127 Well, there may have been some wind. 27:58.133 --> 27:59.403 Let's go back to that. 28:02.167 --> 28:03.667 It looks like it's going straight up. 28:03.667 --> 28:07.027 But if there was a wind down here of course that would tilt 28:07.033 --> 28:08.003 it as it rose. 28:08.000 --> 28:10.600 Let's say the wind is from left to right in this diagram. 28:10.600 --> 28:11.800 That would tilt this plume. 28:11.800 --> 28:13.700 It already has a bit of a tilt. 28:13.700 --> 28:16.600 But still it would rise to this level and then move off 28:16.600 --> 28:17.600 horizontally. 28:17.600 --> 28:20.730 It'll always follow the wind. 28:20.733 --> 28:24.333 But it will continue to rise until it has neutral buoyancy. 28:24.333 --> 28:26.133 And then it'll stay at that level. 28:26.133 --> 28:27.373 Was there another question? 28:30.500 --> 28:31.730 No, OK. 28:37.500 --> 28:39.130 Sandstorms can do a similar thing. 28:39.133 --> 28:40.473 The winds can pick up-- 28:40.467 --> 28:43.127 this is off the west coast of Africa. 28:43.133 --> 28:46.733 The wind has picked up some dust off the Sahara desert and 28:46.733 --> 28:49.303 carried it out over the ocean. 28:49.300 --> 28:52.330 And turbulence is keeping it aloft. 28:52.333 --> 28:55.633 Eventually, gravitational settling will make that stuff 28:55.633 --> 28:58.803 fall back on to the ocean's surface. 28:58.800 --> 29:02.430 So you see the same factors at play in this natural 29:02.433 --> 29:06.273 phenomenon of just picking up dust from the earth's surface. 29:10.800 --> 29:15.130 I want to switch gears and begin to talk about moisture 29:15.133 --> 29:16.373 in the atmosphere. 29:26.133 --> 29:28.603 And we'll continue this subject next time as well. 29:28.600 --> 29:32.030 So you should be reading in your book now about clouds and 29:32.033 --> 29:35.303 precipitation and water vapor in the atmosphere. 29:35.300 --> 29:40.100 And this material will be on the examination as well. 29:40.100 --> 29:42.000 So I want to start with some definitions. 29:42.000 --> 29:43.870 And I haven't written them all out here. 29:43.867 --> 29:46.127 Because I think your book does a good job on this. 29:46.133 --> 29:48.633 Or maybe you already know these. 29:48.633 --> 29:50.133 I'm going to run through them very quickly. 29:50.133 --> 29:53.603 But you should be aware of each of these six things. 29:53.600 --> 29:59.130 They're all alternative ways of describing how much water 29:59.133 --> 30:02.073 vapor you have in the air. 30:02.067 --> 30:08.627 So in fact, if I know any one of these, I can compute, or in 30:08.633 --> 30:12.833 some other way determine, the other five. 30:12.833 --> 30:14.773 But you should be familiar with all six of these. 30:14.767 --> 30:18.927 So one measure of how much water vapor you have in the 30:18.933 --> 30:22.833 atmosphere is the partial pressure. 30:22.833 --> 30:26.233 What contribution are the water vapor molecules making 30:26.233 --> 30:29.833 to the total pressure of the air? 30:29.833 --> 30:32.333 It's probably only a very tiny fraction. 30:32.333 --> 30:36.373 If the pressure in this room is at 1,013 millibars, maybe 30:36.367 --> 30:42.627 only five or six of those millibars are due to the water 30:42.633 --> 30:44.233 vapor molecules. 30:44.233 --> 30:45.533 But you should be able to compute that. 30:48.300 --> 30:51.470 The saturation vapor pressure is not actually a measure of 30:51.467 --> 30:58.767 how much you have. It's the maximum amount you can have. 30:58.767 --> 31:03.067 When you get that amount of water vapor and try to add 31:03.067 --> 31:07.697 more, the excess will condense out. 31:07.700 --> 31:09.670 And this is a strong function of temperature. 31:09.667 --> 31:13.197 I'll be talking about that in just a moment. 31:13.200 --> 31:16.330 The dew point is a measure of how much water vapor you have. 31:16.333 --> 31:23.273 If you cool the air down to the dew point then water vapor 31:23.267 --> 31:25.967 will begin to condense. 31:25.967 --> 31:29.427 So it's the temperature at which water vapor begins to 31:29.433 --> 31:32.333 condense out. 31:32.333 --> 31:37.873 The specific humidity is the ratio of the mass of water 31:37.867 --> 31:39.827 vapor to the mass of air. 31:39.833 --> 31:43.903 It's like a mixing ratio, grams per kilogram, or 31:43.900 --> 31:45.430 kilograms per kilogram. 31:45.433 --> 31:48.373 For every kilogram of air, how much water has 31:48.367 --> 31:50.197 been mixed into it? 31:50.200 --> 31:52.500 That's the specific humidity. 31:52.500 --> 31:58.700 The relative humidity is the ratio of the partial pressure 31:58.700 --> 32:01.170 of water vapor to the saturation value. 32:01.167 --> 32:03.297 It's the ratio of this to this. 32:03.300 --> 32:05.370 It's the relative humidity. 32:05.367 --> 32:08.727 That's important because that tells you how close you are to 32:08.733 --> 32:10.933 saturation. 32:10.933 --> 32:14.803 If you're at 50% relative humidity, that means you could 32:14.800 --> 32:18.700 add twice as much water vapor and just then be bringing it 32:18.700 --> 32:20.670 to the saturation state. 32:20.667 --> 32:25.427 If you're at a 100% relative humidity, there's already just 32:25.433 --> 32:27.603 as much water vapor as can be held. 32:27.600 --> 32:32.930 If you added anymore, you would have to condense out. 32:32.933 --> 32:35.673 And you'll recall the wet bulb depression from the lab 32:35.667 --> 32:36.867 exercise you did. 32:36.867 --> 32:40.197 One way of measuring how much water vapor you have is to 32:40.200 --> 32:44.330 have a wet bulb and a dry bulb thermometer. 32:44.333 --> 32:48.773 If they read the same, you know there's no evaporation 32:48.767 --> 32:52.697 from the wet bulb, which means the humidity is 100%. 32:52.700 --> 32:56.570 If they read differently, that means the air is somewhat dry 32:56.567 --> 33:01.097 is able to evaporate water from the wetted wick. 33:01.100 --> 33:03.970 And that'll give you a different temperature for the 33:03.967 --> 33:05.127 two thermometers. 33:05.133 --> 33:06.773 That's the wet bulb depression. 33:06.767 --> 33:11.427 And from that you can compute any of these others, except 33:11.433 --> 33:11.933 for that one. 33:11.933 --> 33:15.133 You can compute those other four. 33:15.133 --> 33:19.633 Any questions on these measures of water vapor? 33:22.167 --> 33:25.097 I'm kind of assuming that you know this already from a high 33:25.100 --> 33:26.730 school physics or chemistry class. 33:26.733 --> 33:32.673 So this is probably a little bit of a review here. 33:32.667 --> 33:37.367 Some other definitions we're going to need-- 33:37.367 --> 33:44.527 Condensation is when you change water vapor to liquid. 33:44.533 --> 33:47.333 Or sometimes we use that word also when we're 33:47.333 --> 33:49.073 condensing it to ice. 33:49.067 --> 33:53.327 But usually it's used when we condense it to liquid. 33:53.333 --> 33:55.303 Evaporation is the reverse of that. 33:55.300 --> 33:58.930 It's when you take water in the liquid form and evaporate 33:58.933 --> 34:00.733 it, put it back in the vapor state. 34:03.233 --> 34:07.603 When you do either of those things, there is heat either 34:07.600 --> 34:09.830 taken in or released. 34:09.833 --> 34:13.533 And that heat is called the latent heat of condensation. 34:13.533 --> 34:18.173 When you're evaporating water, you have to put heat in. 34:18.167 --> 34:21.027 When you're condensing water, heat comes out. 34:25.033 --> 34:27.833 I want to make a clear distinction between cloud 34:27.833 --> 34:31.273 droplets and raindrops. 34:31.267 --> 34:36.267 A cloud droplet is a typical water drop that you see that 34:36.267 --> 34:37.127 makes up a cloud. 34:37.133 --> 34:40.133 You look up at a cloud in the sky, it is composed of 34:40.133 --> 34:42.333 millions of cloud droplets. 34:42.333 --> 34:46.073 We put the little suffix "let" there to remind us 34:46.067 --> 34:47.597 that these are small. 34:47.600 --> 34:57.770 A typical cloud droplet is about ten microns in diameter. 34:57.767 --> 34:59.697 Ten microns in diameter. 34:59.700 --> 35:02.200 A micron, remember, is a millionth of a meter. 35:05.333 --> 35:09.433 A raindrop is a kind of liquid drop you 35:09.433 --> 35:12.433 find falling to earth. 35:12.433 --> 35:16.033 And they are typically about a millimeter in size. 35:19.167 --> 35:23.527 In other words, about a hundred times larger. 35:23.533 --> 35:27.333 There is a factor of about one hundred in the size of these 35:27.333 --> 35:28.603 two different droplets. 35:36.200 --> 35:37.700 Questions so far on these definitions? 35:40.567 --> 35:44.527 Supercooled liquid water is liquid water that has been 35:44.533 --> 35:48.473 cooled down below the normal freezing point. 35:48.467 --> 35:55.567 Freezing point for fresh water is zero degrees Celsius. 35:55.567 --> 35:59.267 And yet in the atmosphere, we often find liquid water at 35:59.267 --> 36:02.197 temperatures of minus ten, minus 20, 36:02.200 --> 36:05.230 even minus 30 Celsius. 36:05.233 --> 36:09.233 It's in the liquid state, and yet it is below the normal 36:09.233 --> 36:10.633 freezing point of water. 36:10.633 --> 36:17.573 We call water in that state supercooled liquid water. 36:17.567 --> 36:19.727 It wants to freeze. 36:19.733 --> 36:22.403 It's at a temperature where it should be frozen. 36:22.400 --> 36:26.770 But it needs some kind of a trigger to get the freezing 36:26.767 --> 36:28.327 process started. 36:28.333 --> 36:33.003 And it turns out that plays a big role in the generation of 36:33.000 --> 36:38.000 precipitation, as I'll be describing next time. 36:38.000 --> 36:41.700 And then this word riming, which is what happens when a 36:41.700 --> 36:45.230 super cooled droplet hits something. 36:45.233 --> 36:46.673 It wants to freeze. 36:46.667 --> 36:51.627 When it hits an object, it'll probably freeze upon impact 36:51.633 --> 36:56.373 and stick to the object as a frozen piece of water-- 36:56.367 --> 36:58.997 piece of ice stuck to the surface. 36:59.000 --> 37:04.500 That is called riming, that process of having super cooled 37:04.500 --> 37:08.370 water freeze upon impact. 37:08.367 --> 37:09.627 Questions on this? 37:12.500 --> 37:14.230 OK, well, that's a lot of definitions. 37:17.367 --> 37:20.797 The earth, of course, is the water planet. 37:20.800 --> 37:25.100 It's covered by large oceans of water. 37:25.100 --> 37:29.370 The water gets up into the atmosphere and then in areas 37:29.367 --> 37:34.797 where the air is rising, there is adiabatic cooling, which 37:34.800 --> 37:39.300 will drop the air temperature, drop the saturation vapor 37:39.300 --> 37:43.000 pressure, and bring that air-- bring the water vapor in that 37:43.000 --> 37:45.800 air to saturation. 37:45.800 --> 37:49.570 And as that rising air continues, then the excess 37:49.567 --> 37:52.297 water vapor will condense to form cloud droplets. 37:52.300 --> 37:55.500 So you see clouds scattered around in the earth's 37:55.500 --> 37:59.030 atmosphere, you can be almost certain that every place you 37:59.033 --> 38:02.573 see a cloud, whether it's large or small, there is 38:02.567 --> 38:05.067 rising air. 38:05.067 --> 38:09.797 And almost every place you find the sky free of clouds, 38:09.800 --> 38:13.100 there is sinking air. 38:13.100 --> 38:17.630 Because remember, rising air forms clouds 38:17.633 --> 38:18.933 by adiabatic cooling. 38:21.967 --> 38:25.167 So you look at something like this, and well, about half of 38:25.167 --> 38:25.897 this is cloud. 38:25.900 --> 38:26.870 And half of it is clear. 38:26.867 --> 38:28.867 Well, that means half the air is rising. 38:28.867 --> 38:30.197 Half the air is sinking. 38:30.200 --> 38:32.870 The patterns are complicated. 38:32.867 --> 38:36.827 They're connected with a variety of things like fronts, 38:36.833 --> 38:44.333 thunderstorms, deep convection over the equator. 38:44.333 --> 38:48.373 But in all cases, it's rising motion versus sinking motion-- 38:48.367 --> 38:51.827 very clearly displayed when you look at a map like this. 38:55.567 --> 39:01.397 So the key thing to understand about clouds then, or about 39:01.400 --> 39:06.800 water vapor I should say, is how do we take air that is 39:06.800 --> 39:10.600 under saturated or sub-saturated and bring it to 39:10.600 --> 39:11.870 the saturated condition? 39:15.400 --> 39:19.500 And I'm going to run through the three ways that are active 39:19.500 --> 39:22.300 in the earth's atmosphere for bringing air to 39:22.300 --> 39:25.300 the saturated state. 39:25.300 --> 39:28.800 Remember you're starting with air that has a relative 39:28.800 --> 39:32.070 humidity less than a 100%. 39:32.067 --> 39:40.227 And you're asking how can I bring it to 100% and above? 39:40.233 --> 39:44.433 Probably the most obvious way is to add moisture, add water 39:44.433 --> 39:45.703 vapor to that air parcel. 39:51.167 --> 39:53.027 I'll show some examples of that but sea 39:53.033 --> 39:55.573 smoke, contrails, stacks. 39:58.233 --> 40:02.233 Smokestacks sometimes give off clouds in this way. 40:02.233 --> 40:05.873 And human breath, when you breathe out on a cold day, you 40:05.867 --> 40:07.427 can create a little cloud using this 40:07.433 --> 40:09.873 mechanism of adding moisture. 40:09.867 --> 40:12.597 Another ways is to cool by removing heat. 40:12.600 --> 40:16.200 If you have air that a sub-saturated, but you remove 40:16.200 --> 40:20.130 heat, you're going to drop the temperature. 40:20.133 --> 40:24.803 If you drop the temperature, you're going to drop the 40:24.800 --> 40:28.330 saturation vapor pressure. 40:28.333 --> 40:31.133 And you keep doing that, you're going to 40:31.133 --> 40:32.003 bring the two together. 40:32.000 --> 40:34.170 You're going to bring the amount of partial pressure 40:34.167 --> 40:38.697 water vapor together with the saturation partial pressure. 40:38.700 --> 40:39.600 They'll be equal. 40:39.600 --> 40:43.130 And you'll have the saturated condition. 40:43.133 --> 40:45.633 The third method is by adiabatic expansion, that is 40:45.633 --> 40:48.033 by lifting air parcels up in the atmosphere. 40:50.867 --> 40:52.497 I'm going to give you examples of all these. 40:56.933 --> 41:00.173 So first, I'll be adding moisture. 41:00.167 --> 41:04.167 This is something called sea smoke. 41:04.167 --> 41:07.327 Here's an aircraft carrier looking out 41:07.333 --> 41:08.273 over the ocean's surface. 41:08.267 --> 41:11.267 It has a very peculiar appearance. 41:11.267 --> 41:16.667 It appears to be tufts of cloud torn apart by turbulence 41:16.667 --> 41:21.927 but generally hugging the ocean's surface. 41:21.933 --> 41:24.673 What's happening there is you have cold air moving across 41:24.667 --> 41:25.827 the ocean's surface. 41:25.833 --> 41:28.273 And the ocean is warm. 41:28.267 --> 41:32.727 So the ocean is evaporating its moisture into the 41:32.733 --> 41:33.633 atmosphere. 41:33.633 --> 41:38.103 But then as soon as it does that, the air being so cold 41:38.100 --> 41:40.900 with that extra water vapor being added, you've brought 41:40.900 --> 41:42.230 the air to saturation. 41:42.233 --> 41:46.873 And you start to form these little tufts of cloud or fog. 41:46.867 --> 41:49.197 So that's sometimes called sea smoke. 41:49.200 --> 41:50.830 It's not smoke. 41:50.833 --> 41:52.333 You're not burning anything. 41:52.333 --> 41:57.233 But because sometimes that's just the traditional name for 41:57.233 --> 42:00.833 it is sea smoke. 42:00.833 --> 42:03.773 Here's another example from the Northeastern Labrador 42:03.767 --> 42:09.767 current up off the coast of Nova Scotia. 42:09.767 --> 42:15.567 Whenever you get cold air moving over warm water, you 42:15.567 --> 42:18.027 are likely to get this situation. 42:18.033 --> 42:22.503 Water evaporates from the warm water, adds moisture to the 42:22.500 --> 42:25.600 cold air, you've brought that air to saturation. 42:25.600 --> 42:30.930 And little clouds appear to form on the surface. 42:30.933 --> 42:34.973 A contrail is that way. 42:34.967 --> 42:40.127 A contrail is a contraction for condensation trail. 42:40.133 --> 42:46.173 And here's an aircraft producing four contrails, one 42:46.167 --> 42:47.427 from each engine. 42:49.867 --> 42:55.797 So you're burning hydrocarbons in the aircraft engine. 42:55.800 --> 42:58.870 The chemical byproducts of this burning are carbon 42:58.867 --> 43:01.467 dioxide and water vapor. 43:01.467 --> 43:04.997 So the exhaust from the engine is part water vapor. 43:05.000 --> 43:08.830 You're adding it to this cold air, which was sub-saturated 43:08.833 --> 43:09.373 to begin with. 43:09.367 --> 43:11.267 But you've added enough water vapor. 43:11.267 --> 43:14.467 So now you've saturated the air and beyond. 43:14.467 --> 43:17.897 And the excess goes into these tiny condensed particles 43:17.900 --> 43:19.900 either liquid or ice. 43:19.900 --> 43:25.770 Those are probably liquid as a result of adding water vapor 43:25.767 --> 43:28.227 to cold air. 43:28.233 --> 43:29.503 Any questions on this? 43:35.333 --> 43:37.133 There's some contrails from a distance. 43:42.133 --> 43:48.433 Now later on, those contrails may seem to lose their 43:48.433 --> 43:49.603 contrail appearance. 43:49.600 --> 43:50.970 They may spread out. 43:50.967 --> 43:51.897 They may diffuse. 43:51.900 --> 43:53.370 They may go unstable. 43:53.367 --> 43:54.897 They may even lead to what looks like 43:54.900 --> 43:56.070 natural cirrus clouds. 43:56.067 --> 43:58.827 But even that more natural looking cloud, I think, on 43:58.833 --> 44:02.173 this particular day started out as a contrail. 44:02.167 --> 44:05.267 It just spread a bit by turbulent diffusion. 44:08.133 --> 44:11.673 If you have a cooling tower like from a nuclear power 44:11.667 --> 44:14.067 plant, that's not smoke that's coming out of it. 44:14.067 --> 44:18.467 Tha's just hot, moist air. 44:18.467 --> 44:21.827 You're adding water vapor to a dry atmosphere bringing it to 44:21.833 --> 44:25.633 saturation and causing what appears to be a cloud. 44:25.633 --> 44:26.533 Again that's not smoke. 44:26.533 --> 44:32.733 That's just condensed water in the cloud. 44:32.733 --> 44:39.273 And you, yourself on a chilly October or November morning, 44:39.267 --> 44:43.527 if you breathe out, the water vapor that's been added to the 44:43.533 --> 44:49.003 air in your lungs suddenly is added to the atmosphere 44:49.000 --> 44:50.730 bringing it to saturation. 44:50.733 --> 44:54.573 And the excess condenses to form the cloud. 44:54.567 --> 44:57.867 So this first one-- 44:57.867 --> 45:01.127 Well, I think I'll quit there. 45:01.133 --> 45:03.533 We're going to continue this theme next time of looking at 45:03.533 --> 45:06.203 how water vapor acts in the atmosphere. 45:06.200 --> 45:10.470 Our target is to get to a point where we understand how 45:10.467 --> 45:12.127 clouds and precipitation form. 45:12.133 --> 45:15.133 So be reading ahead in your book about clouds and 45:15.133 --> 45:16.403 precipitation.