WEBVTT 00:01.100 --> 00:02.670 RONALD SMITH: Well we're going to spend the day today talking 00:02.667 --> 00:03.497 about clouds. 00:03.500 --> 00:04.900 We started this last time. 00:07.500 --> 00:09.030 So I'll do a little bit of review. 00:13.700 --> 00:17.270 It's all PowerPoint today, which can be fairly boring, so 00:17.267 --> 00:22.027 I hope you'll shout out questions as we go through 00:22.033 --> 00:22.673 this material. 00:22.667 --> 00:25.497 So just remember, the subject today-- and we started last 00:25.500 --> 00:29.170 time-- is clouds and other occurrences of condensed water 00:29.167 --> 00:30.067 in the atmosphere. 00:30.067 --> 00:34.667 So it's all about water in the atmosphere today. 00:34.667 --> 00:38.327 Talked about it being a water planet, but not only in the 00:38.333 --> 00:42.003 oceans but in the atmosphere, as well. 00:42.000 --> 00:45.700 Both water vapor, which is invisible, and the clouds that 00:45.700 --> 00:48.430 form when water vapor condenses. 00:48.433 --> 00:50.433 We went through these definitions. 00:50.433 --> 00:53.733 I won't repeat that unless there are questions. 00:53.733 --> 00:59.303 Are there any questions on these definitions? 00:59.300 --> 01:02.100 You've had a couple days to look at those. 01:02.100 --> 01:02.830 Question, yes? 01:02.833 --> 01:03.573 STUDENT: I have a quick question, whether this lecture 01:03.567 --> 01:06.097 is uploaded on the Classes server? 01:06.100 --> 01:07.970 PROFESSOR: It's already there. 01:07.967 --> 01:10.227 I loaded it this morning, about an hour ago. 01:10.233 --> 01:12.603 It's already there. 01:12.600 --> 01:15.500 A PDF version of it. 01:15.500 --> 01:17.670 Any questions on these definitions? 01:17.667 --> 01:18.567 These are really important. 01:18.567 --> 01:24.197 You can't get away without knowing these six quantities, 01:24.200 --> 01:27.570 and these as well. 01:27.567 --> 01:29.627 But I'll be saying more about this as we 01:29.633 --> 01:31.473 go through the lecture. 01:31.467 --> 01:34.197 But these are really important terms. 01:34.200 --> 01:37.700 I didn't show this last, time but I should have. It's a plot 01:37.700 --> 01:40.670 of saturation vapor pressure for water. 01:40.667 --> 01:44.067 It just says vapor pressure, but it should say saturation 01:44.067 --> 01:48.127 vapor pressure as a function of temperature. 01:48.133 --> 01:51.473 Saturation vapor pressure is defined as the-- 01:51.467 --> 01:54.267 well, there's two equivalent definitions. 01:54.267 --> 01:56.227 I like them both. 01:56.233 --> 02:02.833 It's the partial pressure of water vapor in equilibrium 02:02.833 --> 02:06.373 with a condensed phase. 02:06.367 --> 02:08.927 In other words, if you have a chamber that has some liquid 02:08.933 --> 02:13.233 water in it, and at a certain temperature, and you wait for 02:13.233 --> 02:18.233 a while, there'll be enough evaporation off the surface 02:18.233 --> 02:21.773 and the void will get filled with water vapor. 02:21.767 --> 02:24.967 And if you wait long enough, the partial pressure of water 02:24.967 --> 02:29.197 vapor in that chamber will come to the value given by 02:29.200 --> 02:32.800 this curve, depending on the temperature of the system. 02:32.800 --> 02:36.570 So the higher the temperature, the more water vapor will 02:36.567 --> 02:41.367 evaporate, and the higher the water vapor partial pressure 02:41.367 --> 02:43.467 you will have in the chamber. 02:43.467 --> 02:44.727 Now here's a curious thing. 02:44.733 --> 02:47.773 It actually doesn't matter whether there's air in the 02:47.767 --> 02:51.827 chamber above that liquid water or not, because this is 02:51.833 --> 02:55.733 a property of the water, not of the air. 02:55.733 --> 02:59.373 And I'll often mistakenly say things like, "the air is 02:59.367 --> 03:03.797 saturated." That's not quite right, because it's the water 03:03.800 --> 03:05.770 vapor we're talking about. 03:05.767 --> 03:10.497 Now it's interesting that that water is mixed in to air, but 03:10.500 --> 03:12.200 the air is not really controlling this. 03:12.200 --> 03:15.200 This is the property of water alone. 03:15.200 --> 03:19.200 You'd have the same partial pressure of water vapor above 03:19.200 --> 03:21.570 a liquid at a certain temperature whether there was 03:21.567 --> 03:23.427 air there or not. 03:23.433 --> 03:25.903 Of course, if there was air there, the total pressure 03:25.900 --> 03:30.430 would be higher, but the partial pressure would be-- 03:30.433 --> 03:34.003 of water vapor would be the same value here. 03:34.000 --> 03:34.370 Yes? 03:34.367 --> 03:35.467 STUDENT: What is the difference between saturation 03:35.467 --> 03:36.727 vapor pressure and vapor pressure? 03:39.567 --> 03:45.097 PROFESSOR: The vapor pressure is what you have. Depends on 03:45.100 --> 03:48.470 how much water vapor you have and its temperature. 03:48.467 --> 03:52.867 But the saturation is the value of that quantity that 03:52.867 --> 03:57.627 you would have when there's a condensed phase present, and 03:57.633 --> 03:59.803 they've reached equilibrium. 03:59.800 --> 04:03.200 Or another way to put it is that the saturation vapor 04:03.200 --> 04:08.100 pressure is the maximum you can have before 04:08.100 --> 04:11.500 condensation begins. 04:11.500 --> 04:14.370 In other words, at a certain temperature, let's say 20 04:14.367 --> 04:24.567 degrees Celsius, if I had a vapor pressure of 1 millibar, 04:24.567 --> 04:30.267 that is less than the saturation curve, so nothing's 04:30.267 --> 04:32.627 going to condense. 04:32.633 --> 04:34.333 However, I could bring the air to saturation-- 04:34.333 --> 04:34.403 I just said it again-- 04:34.400 --> 04:40.870 I could bring the water vapor to saturation either by adding 04:40.867 --> 04:47.867 water vapor to that chamber in some way or by cooling it. 04:47.867 --> 04:50.727 In any case I can approach the saturation curve. 04:50.733 --> 04:53.133 And when I get there, whether it's there or there, doesn't 04:53.133 --> 04:59.303 matter, the relative humidity will be 100%. 04:59.300 --> 05:03.770 And if I try to go beyond, using either mechanism, the 05:03.767 --> 05:07.167 excess water vapor will condense out to 05:07.167 --> 05:09.627 form liquid or ice. 05:09.633 --> 05:13.203 So relative humidity is less than 100% here. 05:13.200 --> 05:15.400 When you push it to saturation, no matter how you 05:15.400 --> 05:19.500 do it, either by adding water or by cooling and go beyond, 05:19.500 --> 05:24.000 then you'll get condensed water. 05:24.000 --> 05:25.600 So that's the key idea there. 05:28.533 --> 05:31.033 And I mentioned this last time, so there. 05:31.033 --> 05:34.273 To approach saturation, you can add moisture. 05:34.267 --> 05:36.897 That moves you up on that diagram. 05:36.900 --> 05:39.600 Or you can drop the temperature. 05:39.600 --> 05:42.000 And there's two common ways that the temperature could be 05:42.000 --> 05:44.400 dropped in an atmospheric system. 05:44.400 --> 05:47.470 One would be cool it by removing heat. 05:47.467 --> 05:53.227 Put that little piece of air next to something cold, and so 05:53.233 --> 05:57.803 the heat is drawn out of it by conduction. 05:57.800 --> 05:59.930 And that leads to something we call radiation fog. 05:59.933 --> 06:02.833 I'll give you examples, advection fog, also. 06:02.833 --> 06:07.873 Or you could cool that parcel of air by adiabatic expansion. 06:07.867 --> 06:10.197 That means lifting it. 06:10.200 --> 06:12.470 You don't have to put in contact with the cold surface. 06:12.467 --> 06:17.967 Just lift it, and it'll expand as it goes to lower pressure, 06:17.967 --> 06:20.867 and cool itself by the fact it is doing work on the 06:20.867 --> 06:22.097 environment. 06:22.100 --> 06:23.170 That'll drop the temperature. 06:23.167 --> 06:26.027 And again, you'll push toward saturation. 06:26.033 --> 06:29.673 And if you go beyond, a cloud will form. 06:29.667 --> 06:33.097 So these are the three ways that you approach saturation. 06:33.100 --> 06:36.630 I'm going to give you lots of examples of these today. 06:36.633 --> 06:38.033 Stop me if you have questions. 06:38.033 --> 06:39.673 We did this first few last time. 06:39.667 --> 06:43.227 Sea smoke is where you have a warm water 06:43.233 --> 06:46.003 surface and cold air. 06:46.000 --> 06:49.470 You evaporate water off the warm surface into the air, but 06:49.467 --> 06:53.967 the air can't hold that much water vapor, so you start to 06:53.967 --> 06:57.697 get little liquid droplets forming from the 06:57.700 --> 06:59.230 excess water vapor. 06:59.233 --> 07:03.203 The amount that you've pushed beyond the saturation value. 07:05.900 --> 07:08.070 And this one we talked about last time: contrails. 07:08.067 --> 07:12.597 You're burning hydrocarbon fuels, byproduct of the 07:12.600 --> 07:15.170 burning is carbon dioxide and water vapor. 07:15.167 --> 07:17.697 So you've added water vapor to the air. 07:17.700 --> 07:19.530 You've pushed it beyond saturation. 07:19.533 --> 07:24.203 The excess will condense out in tiny water droplets. 07:24.200 --> 07:26.500 Remember, the vapor itself is invisible. 07:26.500 --> 07:28.930 There's vapor everywhere here. 07:28.933 --> 07:30.733 You only see the condensed particles, 07:30.733 --> 07:33.073 either liquid or ice. 07:33.067 --> 07:33.897 Contrails. 07:33.900 --> 07:35.100 Contrails. 07:35.100 --> 07:36.900 We talked about this. 07:36.900 --> 07:38.770 I think we did. 07:38.767 --> 07:42.697 Nuclear plants have cooling towers to get rid of heat 07:42.700 --> 07:45.370 that's come out of the back side of the turbines. 07:45.367 --> 07:47.967 And what they do is just to evaporate 07:47.967 --> 07:49.627 large amounts of water. 07:49.633 --> 07:50.703 Put it up in the atmosphere. 07:50.700 --> 07:55.800 But as soon as that very humid air reaches the colder 07:55.800 --> 08:01.100 environmental atmosphere, it condenses because you've added 08:01.100 --> 08:01.770 water vapor. 08:01.767 --> 08:03.527 It can't be held, and the excess 08:03.533 --> 08:05.673 goes into liquid droplets. 08:09.200 --> 08:11.230 When you breathe out on a cold winter day, you're doing 08:11.233 --> 08:12.803 exactly the same thing. 08:12.800 --> 08:18.800 Radiation fog is an example of how we're going to cool the 08:18.800 --> 08:23.270 air off ultimately causing fog. 08:23.267 --> 08:27.567 And the way you do that is usually to have a clear night, 08:27.567 --> 08:31.227 no clouds above, so the infrared radiation from the 08:31.233 --> 08:35.303 earth's surface can escape very handily. 08:35.300 --> 08:38.900 Maybe a good half of it escapes to space, which cools 08:38.900 --> 08:44.670 the surface of the earth down several degrees overnight. 08:44.667 --> 08:48.367 The air right in contact with that earth's surface then is 08:48.367 --> 08:49.497 going to lose heat by 08:49.500 --> 08:55.370 conduction to the cold surface. 08:55.367 --> 09:00.427 When you lose heat, you drop the saturation vapor pressure. 09:00.433 --> 09:02.503 You're not necessarily changing the amount of water 09:02.500 --> 09:06.030 vapor in the air, but you're changing the amount that can 09:06.033 --> 09:09.503 be held in the vapor state. 09:09.500 --> 09:14.200 And therefore the fog forms first for the air that's right 09:14.200 --> 09:16.400 in contact with that cooling surface. 09:16.400 --> 09:19.100 That's called radiation fog. 09:19.100 --> 09:24.370 The word radiation goes back to how the heat has been lost 09:24.367 --> 09:26.397 from the system. 09:26.400 --> 09:29.430 But thermodynamically, it's similar to cases we've already 09:29.433 --> 09:30.773 spoken about. 09:30.767 --> 09:36.967 This is a common thing to see, especially in valleys, because 09:36.967 --> 09:40.927 in slightly mountainous terrain the air that's been 09:40.933 --> 09:43.803 cooled by contact with the surface will tend to run 09:43.800 --> 09:46.600 downhill and collect in the valleys. 09:46.600 --> 09:49.870 So you can get radiation fog in flat terrain. 09:49.867 --> 09:53.297 I've seen it many times, but I've seen it even more 09:53.300 --> 09:58.200 frequently in slightly hilly country where the cold air 09:58.200 --> 10:01.800 runs down into the valley and the fog is found most dense, 10:01.800 --> 10:03.670 most thick, down in that valley. 10:03.667 --> 10:05.827 But it's still radiation fog. 10:05.833 --> 10:09.873 Notice the clear sky above, which is usually when you get 10:09.867 --> 10:12.897 a good strong case of radiation fog, it's because 10:12.900 --> 10:15.870 there's no clouds above overnight. 10:15.867 --> 10:17.997 Advection fog is a little different. 10:18.000 --> 10:24.100 There a surface was cold to begin with and then you 10:24.100 --> 10:29.070 brought in moist air by the winds over that surface. 10:29.067 --> 10:33.027 Once you've moved the warm air over the surface, it loses 10:33.033 --> 10:35.503 heat by conduction again. 10:35.500 --> 10:39.200 And the saturation vapor pressure drops. 10:39.200 --> 10:43.700 The relative humidity exceeds 100%, and the fog forms. So 10:43.700 --> 10:48.470 the only difference here is how you generated the cooling. 10:48.467 --> 10:51.327 Instead of doing it by radiation, you did it by 10:51.333 --> 10:54.873 moving warm moist air over a colder surface. 10:57.833 --> 11:01.403 This is quite common in areas of the world ocean where you 11:01.400 --> 11:04.600 have a cold ocean current. 11:04.600 --> 11:07.270 For example, the California current that comes down along 11:07.267 --> 11:10.597 the coast of California is a cold current. 11:10.600 --> 11:14.370 And when warm air masses move over that, whether it's from 11:14.367 --> 11:18.567 further west in the Pacific or whether it comes off the land, 11:18.567 --> 11:21.027 suddenly that air's going to lose heat to 11:21.033 --> 11:23.873 the cold water beneath. 11:23.867 --> 11:25.427 Remember, this is not sea smoke. 11:25.433 --> 11:26.873 Sea smoke was the opposite. 11:26.867 --> 11:30.497 This is cold water, warm air. 11:30.500 --> 11:33.930 Remember, sea smoke was just the opposite of that. 11:33.933 --> 11:37.233 Here you're not gaining water from the ocean. 11:37.233 --> 11:42.703 You're losing heat to the ocean, and causing the fog to 11:42.700 --> 11:45.530 form in that way. 11:45.533 --> 11:47.503 Here's another example of advection 11:47.500 --> 11:49.200 fog along the coastline. 11:49.200 --> 11:51.570 It's quite common, for example, north of here, up in 11:51.567 --> 11:55.167 the Gulf of Maine and further up around Nova Scotia where 11:55.167 --> 11:57.267 the Labrador Current comes down, the 11:57.267 --> 11:59.597 cold Labrador Current. 11:59.600 --> 12:00.600 It's very common. 12:00.600 --> 12:07.300 In fact, you might have advection fog 2/3 of the time. 12:07.300 --> 12:10.670 You might have advection fog over that cold flowing 12:10.667 --> 12:12.967 Labrador Current, because of this effect. 12:12.967 --> 12:15.567 No matter what direction the wind blows in from, it's 12:15.567 --> 12:19.297 probably warmer than that Labrador Current, so it's 12:19.300 --> 12:22.770 going to lose heat to it, and the fog is going to form. 12:22.767 --> 12:25.997 So it's a very foggy area up there where there's a cold 12:26.000 --> 12:28.200 ocean current. 12:28.200 --> 12:29.470 Questions here? 12:33.500 --> 12:34.870 Maybe you've seen this. 12:34.867 --> 12:37.127 Now we're going to talk about clouds, which is the primary 12:37.133 --> 12:40.373 subject of the lecture. 12:40.367 --> 12:48.567 Clouds are formed by rising air and the thermodynamic 12:48.567 --> 12:53.767 mechanism is adiabatic cooling as air rises. 12:53.767 --> 12:57.067 As the air gets cooler, as its temperature drops, the 12:57.067 --> 12:59.867 saturation vapor pressure drops. 12:59.867 --> 13:05.697 And you will reach a humidity of 100% and 13:05.700 --> 13:06.830 then perhaps beyond. 13:06.833 --> 13:10.233 If you try to go beyond, the excess water vapor will 13:10.233 --> 13:14.673 immediately condense out to form a cloud. 13:14.667 --> 13:17.027 So we're going to take a look at a few types of clouds. 13:17.033 --> 13:20.073 But in all cases, the thermodynamics is the same. 13:20.067 --> 13:23.597 So here is our friend, the fair weather cumulus cloud 13:23.600 --> 13:30.670 with the flat base, the cauliflower turbulent tops, 13:30.667 --> 13:34.327 and probably the most common type of cloud. 13:37.600 --> 13:41.700 I have a little time lapse movie of-- we did a project in 13:41.700 --> 13:45.170 my group a few months ago down in the Caribbean, looking at 13:45.167 --> 13:49.367 how clouds and precipitation are generated in the tropical 13:49.367 --> 13:52.127 belt of our planet. 13:52.133 --> 13:56.073 We flew aircraft through clouds and so on. 13:56.067 --> 13:57.867 If I can get this to work, I'm going to show you some 13:57.867 --> 14:01.997 time-lapse photography of cumulus clouds over the island 14:02.000 --> 14:03.470 of Dominica. 14:03.467 --> 14:04.667 The sun just rose. 14:04.667 --> 14:09.097 That black to illuminated thing was the sun rising in 14:09.100 --> 14:14.370 the east. You're beginning to see cumulus clouds forming 14:14.367 --> 14:17.927 here, bubbling up, almost like boiling in some sense. 14:17.933 --> 14:24.033 This is heat from the land rising, but then as the air 14:24.033 --> 14:26.503 rises, it cools. 14:26.500 --> 14:31.530 The humidity exceeds 100%, and the cloud forms. But then the 14:31.533 --> 14:34.903 air rises into the clouds and spreads out and descends. 14:34.900 --> 14:39.530 So maybe you can see it, but clouds are also disappearing. 14:39.533 --> 14:43.003 As fast as they're being produced in the updrafts, 14:43.000 --> 14:45.930 they're being eliminated by the down drafts. 14:45.933 --> 14:48.873 Because this adiabatic cooling and heating is 14:48.867 --> 14:50.567 a reversible process. 14:50.567 --> 14:51.697 You cool when you lift. 14:51.700 --> 14:53.570 You warm when you descend. 14:53.567 --> 14:56.997 And so you're going to form cumulus clouds and the parcels 14:57.000 --> 14:59.530 are just moving through these things. 14:59.533 --> 15:04.403 Cloudy on the way up, and clear air on the way down. 15:04.400 --> 15:08.500 The time is in the upper box in local time, so now it's the 15:08.500 --> 15:12.570 evening, just before the sunset, when the heating of 15:12.567 --> 15:15.667 the island was reduced so was the convection. 15:15.667 --> 15:21.897 So this was driven by the sun heating the island. 15:21.900 --> 15:23.100 The warm air rising. 15:23.100 --> 15:26.500 But then in the rising process, the water vapor 15:26.500 --> 15:31.870 became saturated, condensed, and the cloud was formed. 15:31.867 --> 15:33.627 Are there any questions on that short-- 15:33.633 --> 15:34.903 on that short video? 15:39.767 --> 15:43.397 When you are looking at cumulus clouds yourself, you 15:43.400 --> 15:46.700 normally see a fixed pattern. 15:46.700 --> 15:48.930 You may have noticed, well it's still here in fact, the 15:48.933 --> 15:50.573 speedup ratio on that time-lapse 15:50.567 --> 15:52.827 photography was 720. 15:52.833 --> 15:55.033 That's a big speed-up ratio. 15:55.033 --> 16:00.103 So things are happening in this time-lapse film 700 times 16:00.100 --> 16:03.670 faster than they do in nature. 16:03.667 --> 16:06.097 But that makes it wonderful, because you can see the 16:06.100 --> 16:07.670 process happening. 16:07.667 --> 16:09.967 When you look at it yourself, you see a 16:09.967 --> 16:13.997 static cloud pattern. 16:14.000 --> 16:15.500 If you try to watch it, sometimes 16:15.500 --> 16:16.900 you can see the changes. 16:16.900 --> 16:19.800 But it's hard, because they're slow based on 16:19.800 --> 16:22.970 the human time scale. 16:22.967 --> 16:26.827 You have to have a great deal of attention focused on that 16:26.833 --> 16:30.873 cloud to be able to watch it evolve and change over a 16:30.867 --> 16:32.597 period of an hour or two. 16:32.600 --> 16:34.670 Whereas with time lapse, we see it all before us 16:34.667 --> 16:40.167 happening, and we can easily understand it. 16:40.167 --> 16:44.327 I wanted to give you a couple other examples of what we did 16:44.333 --> 16:46.233 in this project. 16:46.233 --> 16:51.273 Here, for example, is a short aircraft flight through a 16:51.267 --> 16:54.497 couple of those cumulus clouds. 16:54.500 --> 16:57.330 The aircraft is equipped with all kinds of wonderful 16:57.333 --> 16:58.273 instruments on board. 16:58.267 --> 16:59.567 Let me just show you what's plotted here. 16:59.567 --> 17:03.127 So this is a horizontal distance in kilometers so the 17:03.133 --> 17:06.273 airplane flew about 30 kilometers, and passed 17:06.267 --> 17:09.897 through, depending how you count them, two or four 17:09.900 --> 17:10.670 different clouds. 17:10.667 --> 17:13.967 Maybe one, two, three, four, or something like that. 17:13.967 --> 17:17.767 What's plotted in blue, LWC, is the liquid water 17:17.767 --> 17:19.367 concentration. 17:19.367 --> 17:23.827 In those cloud drops, how much total liquid is there in units 17:23.833 --> 17:30.503 of grams of water, liquid water, per cubic meter of air? 17:30.500 --> 17:31.930 We have instruments on-board the aircraft 17:31.933 --> 17:33.303 that can measure this. 17:33.300 --> 17:36.900 And so when it's zero, of course you're not in a cloud. 17:36.900 --> 17:41.800 A cloud is defined by its liquid water content. 17:41.800 --> 17:44.400 If there's no liquid water, you're not in a cloud. 17:44.400 --> 17:49.030 So we entered the cloud there, we had values of one to two 17:49.033 --> 17:53.033 grams per cubic meter of liquid. 17:53.033 --> 17:55.573 Then we're briefly out of the cloud, back into another one. 17:55.567 --> 17:56.627 Out, in. 17:56.633 --> 17:57.673 Out, in. 17:57.667 --> 17:59.127 And finally, out. 17:59.133 --> 18:01.103 Here's the vertical motion. 18:01.100 --> 18:04.300 The air is moving up and down. 18:04.300 --> 18:06.900 So not much vertical motion outside the cloud, but inside 18:06.900 --> 18:08.900 the cloud it's noisy. 18:08.900 --> 18:10.170 It's turbulent. 18:10.167 --> 18:14.667 But most of the motions are upwards. 18:14.667 --> 18:19.597 There are few little down bursts, but generally, most of 18:19.600 --> 18:23.870 the air inside a cloud is moving upwards, as I argued it 18:23.867 --> 18:29.027 must be because it's adiabatic cooling that is responsible 18:29.033 --> 18:32.073 for the cloud. 18:32.067 --> 18:34.727 Here's the water vapor mixing ratio. 18:34.733 --> 18:37.833 We call that the specific humidity. 18:37.833 --> 18:41.403 It was to units of grams of water vapor 18:41.400 --> 18:43.530 per kilogram of air-- 18:43.533 --> 18:46.503 how much water is mixed into the air. 18:46.500 --> 18:51.700 It was 11 outside the cloud, and about 13 inside the cloud. 18:51.700 --> 18:54.230 And then we also had instruments that could measure 18:54.233 --> 18:58.773 the droplet diameter of the cloud droplets. 18:58.767 --> 19:02.197 They're ranging between about 16 and 28 microns. 19:02.200 --> 19:04.400 A micron is a millionth of a meter. 19:04.400 --> 19:06.330 These are small droplets, so small. 19:06.333 --> 19:08.133 These are typical cloud droplets. 19:11.667 --> 19:13.397 They're so small that they don't really 19:13.400 --> 19:14.700 fall out very much. 19:14.700 --> 19:17.200 They just follow the air. 19:17.200 --> 19:19.930 They're so tiny that their gravitational acceleration is 19:19.933 --> 19:24.273 quite small, and they mostly just follow the air parcels. 19:24.267 --> 19:28.327 But you can measure them as you fly through a cloud, so 19:28.333 --> 19:29.503 that gives you an idea of that. 19:29.500 --> 19:34.200 And then one more diagram from the same experiment we did. 19:34.200 --> 19:38.430 We found it under certain conditions, some clouds would 19:38.433 --> 19:41.073 have-- the cloud droplet diameter would be larger, in 19:41.067 --> 19:42.697 other cases, smaller. 19:42.700 --> 19:45.130 This is the probability density of finding a 19:45.133 --> 19:48.573 particular cloud droplet diameter. 19:48.567 --> 19:52.167 On one research flight, Research Flight 13, we found 19:52.167 --> 19:56.367 cloud droplets that were on the order of 20 to 25 microns. 19:56.367 --> 20:00.767 On another day, more like 10 to 15. 20:00.767 --> 20:03.497 And that has a lot to do with then whether it's going to 20:03.500 --> 20:04.300 rain or not. 20:04.300 --> 20:07.770 Because in order to produce rain-- 20:07.767 --> 20:10.467 let me just remind you of something. 20:10.467 --> 20:16.167 A raindrop is about 100 times larger than a cloud droplet. 20:16.167 --> 20:17.797 100 times. 20:17.800 --> 20:21.230 The diameter has a ratio of about 100 times. 20:21.233 --> 20:23.473 In order to make a raindrop, you have to bring together a 20:23.467 --> 20:26.897 very large number of cloud droplets. 20:26.900 --> 20:28.100 How many would you guess? 20:28.100 --> 20:28.970 How many would you have to bring 20:28.967 --> 20:31.067 together to form one raindrop? 20:34.433 --> 20:35.603 I thought you were going to say. 20:35.600 --> 20:40.230 Remember, I said the diameters had a ratio of 100 to 1. 20:40.233 --> 20:43.073 But it's really volumes we're talking about. 20:43.067 --> 20:45.497 In order to construct a raindrop, you're going to have 20:45.500 --> 20:50.030 to bring together the volume of a raindrop. 20:50.033 --> 20:54.673 So how many droplets of 100 times smaller do you have to 20:54.667 --> 20:57.327 bring together to form the volume of a raindrop? 20:57.333 --> 20:58.533 STUDENT: 100 cubed. 20:58.533 --> 21:00.573 PROFESSOR: 100 cubed. 21:00.567 --> 21:01.727 Right. 21:01.733 --> 21:04.833 That is to say, 10 to the 6, or a million. 21:04.833 --> 21:07.903 You've got to bring together a million of these 21:07.900 --> 21:09.530 to make one a raindrop. 21:09.533 --> 21:10.403 Please remember that. 21:10.400 --> 21:15.800 Because one of the longest questions in all of 21:15.800 --> 21:19.100 meteorology, goes back to the days of the Greeks, is why do 21:19.100 --> 21:21.430 most clouds not rain? 21:21.433 --> 21:23.803 But a few do. 21:23.800 --> 21:26.630 In some cases, there's a mechanism that can bring 21:26.633 --> 21:31.503 together a million cloud droplets to form one raindrop. 21:31.500 --> 21:34.230 But it depends to some extent on how the cloud droplets, how 21:34.233 --> 21:35.733 big they are to begin with. 21:35.733 --> 21:40.173 And we found some variability from case to case in that 21:40.167 --> 21:41.827 cloud droplet size. 21:41.833 --> 21:43.833 Again, the units here are microns. 21:43.833 --> 21:44.903 These are the small ones. 21:44.900 --> 21:46.230 These are the cloud droplets. 21:46.233 --> 21:50.973 The ones that just hang up there in the sky. 21:50.967 --> 21:52.227 Any questions here? 21:54.667 --> 21:55.127 OK. 21:55.133 --> 21:57.003 Now let's look at some other cloud types. 21:57.000 --> 21:58.430 I'm not going to go through this exhaustively. 22:01.967 --> 22:04.427 This is a type of cloud that is caused-- 22:04.433 --> 22:08.233 called, in the scientific literature it's called, 22:08.233 --> 22:10.173 alto-cumulus. 22:10.167 --> 22:15.097 Alto meaning mid-level cloud, about halfway up through the 22:15.100 --> 22:19.300 troposphere, and cumulus because it has some turbulent 22:19.300 --> 22:22.500 structure to it. 22:22.500 --> 22:27.070 This particular type of alto-cumulus, which has a kind 22:27.067 --> 22:30.067 of banded nature to it, is called, 22:30.067 --> 22:33.097 colloquially, mackerel sky. 22:33.100 --> 22:37.170 And you find that term very often in the literature of the 22:37.167 --> 22:41.797 sea, for example, but you find it commonly in other types of 22:41.800 --> 22:44.000 literature, as well. 22:44.000 --> 22:45.300 Now you know what's going on here. 22:45.300 --> 22:46.230 Well, you don't know everything. 22:46.233 --> 22:48.133 But you know that the air is rising 22:48.133 --> 22:51.733 there, and sinking there. 22:51.733 --> 22:52.933 Rising to give you the clouds. 22:52.933 --> 22:54.433 Sinking to give you the clear air. 22:54.433 --> 22:57.803 You don't necessarily know what's causing that particular 22:57.800 --> 23:00.570 pattern of rising and sinking air. 23:00.567 --> 23:03.927 I think it has to do a little bit with wind shear and maybe 23:03.933 --> 23:07.903 a little bit with the cooling of the tops by radiation. 23:07.900 --> 23:09.430 But I'm not sure in this case. 23:09.433 --> 23:13.703 But I do know that that's an alto-cumulus cloud of the type 23:13.700 --> 23:16.700 that is commonly referred to as mackerel sky. 23:16.700 --> 23:18.230 And where does the word "mackerel" come 23:18.233 --> 23:21.033 from, you may be asking? 23:21.033 --> 23:27.133 Well, the fishermen that gave that cloud its colloquial name 23:27.133 --> 23:32.503 were catching Atlantic mackerel, and that pattern on 23:32.500 --> 23:38.570 the back of the fish reminded them of that pattern. 23:38.567 --> 23:41.827 It's not the scientific name, but it's a common colloquial 23:41.833 --> 23:46.073 name for that kind of cloud pattern. 23:46.067 --> 23:47.497 This is cirro-stratus. 23:47.500 --> 23:48.230 So what does that mean? 23:48.233 --> 23:52.473 Cirro means high ice cloud. 23:52.467 --> 23:56.567 Stratus means without structure, horizontal, a 23:56.567 --> 24:01.697 fairly homogeneous horizontal layer of cloud. 24:01.700 --> 24:04.070 You see a little bit of light and dark patterns, but 24:04.067 --> 24:09.397 generally that is a layer of rather continuous cloud. 24:09.400 --> 24:13.330 It's a little bit filmy, which reminds you that 24:13.333 --> 24:14.603 it's probably ice. 24:14.600 --> 24:16.830 And if it's ice, it's certainly high. 24:16.833 --> 24:19.833 It's certainly up in the upper part of the-- 24:19.833 --> 24:21.103 of the troposphere. 24:25.100 --> 24:28.700 Here is a fair weather cumulus that is beginning to develop 24:28.700 --> 24:30.300 vertically. 24:30.300 --> 24:33.430 There's enough latent heat being given off when the water 24:33.433 --> 24:35.173 vapor is condensing. 24:35.167 --> 24:38.897 And that heat is keeping the cloudy air warm enough. 24:38.900 --> 24:42.500 So it's not just stopping, giving you a fair weather 24:42.500 --> 24:44.070 cumulus, and then the air descending again. 24:44.067 --> 24:46.927 Instead, it's continued to rise. 24:46.933 --> 24:49.303 I don't believe it's at the tropopause yet, so I'm going 24:49.300 --> 24:50.200 to call that-- 24:50.200 --> 24:52.370 and it's not raining out the bottom-- so I'm going to call 24:52.367 --> 24:54.897 that cumulus congestious. 24:54.900 --> 24:57.130 A growing cumulus cloud. 24:59.700 --> 25:03.600 But now here's one that is raining out the bottom. 25:03.600 --> 25:06.100 You see it here. 25:06.100 --> 25:08.600 I don't see a good anvil yet. 25:08.600 --> 25:12.730 But because it's raining, and nimbus means rain, I would 25:12.733 --> 25:15.233 definitely call that cumulo-nimbus. 25:15.233 --> 25:17.303 It's a cumulus cloud that's raining, and 25:17.300 --> 25:18.830 typically these are deep. 25:18.833 --> 25:23.473 They go from one kilometer for the base up to 8, 10, 12 25:23.467 --> 25:25.567 kilometers at the top. 25:25.567 --> 25:29.697 So that's a deep cloud spanning the entire 25:29.700 --> 25:31.470 troposphere, from bottom to top. 25:31.467 --> 25:31.727 Question? 25:31.733 --> 25:34.373 STUDENT: Can you explain again what about the latent heat 25:34.367 --> 25:36.597 causes it to keep on rising? 25:39.367 --> 25:41.967 PROFESSOR: Remember we talked about-- we defined a stable 25:41.967 --> 25:45.697 atmosphere as an atmosphere where a rising parcel finds 25:45.700 --> 25:49.670 itself colder than its environment, more dense, and 25:49.667 --> 25:51.497 wants to fall back. 25:51.500 --> 25:54.570 Well that might happen in this environment, in the dry air, 25:54.567 --> 25:57.497 but inside the cloud that air parcel has 25:57.500 --> 25:59.270 another source of heat. 25:59.267 --> 26:03.867 As it rises, water vapor is condensing, adding heat to the 26:03.867 --> 26:06.327 air parcel, making it warmer, and that allows it 26:06.333 --> 26:09.473 to continue to rise. 26:09.467 --> 26:14.667 So we would call that a moist unstable atmosphere where, due 26:14.667 --> 26:18.097 to that extra latent heat, the parcel is able to rise and 26:18.100 --> 26:18.930 form that cloud. 26:18.933 --> 26:20.203 STUDENT: So when it condenses, heat is given off? 26:22.600 --> 26:23.600 PROFESSOR: Heat is given off. 26:23.600 --> 26:23.900 Right. 26:23.900 --> 26:25.170 Exactly. 26:31.333 --> 26:32.603 Here's another cumulo-nimbus. 26:32.600 --> 26:36.300 Now the bottom of it is obscured by other clouds, so 26:36.300 --> 26:39.970 I've taken a bit of a risk in calling it nimbus, because I 26:39.967 --> 26:42.097 can't see the rain coming out the bottom. 26:42.100 --> 26:44.700 But I can see the anvil. 26:44.700 --> 26:49.830 That air has risen, hit the stable stratosphere, and 26:49.833 --> 26:54.373 spread out a little bit in the shape of a blacksmith's anvil. 26:54.367 --> 26:57.997 So we call it an anvil. 26:58.000 --> 27:01.500 Usually by the time it's done that, there is rain coming out 27:01.500 --> 27:01.970 of the bottom. 27:01.967 --> 27:05.397 So I'm going to take a chance and call that one also 27:05.400 --> 27:07.030 cumulo-nimbus. 27:07.033 --> 27:09.933 It's probably going to have liquid water down here 27:09.933 --> 27:11.803 probably some ice at the top. 27:15.200 --> 27:20.300 Sometimes underneath the anvil you get a rather famous and 27:20.300 --> 27:24.470 interesting cloud structure called mammatus. 27:24.467 --> 27:28.567 Air that's been carried up to the tropopause, and spread out 27:28.567 --> 27:31.797 because it can't rise any further, then begins to 27:31.800 --> 27:35.500 descend again in lumps. 27:35.500 --> 27:40.630 And when the lighting is correct, as it was here, when 27:40.633 --> 27:45.003 the sun is low in the sky and is illuminating these things 27:45.000 --> 27:47.770 kind of from the side, they take a rather dramatic 27:47.767 --> 27:53.667 appearance of smooth descending blobs of cloudy air 27:53.667 --> 27:55.267 coming out of the bottom of the anvils. 27:55.267 --> 27:57.827 It's called mammatus. 27:57.833 --> 28:00.433 The origin is, I think, evident. 28:04.267 --> 28:08.527 My favorite cloud is the lenticular cloud. 28:08.533 --> 28:11.533 When air is forced to rise over a mountain range, it's 28:11.533 --> 28:15.873 lifted temporarily, and then sinks on the backside. 28:15.867 --> 28:20.397 And when it's lifted it'll reach the saturation point, 28:20.400 --> 28:23.030 perhaps cloud will form. 28:23.033 --> 28:26.973 But then as that air descends back down the lee side of the 28:26.967 --> 28:32.697 mountain, everything reverses, and the excess water will 28:32.700 --> 28:34.970 evaporate, and you get back to clear sky. 28:34.967 --> 28:36.667 So we're only seeing half of it here. 28:36.667 --> 28:44.727 But this is a case looking northwest from a point that is 28:44.733 --> 28:46.533 east of the Sierra Nevada range. 28:46.533 --> 28:50.933 So the air is rising here, forming that cloud-- 28:50.933 --> 28:53.533 we don't see the other side of it here-- the remarkable thing 28:53.533 --> 28:58.903 is that cloud stays more or less steady as the air 28:58.900 --> 29:02.100 continues to flow through it. 29:02.100 --> 29:04.930 So the cloud is fixed, but new air parcels are entering and 29:04.933 --> 29:07.733 leaving, entering and leaving, entering and leaving, and the 29:07.733 --> 29:10.173 cloud just stays there more or less in the same place. 29:10.167 --> 29:12.927 You may see it wiggle around a little bit, but generally it 29:12.933 --> 29:13.903 just stays there. 29:13.900 --> 29:16.930 Remarkable to see. 29:16.933 --> 29:19.433 Here's one over Mount Fuji. 29:19.433 --> 29:22.003 Lenticular cloud. 29:22.000 --> 29:23.430 Sometimes it's even hard to know which 29:23.433 --> 29:24.433 way the air is blowing. 29:24.433 --> 29:26.473 I think the air is from left to right here. 29:26.467 --> 29:30.797 It's rising, a cloud is forming, then when the air 29:30.800 --> 29:35.700 descends, that condensed water evaporates again, and the 29:35.700 --> 29:38.530 parcel goes back to just clear air. 29:38.533 --> 29:42.573 Water vapor only, no condensed water. 29:42.567 --> 29:45.697 These occur in a variety of different configurations. 29:45.700 --> 29:47.330 I like this one. 29:47.333 --> 29:52.973 This is called the pile of plates lenticular. 29:52.967 --> 29:54.497 Now why does it look like this? 29:54.500 --> 29:56.070 There's one mountain range. 29:56.067 --> 29:58.297 The air is rising up over it and then sinking. 29:58.300 --> 29:59.470 Why that structure? 29:59.467 --> 30:01.367 That structure can only be there-- 30:01.367 --> 30:03.027 I thought about this for years-- there's only one 30:03.033 --> 30:05.333 possible explanation that I can come up with. 30:05.333 --> 30:09.473 And that is at the water vapor upwind of this region was 30:09.467 --> 30:11.727 layered a little bit. 30:11.733 --> 30:13.973 A moist layer dry layer, moist layer, dry layer. 30:13.967 --> 30:14.527 Moist. Dry. 30:14.533 --> 30:15.933 Moist. Dry. 30:15.933 --> 30:20.203 Then when the whole thing was lifted, the moist layers came 30:20.200 --> 30:23.100 to saturation. 30:23.100 --> 30:25.030 The dry layers, with roughly the same amount 30:25.033 --> 30:26.273 of lifting did not. 30:29.767 --> 30:30.967 The moist layers formed a cloud. 30:30.967 --> 30:34.197 The dry layers formed the interleaving dry air. 30:34.200 --> 30:37.100 And then, of course, when the air descends back down the lee 30:37.100 --> 30:39.730 side, all the condensed water re-evaporates. 30:42.500 --> 30:46.030 These things are sometimes found right, almost connected 30:46.033 --> 30:47.573 with a mountain range as you see them there. 30:47.567 --> 30:50.897 But sometimes they're quite some distance above or even 30:50.900 --> 30:54.000 displaced from the mountain, in which case they have been 30:54.000 --> 30:57.470 very frequently identified as UFOs, 30:57.467 --> 30:59.327 unidentified flying objects. 30:59.333 --> 31:05.103 In fact, a very large fraction of the UFO reports over the 31:05.100 --> 31:11.200 last 100 years can ultimately be explained in terms of 31:11.200 --> 31:13.470 lenticular clouds. 31:13.467 --> 31:17.327 So be skeptical of the UFOs enough. 31:17.333 --> 31:20.003 The first place I would look for an explanation of a new 31:20.000 --> 31:23.230 sighting would be to see if it's in a mountainous area, 31:23.233 --> 31:25.673 see if the atmospheric conditions were right to form 31:25.667 --> 31:31.967 this kind of local smooth uplift more or less stationary 31:31.967 --> 31:38.397 because of the way this cloud is formed. 31:38.400 --> 31:43.370 Now that covers most of the clouds, but I wanted to be 31:43.367 --> 31:47.167 sure your background was a little bit broader than this. 31:47.167 --> 31:49.527 So I want to talk about vortices for a minute. 31:49.533 --> 31:49.933 Question? 31:49.933 --> 31:50.173 Yes. 31:50.167 --> 31:52.867 STUDENT: You said there was the right atmospheric 31:52.867 --> 31:54.797 condition to form those lenticular clouds. 31:54.800 --> 31:56.270 What is that? 31:56.267 --> 32:00.267 PROFESSOR: Well if the air was too dry to begin with, and you 32:00.267 --> 32:03.767 only lifted it a few hundred meters, you wouldn't bring it 32:03.767 --> 32:04.667 to saturation. 32:04.667 --> 32:06.927 It wouldn't be enough. 32:06.933 --> 32:10.303 It's got to be humid enough, at least in those layers, and 32:10.300 --> 32:13.500 the mountain's got to be high enough to lift it to 32:13.500 --> 32:14.600 saturation. 32:14.600 --> 32:16.970 You don't bring it to saturation, then you won't get 32:16.967 --> 32:19.727 the cloud forming. 32:19.733 --> 32:23.973 Anything else on these clouds? 32:23.967 --> 32:26.767 So, vortices. 32:26.767 --> 32:30.567 Intense vortices, no matter how they're formed, have low 32:30.567 --> 32:33.267 pressure in the middle. 32:33.267 --> 32:36.297 If you don't believe this, take a bucket of water and 32:36.300 --> 32:40.300 reach in with your hand and get it swirling very fast. 32:40.300 --> 32:43.800 You'll notice that the surface of the water in the middle is 32:43.800 --> 32:46.130 drawn down in the middle. 32:46.133 --> 32:48.303 You can do it with a glass of water, too, at lunch. 32:48.300 --> 32:51.000 Just get it swirling, and you'll see that in the middle, 32:51.000 --> 32:52.930 the water's drawn down. 32:52.933 --> 32:55.773 Hydrostatically that tells you there's a lower pressure in 32:55.767 --> 32:57.497 the middle of the vortex. 32:57.500 --> 33:01.070 It has to be that way for centrifugal force to be 33:01.067 --> 33:03.967 balanced by a pressure gradient force in there. 33:03.967 --> 33:05.997 But that's not our primary focus. 33:06.000 --> 33:09.330 So if you accept the fact that there's low pressure in the 33:09.333 --> 33:11.703 middle of a vortex, if you take an air parcel that has a 33:11.700 --> 33:15.470 certain amount of water vapor in it, and you move it into 33:15.467 --> 33:19.127 the center that vortex somehow, its 33:19.133 --> 33:21.403 pressure will drop. 33:21.400 --> 33:24.100 Well, what happens when you drop the pressure of air? 33:27.133 --> 33:32.273 It expands and cools by adiabatic expansion. 33:32.267 --> 33:34.667 Until this moment, I've been talking about adiabatic 33:34.667 --> 33:38.067 expansion as if the only way you can do it is by lifting 33:38.067 --> 33:42.067 air into lower pressure regions. 33:42.067 --> 33:44.327 But here's another way you could do it just by taking a 33:44.333 --> 33:48.273 parcel of air from outside and putting it inside a vortex. 33:48.267 --> 33:49.867 You can drop its pressure. 33:49.867 --> 33:52.567 You could make it expand adiabatically, drop the 33:52.567 --> 33:54.127 temperature, drop the saturation. 33:54.133 --> 33:58.233 The whole story continues, is the same, drop the saturation 33:58.233 --> 34:02.733 vapor pressure and a cloud can form. 34:02.733 --> 34:03.973 So a tornado. 34:07.233 --> 34:08.733 The main-- this part of a tornado is called the 34:08.733 --> 34:10.173 condensation funnel. 34:10.167 --> 34:13.967 Now sometimes another funnel is identified near the bottom, 34:13.967 --> 34:15.697 and you can begin to see evidence of it 34:15.700 --> 34:17.700 here, the dark region. 34:17.700 --> 34:20.800 That's called the debris funnel. 34:20.800 --> 34:23.230 That's material that's been kicked up off the surface and 34:23.233 --> 34:24.533 is being carried upwards. 34:24.533 --> 34:25.503 I'm not talking about that. 34:25.500 --> 34:27.200 I'm talking about this, actually much 34:27.200 --> 34:28.700 more spectacular funnel. 34:28.700 --> 34:31.300 That is just a cloud. 34:31.300 --> 34:36.930 That's just condensed water vapor to form cloud droplets, 34:36.933 --> 34:39.903 because air from out here is getting into there, and 34:39.900 --> 34:42.530 there's low pressure in the middle. 34:42.533 --> 34:46.533 That's just another example of adiabatic expansion causing a 34:46.533 --> 34:49.103 cloud to form, in this case, the 34:49.100 --> 34:53.100 condensation funnel of a tornado. 34:53.100 --> 34:55.200 Questions on that? 34:55.200 --> 34:57.370 We'll talk more about tornadoes later in the course. 35:01.433 --> 35:03.603 Pretend you didn't look at that for a moment. 35:03.600 --> 35:06.970 As you first looked at that image you may say, well he 35:06.967 --> 35:09.027 already showed us that, didn't he? 35:09.033 --> 35:13.373 He showed us contrails earlier in the day. 35:13.367 --> 35:16.927 But take a closer look at that airplane and where the 35:16.933 --> 35:20.303 vortices are coming from. 35:20.300 --> 35:22.100 And they're vortices, these little cloud lines. 35:22.100 --> 35:25.100 They're not coming from the engines. 35:25.100 --> 35:29.070 Actually, they're coming from the ends of the flaps. 35:29.067 --> 35:31.497 That pilot, he's got his landing gear down. 35:31.500 --> 35:35.300 He's got his flaps down to get extra lift. 35:35.300 --> 35:38.430 And he's producing little wingtip, vortices, not exactly 35:38.433 --> 35:41.673 off the wingtip, but off the edge of the flaps. 35:41.667 --> 35:44.667 That's a trailing vortex. 35:44.667 --> 35:47.627 There's low pressure inside. 35:47.633 --> 35:50.133 And the low pressure is causing some water vapor to 35:50.133 --> 35:53.633 condense, to form cloud droplets inside 35:53.633 --> 35:56.173 that trailing vortex. 35:56.167 --> 35:59.367 So be sure you can distinguish that from the-- 35:59.367 --> 36:00.427 from the contrails. 36:00.433 --> 36:00.933 Yes? 36:00.933 --> 36:04.203 STUDENT: How does that create low pressure? 36:04.200 --> 36:08.900 PROFESSOR: Well, you know as a-- in order for an aircraft 36:08.900 --> 36:13.270 to fly it has to have fast air moving over the top of the 36:13.267 --> 36:19.867 wing, which by Bernoulli's law gives you low pressure, and 36:19.867 --> 36:23.567 higher pressure at the bottom, near the end of that wing. 36:23.567 --> 36:25.567 Because you have high pressure and low pressure, the high 36:25.567 --> 36:28.697 pressure air whips surround and forms the trailing vortex 36:28.700 --> 36:31.200 that comes off the wing tip. 36:31.200 --> 36:33.700 But remember, just like the tornado the fact that there's 36:33.700 --> 36:37.500 swirling air in that ensures that there's going to be lower 36:37.500 --> 36:39.400 pressure in the middle than there is outside. 36:39.400 --> 36:41.170 STUDENT: How common is this? 36:41.167 --> 36:45.997 PROFESSOR: Well again, the humidity has to be high in 36:46.000 --> 36:49.000 order for that little bit of pressure drop. 36:49.000 --> 36:53.070 So I'd say a picture like this is relatively rare, because 36:53.067 --> 36:56.267 you probably would have to have humidity higher than 95% 36:56.267 --> 37:01.597 preexisting in order to get that kind of a phenomenon. 37:06.900 --> 37:11.970 There's another way to get the sudden drop in pressure. 37:11.967 --> 37:15.397 And this is off of an aircraft flying supersonically. 37:15.400 --> 37:17.400 It's flying very rapidly through the atmosphere, faster 37:17.400 --> 37:18.730 than the speed of sound. 37:18.733 --> 37:22.603 It has its own characteristic flow field around aircraft 37:22.600 --> 37:26.000 when it's flying that fast, and its own characteristic 37:26.000 --> 37:28.270 pressure anomalies. 37:28.267 --> 37:33.867 Normally a supersonic aircraft where it parts the air gets 37:33.867 --> 37:38.567 high pressure, and where the air is coming back again near 37:38.567 --> 37:41.297 the rear of the aircraft, you're getting lower pressure, 37:41.300 --> 37:44.430 even lower than ambient. 37:44.433 --> 37:45.773 Even lower than ambient. 37:48.333 --> 37:51.103 And we will see what this does to the 37:51.100 --> 37:54.470 thermodynamics of the air. 37:54.467 --> 37:59.067 Air entering that lower pressure region on the back of 37:59.067 --> 38:04.397 the aircraft will adiabatically expand and so 38:04.400 --> 38:05.030 on, and so on. 38:05.033 --> 38:06.233 And a cloud will form. 38:06.233 --> 38:08.703 It's the same procedure we've talked about. 38:08.700 --> 38:10.030 Let's take a look at what that looks like. 38:12.600 --> 38:16.030 How many have seen that kind of diagram before? 38:16.033 --> 38:17.303 Only a few of you. 38:17.300 --> 38:21.930 I am intrigued by this, partly as a pilot but mostly just 38:21.933 --> 38:26.503 as-- just for the beauty and wonder of that. 38:26.500 --> 38:29.000 Your first thought may be, well, there is something 38:29.000 --> 38:33.630 there, and the aircraft is just flying through it, like 38:33.633 --> 38:36.373 some kind of a membrane, or something in the aircraft. 38:36.367 --> 38:36.597 No. 38:36.600 --> 38:37.130 No. 38:37.133 --> 38:39.973 The aircraft is flying supersonically, perhaps 1000 38:39.967 --> 38:41.197 miles per hour. 38:41.200 --> 38:45.600 And this is moving along with it, exactly following it. 38:45.600 --> 38:47.430 Tied to it. 38:47.433 --> 38:49.503 Notice there's another one, little one, 38:49.500 --> 38:50.670 just behind the cockpit. 38:50.667 --> 38:53.397 There's a little bit of a low pressure area there, and a 38:53.400 --> 38:56.700 bigger low pressure area there on the rear of the aircraft. 38:56.700 --> 39:00.770 And the air parcels, which were sub-saturated here, have 39:00.767 --> 39:02.997 entered this low pressure area. 39:03.000 --> 39:05.100 And the story is the same. 39:05.100 --> 39:08.700 Adiabatic expansion, cloud formation, and then when you 39:08.700 --> 39:11.230 get beyond, that the pressure jumps up again. 39:11.233 --> 39:15.003 The process reverses, and the cloud disappears. 39:15.000 --> 39:15.730 Question in the back. 39:15.733 --> 39:18.873 STUDENT: What is the difference between adiabatic 39:18.867 --> 39:21.797 cooling and adiabatic expansion? 39:21.800 --> 39:25.700 PROFESSOR: Adiabatic expansion is, they're 39:25.700 --> 39:27.200 almost the same thing. 39:27.200 --> 39:29.530 When you have adiabatic expansion, you 39:29.533 --> 39:31.833 get adiabatic cooling. 39:31.833 --> 39:35.803 Adiabatic expansion is defined as expanding air without 39:35.800 --> 39:36.570 adding heat. 39:36.567 --> 39:39.797 The word adiabatic means without adding heat. 39:39.800 --> 39:43.300 When you do that, when you expand air without adding 39:43.300 --> 39:47.500 heat, you cool it by the work that it does on its 39:47.500 --> 39:48.270 environment. 39:48.267 --> 39:49.867 In other words, you drop the temperature. 39:49.867 --> 39:52.697 So those are almost synonymous, but one 39:52.700 --> 39:54.500 leads to the other. 39:54.500 --> 39:55.730 Does that help? 39:58.733 --> 40:02.003 Then one more example of this. 40:02.000 --> 40:06.900 I call that the fighter plane wearing a tutu picture. 40:06.900 --> 40:09.570 Again, this is moving with that aircraft. 40:09.567 --> 40:13.697 This is not so common because the humidity probably has to 40:13.700 --> 40:19.630 be something like 95%, 96%, 97% in order to get that 40:19.633 --> 40:23.003 phenomenon to occur when the aircraft flies through it. 40:23.000 --> 40:25.100 STUDENT: Isn't that at higher altitudes? 40:25.100 --> 40:26.800 PROFESSOR: Not necessarily. 40:26.800 --> 40:29.230 It could be at any altitude, but the relative humidity 40:29.233 --> 40:33.703 would have to be high to begin with, the relative humidity. 40:33.700 --> 40:34.930 Questions on this? 40:37.400 --> 40:40.100 Haze and pollution. 40:40.100 --> 40:41.130 We talked about this a little bit. 40:41.133 --> 40:45.633 If the air is polluted and the particles are hydroscopic, 40:45.633 --> 40:49.233 that is, they like to attract water, then even if the 40:49.233 --> 40:52.333 relative humidity is less than 100%, they may attract a 40:52.333 --> 40:55.933 little bit of water on to them and make themselves bigger. 40:55.933 --> 41:00.573 When they do that it's called haze, and it reduces the 41:00.567 --> 41:02.897 visual range. 41:02.900 --> 41:06.400 Take a look at this photo. 41:06.400 --> 41:10.100 Here's a nearby forest. There's a forest on a somewhat 41:10.100 --> 41:12.100 more distant hill. 41:12.100 --> 41:15.170 And a forest on an even more distant hill. 41:15.167 --> 41:19.797 This is up in Maine, I think, one of my summer vacations. 41:19.800 --> 41:22.830 Those three surfaces are pretty much identical, and yet 41:22.833 --> 41:25.473 that looks lighter and bluer than this. 41:25.467 --> 41:28.427 And this looks lighter and bluer than that. 41:28.433 --> 41:33.233 What's happening is if there's haze in the air, and sunlight 41:33.233 --> 41:37.373 is being scattered by that haze to your eye. 41:37.367 --> 41:39.297 So the further away the object is-- the further away the 41:39.300 --> 41:48.470 object is, it's not the darker that it looks, the lighter it 41:48.467 --> 41:54.727 looks because you're adding what I call path radiance to 41:54.733 --> 41:56.133 what you're seeing. 41:56.133 --> 41:59.833 And of course as the air becomes more and more humid, 41:59.833 --> 42:01.673 those particles will grow a little bit more. 42:01.667 --> 42:03.327 That mountain may disappear. 42:03.333 --> 42:06.133 That mountain may come to look like that mountain. 42:06.133 --> 42:08.873 And so this can be sometimes-- 42:08.867 --> 42:11.227 of course, you need the particles, but the humidity 42:11.233 --> 42:13.303 plays a role as well. 42:13.300 --> 42:18.470 How much water condenses onto those haze particles. 42:18.467 --> 42:20.627 Clouds seen from space. 42:20.633 --> 42:25.873 These are some fair weather cumulus clouds over Central 42:25.867 --> 42:27.897 South America, equatorial South America 42:27.900 --> 42:30.830 near the Ucayali River. 42:30.833 --> 42:33.603 They're just fair weather cumulus clouds, but they're 42:33.600 --> 42:35.170 forming in rows. 42:35.167 --> 42:38.827 Those are called cloud streets in the literature. 42:38.833 --> 42:42.673 And they are just fair weather cumulus clouds caused by the 42:42.667 --> 42:44.367 heating of the ground by the sun. 42:44.367 --> 42:49.227 But if there's a wind blowing, cumulus clouds tend to line up 42:49.233 --> 42:52.933 in rows, or we use the word streets. 42:52.933 --> 42:56.273 And so I know from this image that the wind is blowing 42:56.267 --> 43:01.367 either in that direction or in that direction because clouds 43:01.367 --> 43:05.967 line up with the wind to form that kind of a pattern. 43:05.967 --> 43:09.467 If I would guess, I'd suspect it's from upper-left to 43:09.467 --> 43:10.167 lower-right. 43:10.167 --> 43:11.927 But I know it's always parallel to the 43:11.933 --> 43:13.673 wind in that case. 43:13.667 --> 43:17.467 This is hard to see sometimes from the ground because you 43:17.467 --> 43:19.367 may see a cloud over there, and a cloud over there, but 43:19.367 --> 43:21.967 it's hard to make out this pattern, because you're too 43:21.967 --> 43:23.767 close to it. 43:23.767 --> 43:26.967 Being up in a satellite, you get to see the whole pattern. 43:26.967 --> 43:29.397 And, of course, that's really true for larger scale cloud 43:29.400 --> 43:30.470 patterns like this. 43:30.467 --> 43:32.627 Here's North America. 43:32.633 --> 43:36.073 Here's a beautiful mid-latitudes cyclone. 43:36.067 --> 43:39.997 The cyclone center is here, occluded front back to here, 43:40.000 --> 43:41.870 warm front, cold front. 43:41.867 --> 43:44.067 Now if you're at any location all you see 43:44.067 --> 43:49.067 is overcast or clear. 43:49.067 --> 43:51.397 But from far enough away, from up in space, you could see 43:51.400 --> 43:53.200 this is part of a beautiful-- 43:53.200 --> 43:55.900 we call that a comma cloud, because as 43:55.900 --> 43:56.800 the shape of a comma. 43:56.800 --> 43:59.770 But that's a beautiful indication of what's called a 43:59.767 --> 44:02.227 mid-latitude frontal cyclone. 44:02.233 --> 44:05.333 And we'll be talking about the structure of these later on in 44:05.333 --> 44:06.973 the course. 44:06.967 --> 44:11.567 Thunderstorms look like this from space. 44:11.567 --> 44:13.097 They have an anvil. 44:13.100 --> 44:14.270 We've spoken about that. 44:14.267 --> 44:18.127 That's kind of a flat part where it's hit the tropopause 44:18.133 --> 44:23.003 and can't go any higher so it spreads out. 44:23.000 --> 44:26.570 The main updraft is usually where this bumpy part is. 44:26.567 --> 44:30.327 The main updraft is there and hits, spreads out, and forms a 44:30.333 --> 44:33.933 big, big anvil. 44:33.933 --> 44:36.573 Hurricane looks like this. 44:36.567 --> 44:39.397 There's the eye of the hurricane rising air, big 44:39.400 --> 44:41.370 anvil spiraling outwards. 44:41.367 --> 44:46.167 Clear skies from scattered cumulus other places, but, 44:46.167 --> 44:49.767 again, you need to be on a satellite to see that big, 44:49.767 --> 44:51.627 that big pattern there. 44:51.633 --> 44:54.503 We're almost done, but I want to remind you that we're 44:54.500 --> 44:55.230 talking about clouds. 44:55.233 --> 44:59.273 We're talking about liquid and/or ice. 44:59.267 --> 45:01.867 It's a condensed phase of water, but it could be liquid 45:01.867 --> 45:03.197 and/or ice. 45:03.200 --> 45:07.800 If the temperature is higher than zero degrees Celsius, 45:07.800 --> 45:10.530 it's always going to be liquid. 45:10.533 --> 45:15.833 If the temperature is colder than minus 40 Celsius, it's 45:15.833 --> 45:18.133 always going to be ice. 45:18.133 --> 45:20.473 But if it's in this intermediate range, this 45:20.467 --> 45:25.267 rather wide intermediate range, of zero Celsius down to 45:25.267 --> 45:29.627 minus 40, it could be either liquid or ice. 45:29.633 --> 45:32.233 If it's liquid, we call it supercooled liquid because it 45:32.233 --> 45:37.373 is below its normal freezing point. 45:37.367 --> 45:38.997 There's a definition. 45:39.000 --> 45:42.830 It's very dangerous for ships and aircraft, because that ice 45:42.833 --> 45:44.273 will freeze on contact. 45:44.267 --> 45:47.597 For example, here's a wing of an aircraft that's been flying 45:47.600 --> 45:52.530 through a cloud that had supercooled water in it. 45:52.533 --> 45:56.873 Those little cloud particles froze on contact with the wing 45:56.867 --> 46:03.497 and formed this rather ugly and poorly aerodynamic shape, 46:03.500 --> 46:05.870 making it very difficult for that aircraft to fly. 46:05.867 --> 46:09.167 A large number of accidents and fatalities have arisen 46:09.167 --> 46:11.467 from this kind of a problem. 46:11.467 --> 46:14.067 The only thing to do is to quickly get out of that 46:14.067 --> 46:17.067 condition and help to melt it off, or have some special 46:17.067 --> 46:20.527 technology on the wing that will throw off that ice, 46:20.533 --> 46:25.473 either by heating or by what's called a-- 46:25.467 --> 46:29.597 well it's a rubber thing that can be expanded to actually 46:29.600 --> 46:31.500 physically knock off the ice. 46:31.500 --> 46:35.200 A boot it's called a boot in that case. 46:35.200 --> 46:42.070 When riming forms on the ship, you get supercooled cloud 46:42.067 --> 46:45.597 droplets passing over a ship and it forms. That's very 46:45.600 --> 46:48.430 dangerous because that'll add mass to the 46:48.433 --> 46:51.003 upper part of the ship. 46:51.000 --> 46:53.870 And the moment you add too much to that, the ship will 46:53.867 --> 46:59.027 then spontaneously turn over, and everyone will be lost. You 46:59.033 --> 47:01.703 really have to get the sailors out there chipping away at 47:01.700 --> 47:04.830 that ice, because it has a very dangerous weight to the 47:04.833 --> 47:07.833 top of the ship when supercooled water is 47:07.833 --> 47:10.903 collecting like that. 47:10.900 --> 47:12.800 Here's a case you can see from time to time. 47:12.800 --> 47:18.430 Here's a layer of probably alto-cumulus that's liquid. 47:21.067 --> 47:25.097 Then, for some reason, and this is below the freezing 47:25.100 --> 47:27.700 point, maybe it's minus 20 Celsius. 47:27.700 --> 47:31.330 For some reason in some area that liquid has frozen 47:31.333 --> 47:34.403 spontaneously forming snowflakes and they're falling 47:34.400 --> 47:37.430 out, leaving a little clear space in the cloud, and a 47:37.433 --> 47:43.033 little plume of falling snowflakes out of that region. 47:43.033 --> 47:44.973 Look for this over the next winter. if you look hard 47:44.967 --> 47:47.227 enough, you'll probably find a case or two of this.