WEBVTT 01:08.333 --> 01:12.573 RONALD SMITH: Well, when I lectured last on 01:12.567 --> 01:15.497 Wednesday of last week, we went through a long PowerPoint 01:15.500 --> 01:19.800 presentation about water in the atmosphere. 01:19.800 --> 01:22.830 And there's a lot of information in that. 01:22.833 --> 01:24.933 And of course, that lecture's been posted. 01:24.933 --> 01:27.073 But today we're going to continue on from there. 01:27.067 --> 01:29.827 We're not finished with water in the atmosphere. 01:29.833 --> 01:31.603 We're going to move more towards the subject of 01:31.600 --> 01:33.170 precipitation-- 01:33.167 --> 01:35.027 how precipitation forms. 01:35.033 --> 01:42.133 And let me begin by just describing what you see when 01:42.133 --> 01:44.003 you look at a cloud. 01:44.000 --> 01:46.430 So I've got a little cartoon here. 01:46.433 --> 01:47.933 I'm going to make a cloud in just a minute. 01:47.933 --> 01:51.733 But what do you see when you look at a cloud? 01:51.733 --> 01:52.403 There's a cloud. 01:52.400 --> 01:53.630 There's you as the observer. 01:56.833 --> 01:59.673 You are-- when you look away from the cloud, you're seeing 01:59.667 --> 02:03.797 sunlight that has scattered off air molecules. 02:06.567 --> 02:09.927 And that light looks blue to you. 02:09.933 --> 02:13.703 When you look at the cloud itself, you see sunlight that 02:13.700 --> 02:18.400 is scattered off tiny condensed water droplets. 02:18.400 --> 02:21.400 And that cloud looks white to you. 02:21.400 --> 02:25.300 Under certain circumstances, if you look at the rain 02:25.300 --> 02:29.930 falling from the cloud and the light, the orientation of the 02:29.933 --> 02:34.503 illumination is just right, you'll see a rainbow. 02:34.500 --> 02:36.170 So what's going on? 02:36.167 --> 02:39.167 Why do you see three such very different things when you look 02:39.167 --> 02:41.627 at a cloud? 02:41.633 --> 02:44.933 Well, it has to do with the different types of the way 02:44.933 --> 02:48.333 light scatters from particles. 02:48.333 --> 02:51.833 And so I wanted to just run through this before we got any 02:51.833 --> 02:53.673 further into clouds. 02:53.667 --> 02:56.967 If you imagine a photon of light with a certain 02:56.967 --> 03:01.367 wavelength lambda approaching a particle, it's going to be 03:01.367 --> 03:05.427 scattered off by that particle in some direction. 03:05.433 --> 03:09.603 What matters is the ratio of the wavelength of the light to 03:09.600 --> 03:12.130 the diameter of the droplet, that ratio. 03:15.000 --> 03:21.030 If the wavelength is much, much larger than the diameter 03:21.033 --> 03:24.833 of the droplet, that's called Rayleigh scattering, named 03:24.833 --> 03:29.803 after the famous English physicist. And under that 03:29.800 --> 03:37.300 condition, the scattering, you say S, is proportional to one 03:37.300 --> 03:40.400 over the wavelength to the fourth power. 03:40.400 --> 03:44.730 In other words, the shorter the wavelength, the more is 03:44.733 --> 03:47.873 the scattering under that condition. 03:47.867 --> 03:51.397 The shorter the wavelength for the same particle, the more is 03:51.400 --> 03:53.770 the scattering. 03:53.767 --> 03:57.597 Well, that's why the sky is blue because the visible 03:57.600 --> 04:03.270 spectrum includes red on the long wave side, green in the 04:03.267 --> 04:05.797 middle, blue on the short wave side. 04:05.800 --> 04:10.700 So when light comes from the sun and encounters these small 04:10.700 --> 04:14.630 air molecules, the blue light is scattered more than the 04:14.633 --> 04:16.903 green and the red. 04:16.900 --> 04:18.500 And therefore, the light scattered to your eye-- 04:18.500 --> 04:21.830 what you saw this morning as you walked up, the blue sky. 04:21.833 --> 04:26.233 That's because that scattering is from very small molecules 04:26.233 --> 04:30.933 which places you into the Rayleigh scattering regime, 04:30.933 --> 04:35.373 where short wavelengths scatter more intensely than 04:35.367 --> 04:36.867 the longer wavelengths. 04:36.867 --> 04:38.197 So that's why this sky is blue. 04:38.200 --> 04:43.070 Now the cloud particles in the cloud are 04:43.067 --> 04:47.767 much larger than molecules. 04:47.767 --> 04:55.567 They're typically 10 microns in size, which is of such a 04:55.567 --> 04:58.997 wavelength that it puts you into the so-called Mie 04:59.000 --> 05:02.830 scattering regime where wavelength is within one or 05:02.833 --> 05:05.333 two orders of magnitude of the diameter. 05:05.333 --> 05:08.173 It doesn't have to be close to the same as the diameter but 05:08.167 --> 05:13.727 within a factor of 10 or 100 of the-- 05:13.733 --> 05:15.073 I see you straining to see them. 05:15.067 --> 05:20.127 Let me pull this down, might help with it. 05:20.133 --> 05:23.303 Mie scattering, if you're in that regime, all wavelengths 05:23.300 --> 05:26.130 are scattered roughly equally. 05:26.133 --> 05:30.003 Red, green, blue light are scattered equally. 05:30.000 --> 05:34.900 That is whatever color you have illuminating the object, 05:34.900 --> 05:36.930 that's the same color you will get 05:36.933 --> 05:38.833 reflecting from the object. 05:38.833 --> 05:42.503 So if you're illuminating the object with white light, then 05:42.500 --> 05:45.870 the scattered light will be white as well, have the same 05:45.867 --> 05:50.127 mix of the different colors in the spectrum. 05:50.133 --> 05:53.173 So the cloud appears white to your eye because it's 05:53.167 --> 05:56.067 scattering all those different solar wavelengths equally. 05:58.933 --> 06:02.403 The other case-- the third case is when the diameter of 06:02.400 --> 06:05.970 the object is much larger than the wavelength of light. 06:05.967 --> 06:10.497 That would apply, by the way, for a raindrop which has a 06:10.500 --> 06:13.000 diameter of about one millimeter. 06:13.000 --> 06:16.700 And remember the wavelength we're talking about, for 06:16.700 --> 06:19.330 example, visible light, is only 0.5 microns. 06:19.333 --> 06:22.773 So that would easily satisfy that. 06:22.767 --> 06:25.867 And that's the case where you can trace-- 06:25.867 --> 06:29.127 it's called geometric optics is the physical name for that 06:29.133 --> 06:30.673 scattering regime. 06:30.667 --> 06:34.427 That means you can actually trace a ray of light that 06:34.433 --> 06:38.603 comes into the raindrop, refracts as it enters, bounces 06:38.600 --> 06:42.600 off one side, refracts again, and comes back to your eye 06:42.600 --> 06:46.830 splitting the color slightly because refractive index-- the 06:46.833 --> 06:49.403 bending of the light is a function of wavelength, 06:49.400 --> 06:52.730 therefore, giving you this special rainbow effect. 06:52.733 --> 06:54.533 You couldn't get that from cloud droplets 06:54.533 --> 06:56.303 because they're too small. 06:56.300 --> 07:00.200 But the fact that they're larger allows you to see that 07:00.200 --> 07:03.500 splitting of the light and the rainbow forming in that way. 07:03.500 --> 07:11.730 So that is basically what you see when you look at a cloud, 07:11.733 --> 07:15.103 and the sky, and the raindrops. 07:15.100 --> 07:16.470 A couple of other points though. 07:16.467 --> 07:19.927 There's another way that meteorologists look at clouds, 07:19.933 --> 07:22.433 and that's with radar-- 07:22.433 --> 07:24.673 meteorological radar. 07:24.667 --> 07:26.397 I've sketched that here. 07:26.400 --> 07:28.070 You've got a dish antenna. 07:28.067 --> 07:29.927 You make a microwave signal. 07:29.933 --> 07:31.573 Usually that wavelength is about 10 07:31.567 --> 07:34.427 centimeters, about like that. 07:34.433 --> 07:36.673 You send it towards the cloud. 07:36.667 --> 07:43.527 And it may or may not scatter back to your radar system. 07:43.533 --> 07:47.373 If it does, you can determine the distance from the time it 07:47.367 --> 07:49.927 takes the radar signal to get there and back. 07:49.933 --> 07:52.633 And you can learn something about the 07:52.633 --> 07:53.773 particles in that cloud. 07:53.767 --> 07:58.327 But remember, with a longer wavelength, the same cloud 07:58.333 --> 08:02.073 droplets, and even the raindrops, are going to be in 08:02.067 --> 08:03.567 the Rayleigh scattering regime. 08:03.567 --> 08:06.127 So with this longer wavelength, you're almost 08:06.133 --> 08:10.003 always going to be in the Rayleigh scattering regime 08:10.000 --> 08:13.500 because your wavelength here is so much larger than the 08:13.500 --> 08:15.970 wavelength of visible light. 08:15.967 --> 08:21.597 So with that strong inverse dependence on wavelength, and 08:21.600 --> 08:26.830 the diameter comes in too, it turns out that cloud droplets 08:26.833 --> 08:32.803 do not scatter radar effectively, but raindrops do. 08:32.800 --> 08:36.370 So when you're looking at a radar image of a cloud, you 08:36.367 --> 08:39.067 are seeing only where it is raining. 08:39.067 --> 08:42.767 You do not see the cloud you see with the visible eye. 08:42.767 --> 08:45.527 With the visible eye, you see this white puffy thing full of 08:45.533 --> 08:46.633 cloud droplets. 08:46.633 --> 08:47.833 Radar doesn't see that. 08:47.833 --> 08:48.803 Radar goes right through. 08:48.800 --> 08:53.670 But the radar will bounce off from the raindrops or the 08:53.667 --> 08:55.867 snowflakes within that cloud. 08:55.867 --> 08:58.827 So remember when using a meteorological radar, you are 08:58.833 --> 09:04.403 seeing precipitation rather than clouds per se. 09:04.400 --> 09:08.230 Of course, clouds produced precipitation, but they are 09:08.233 --> 09:10.273 different things. 09:10.267 --> 09:11.497 Any questions on that? 09:14.033 --> 09:14.303 OK. 09:14.300 --> 09:16.400 Well, that is background. 09:16.400 --> 09:20.330 We can do a little experiment to make a cloud. 09:20.333 --> 09:24.333 I've got a vacuum pump hidden under here and a little grey 09:24.333 --> 09:27.573 chamber that I'm going to evacuate. 09:27.567 --> 09:31.827 I'm going to hook this up to the jar in this way. 09:31.833 --> 09:38.173 And then by twisting a valve, I'm going to allow some of the 09:38.167 --> 09:44.067 air from this jug to pass into this evacuated cylinder. 09:44.067 --> 09:48.827 When I do that, let's say for the sake of argument that I 09:48.833 --> 09:52.233 take about that much of the air from here on up and 09:52.233 --> 09:54.173 suddenly remove it. 09:54.167 --> 09:58.267 Well, then the rest of the air in the jug is going to expand 09:58.267 --> 09:59.227 to fill the jug. 09:59.233 --> 10:03.973 Now what happens when air expands suddenly? 10:03.967 --> 10:06.567 We've been over and over this, but now you're going to see it 10:06.567 --> 10:07.897 in this new context. 10:07.900 --> 10:12.900 When air expands, it does work on its environment. 10:12.900 --> 10:16.000 And it adiabatically cools. 10:16.000 --> 10:19.600 Its temperature drops because of that work 10:19.600 --> 10:21.230 it's done in expansion. 10:21.233 --> 10:25.573 When the temperature drops, the saturation vapor pressure 10:25.567 --> 10:26.327 drops as well. 10:26.333 --> 10:29.933 That is the amount of water vapor that can be held in the 10:29.933 --> 10:33.673 vapor state suddenly decreases. 10:33.667 --> 10:38.567 Well, if Ive got-- if this is the saturation vapor pressure, 10:38.567 --> 10:40.697 and this is the amount of water vapor that I actually 10:40.700 --> 10:44.500 have, the partial pressure of water vapor, and I suddenly 10:44.500 --> 10:48.200 drop the temperature, that's going to drop this value down. 10:48.200 --> 10:51.930 I haven't changed the amount of water in the air, but I've 10:51.933 --> 10:55.903 decreased the amount that can be held in the vapor state. 10:55.900 --> 10:59.070 And if I move it down to this level and slightly beyond, 10:59.067 --> 11:02.727 that excess must suddenly condense. 11:02.733 --> 11:05.473 It cannot remain in the vapor state. 11:05.467 --> 11:10.067 And therefore, I'm going to force a cloud to form in very 11:10.067 --> 11:13.297 much the same way that the atmosphere does, 11:13.300 --> 11:15.470 by adiabatic expansion. 11:15.467 --> 11:17.997 The only difference is instead of getting the adiabatic 11:18.000 --> 11:21.930 expansion by lifting a parcel up into the atmosphere, I'm 11:21.933 --> 11:25.103 doing it with this little apparatus of suddenly dropping 11:25.100 --> 11:26.830 the pressure. 11:26.833 --> 11:28.073 Are there questions on this? 11:30.400 --> 11:37.630 Now I'm going to give it a few condensation nuclei because I 11:37.633 --> 11:41.903 prefer to form a large number of small droplets rather than 11:41.900 --> 11:45.830 a small number of large droplets. 11:45.833 --> 11:48.373 It'll make the cloud more visible that way. 11:55.367 --> 11:59.927 So I'm just going to blow a little bit of smoke in there. 11:59.933 --> 12:11.733 You won't be able to see this, but these will be very tiny-- 12:11.733 --> 12:14.233 these will be very tiny aerosol particles just to give 12:14.233 --> 12:16.533 the water something to condense on to. 12:31.933 --> 12:35.233 What you hear is a vacuum pump. 12:35.233 --> 12:37.373 I have a little gauge here to tell me whether or not it's 12:37.367 --> 12:40.397 working, whether this chamber is being evacuated. 12:53.767 --> 12:56.067 I put it on this overhead projector to get nice 12:56.067 --> 12:58.727 illumination of the chamber. 12:58.733 --> 13:00.103 And you see it's empty now. 13:00.100 --> 13:01.370 There's nothing there. 13:08.600 --> 13:10.600 OK, I think we have a little bit of a vacuum in here. 13:10.600 --> 13:14.670 So I'm going to try suddenly opening-- 13:14.667 --> 13:16.267 right now this valve is closed. 13:16.267 --> 13:19.167 So there's low pressure here but normal-- 13:19.167 --> 13:19.697 look-- 13:19.700 --> 13:22.670 normal atmospheric pressure in the jar. 13:22.667 --> 13:25.567 But when I open that valve, some of the air is going to be 13:25.567 --> 13:26.167 sucked off. 13:26.167 --> 13:28.827 And we'll see this doesn't always work. 13:28.833 --> 13:32.133 But we'll see if we can form a cloud this morning. 13:37.200 --> 13:44.570 Three, two, one, there's a cloud. 13:47.433 --> 13:50.303 Now this is a reversible process where I suddenly to 13:50.300 --> 13:53.370 bring that parcel back down to the atmosphere or here just 13:53.367 --> 13:56.397 let the pressure build up, it would compress warm 13:56.400 --> 13:57.300 and remove the cloud. 13:57.300 --> 13:59.970 So on the count of three, I'm just going to rip the top out 13:59.967 --> 14:04.067 of there and let atmospheric pressure push that down again. 14:04.067 --> 14:06.027 Three, two, one. 14:08.767 --> 14:12.167 I'm going to repeat that again. 14:12.167 --> 14:13.727 Let me close this off. 14:19.633 --> 14:21.203 Let me take questions for a minute about 14:21.200 --> 14:22.030 what I've done here. 14:22.033 --> 14:25.273 Are there any questions about what I'm actually doing 14:25.267 --> 14:26.527 physically up here? 14:30.367 --> 14:30.697 Question? 14:30.700 --> 14:31.900 STUDENT: Can you explain it again? 14:31.900 --> 14:36.530 PROFESSOR: So I'm evacuating this grey cylinder. 14:36.533 --> 14:40.073 It's just a little reservoir of low pressure. 14:40.067 --> 14:43.197 When I open the valve, then some of this atmospheric 14:43.200 --> 14:46.800 pressure is going to bleed off into here allowing the rest of 14:46.800 --> 14:50.200 the air to expand suddenly. 14:50.200 --> 14:53.430 That's just what happens when an air parcel rises up in the 14:53.433 --> 14:57.473 atmosphere and expands, cools adiabatically, drops the 14:57.467 --> 15:01.567 saturation vapor pressure, and the excess water vapor must 15:01.567 --> 15:04.167 then immediately condense. 15:04.167 --> 15:07.097 And it's finding one of the small particles in there, 15:07.100 --> 15:10.400 condensation nuclei, to condense on. 15:10.400 --> 15:12.630 Notice how small these particles are. 15:12.633 --> 15:14.303 You may have seen them swirling around. 15:14.300 --> 15:17.300 But they didn't fall out. 15:17.300 --> 15:21.030 Let's watch that the next time I do the experiment here. 15:21.033 --> 15:22.303 I think we're set. 15:26.600 --> 15:29.900 I haven't got a very good vacuum. 15:29.900 --> 15:31.530 Maybe this wasn't closed properly. 15:39.867 --> 15:41.127 It's OK. 15:46.867 --> 15:49.167 Three, two, one. 15:53.900 --> 15:57.330 Now there are tiny particles in there, but you don't see 15:57.333 --> 15:59.173 them falling. 15:59.167 --> 16:01.427 Sometimes you see them swirling around a little bit. 16:01.433 --> 16:03.833 There's some air currents in there. 16:03.833 --> 16:08.573 But they're so small that they don't really fall out 16:08.567 --> 16:10.267 gravitationally. 16:10.267 --> 16:12.497 I'll reverse the process now. 16:12.500 --> 16:15.430 Three, two, one. 16:15.433 --> 16:17.503 And it's gone. 16:17.500 --> 16:18.770 OK. 16:27.333 --> 16:31.433 That is how a cloud forms. Are there questions on that? 16:31.433 --> 16:33.673 They're must be some mysteries on what I've done here. 16:36.533 --> 16:37.833 Anything at all? 16:37.833 --> 16:40.073 STUDENT: Does the jar feel cold to the touch? 16:40.067 --> 16:40.497 PROFESSOR: I'm sorry. 16:40.500 --> 16:41.430 STUDENT: Does the jar feel cold to the touch? 16:41.433 --> 16:42.473 PROFESSOR: No. 16:42.467 --> 16:44.867 I don't think the temperature drop is enough. 16:44.867 --> 16:51.127 And the mass of the jar itself is too large, I think, for 16:51.133 --> 16:52.433 that to be detectable, at least by 16:52.433 --> 16:53.533 my hand on the outside. 16:53.533 --> 16:58.003 If we had a proper sensor in there, we could probably 16:58.000 --> 17:01.630 measure, if it was a fast response instrument, we could 17:01.633 --> 17:04.503 probably measure a several degree drop. 17:04.500 --> 17:04.670 I'm guessing-- 17:04.667 --> 17:05.867 I haven't done the calculation. 17:05.867 --> 17:09.267 I'm guessing it would be three or four degrees Celsius. 17:09.267 --> 17:12.327 I've got a little bit of liquid water in here. 17:12.333 --> 17:15.373 So this air is pretty humid to start with. 17:15.367 --> 17:15.427 Probably-- 17:15.433 --> 17:18.673 I'm guessing the air in there is probably 90% or 95% 17:18.667 --> 17:22.197 relative humidity, so I don't have to cool it too much to 17:22.200 --> 17:23.500 bring it to saturation. 17:23.500 --> 17:25.830 But I doubt that I can feel that 17:25.833 --> 17:26.933 temperature difference there. 17:26.933 --> 17:29.533 But there is one, but I think it would have to have a good 17:29.533 --> 17:31.673 sensor inside to feel that. 17:31.667 --> 17:32.827 That's a good question though. 17:32.833 --> 17:35.773 Temperature is a key here, and that part we can't see. 17:35.767 --> 17:39.327 We could only see the response of the water vapor to that 17:39.333 --> 17:42.173 drop in temperature. 17:42.167 --> 17:42.527 Yes? 17:42.533 --> 17:44.373 STUDENT: Why do the particles start swirling after 17:44.367 --> 17:45.867 you have the cloud? 17:45.867 --> 17:48.167 PROFESSOR: Well, when I suck the air out of 17:48.167 --> 17:51.767 there, remember if I suck it out, it's not going to come 17:51.767 --> 17:52.527 out smoothly. 17:52.533 --> 17:55.203 It's going to come out suddenly in one region more 17:55.200 --> 17:55.700 than another. 17:55.700 --> 17:58.830 And that's going to produce some eddies in there just 17:58.833 --> 18:01.003 because I've suddenly drawn air out of 18:01.000 --> 18:03.170 one part of the chamber. 18:06.167 --> 18:06.967 So that's just leftover from-- it could also be a little bit 18:06.967 --> 18:08.227 because I'm heating it from the bottom. 18:08.233 --> 18:10.703 It could be a little bit of thermal convection. 18:10.700 --> 18:13.570 But I suspect it's mostly just because I drew the air out 18:13.567 --> 18:19.067 very suddenly from one location in the bottle. 18:19.067 --> 18:20.997 Anything else on that? 18:21.000 --> 18:21.330 Yes? 18:21.333 --> 18:21.833 STUDENT: For the equation S approximately 1 over lamda to 18:21.833 --> 18:22.103 the fourth, is that for all the scattering or just for the 18:22.100 --> 18:23.330 Rayleigh scattering? 18:31.300 --> 18:33.970 PROFESSOR: That's just for Rayleigh scattering. 18:33.967 --> 18:34.267 Yes. 18:34.267 --> 18:37.427 In fact, for Mie scattering, you would write S is 18:37.433 --> 18:41.003 approximately independent of wavelength. 18:41.000 --> 18:45.630 And for geometric optics, it's more complicated because the 18:45.633 --> 18:49.103 ray is actually bouncing around inside the raindrop and 18:49.100 --> 18:50.630 scattering in a different way. 18:50.633 --> 18:54.533 So that applies only to the Rayleigh scattering. 18:54.533 --> 18:56.673 That's the property of-- that is why the sky is blue when 18:56.667 --> 18:58.597 you scatter off very small particles. 19:01.200 --> 19:02.930 OK. 19:02.933 --> 19:06.473 Anything else on the experiment? 19:06.467 --> 19:10.197 Now the subject that we have to deal with today is how you 19:10.200 --> 19:14.930 take a cloud that is composed of these tiny cloud droplets, 19:14.933 --> 19:19.603 10 microns in size, too small to fall gravitationally and 19:19.600 --> 19:22.930 occasionally get rain out of these clouds. 19:22.933 --> 19:26.503 As I mentioned last time, if you compare the diameter of a 19:26.500 --> 19:29.300 droplet to the diameter of a raindrop, there's a factor of 19:29.300 --> 19:31.530 100 difference. 19:31.533 --> 19:34.903 Their volumes are different by a factor of a million. 19:34.900 --> 19:38.200 That's 100 cubed, because the volume of the sphere goes like 19:38.200 --> 19:40.700 the cube of the radius. 19:40.700 --> 19:44.300 So we'd have to bring together a million cloud droplets to 19:44.300 --> 19:47.300 form one raindrop. 19:47.300 --> 19:50.000 How and under what circumstances are 19:50.000 --> 19:50.970 we going to do this? 19:50.967 --> 19:51.967 There are two theories. 19:51.967 --> 19:53.597 Your book is good in this. 19:53.600 --> 19:58.300 There are two outstanding theories for this. 19:58.300 --> 19:59.800 The first one is called collision-coalescence. 20:03.333 --> 20:08.503 If you have tiny cloud droplets, but they're not all 20:08.500 --> 20:12.370 the same size, some are a little bit larger than others, 20:12.367 --> 20:14.097 they're not falling very fast. 20:14.100 --> 20:17.070 But the large one is going to be falling a little bit faster 20:17.067 --> 20:19.667 than the smaller ones. 20:19.667 --> 20:22.167 And therefore, up in the cloud, the larger ones are 20:22.167 --> 20:25.167 going to overtake the smaller ones. 20:25.167 --> 20:30.867 And when they collide, they may coalesce. 20:30.867 --> 20:31.427 They might not. 20:31.433 --> 20:32.533 They might bounce off each other. 20:32.533 --> 20:35.633 But with some degree of efficiency, they will coalesce 20:35.633 --> 20:38.133 and make a larger droplet. 20:38.133 --> 20:41.773 Well, now it's going to fall even faster. 20:41.767 --> 20:45.027 And so it's going to sweep up smaller droplets at an 20:45.033 --> 20:47.233 increasing rate. 20:47.233 --> 20:49.833 And before you know it, you could sweep up a million 20:49.833 --> 20:52.133 droplets and form a raindrop. 20:52.133 --> 20:55.733 You may have seen something like this on a cold winter day 20:55.733 --> 21:01.303 with droplets condensing inside of your window at home. 21:01.300 --> 21:04.970 Sometimes a drop will start to move down the window. 21:04.967 --> 21:08.027 And then as it collects up other droplets, suddenly it 21:08.033 --> 21:10.003 will fall right to the bottom of the window. 21:10.000 --> 21:11.570 That's kind of what I'm talking about with 21:11.567 --> 21:13.597 collision-coalescence. 21:13.600 --> 21:17.800 It's not a very efficient process most of the time, 21:17.800 --> 21:21.830 because these cloud droplets are too similar to each other. 21:21.833 --> 21:25.503 There's not enough of a range of large to small particles to 21:25.500 --> 21:26.230 get this going. 21:26.233 --> 21:31.803 But on occasion, especially over tropical oceans, this 21:31.800 --> 21:33.730 mechanism is thought to dominate. 21:36.967 --> 21:40.397 The other mechanism, a little more complicated, so follow 21:40.400 --> 21:42.430 this argument closely. 21:42.433 --> 21:46.803 It assumes that you have supercooled water droplets, 21:46.800 --> 21:49.800 tiny droplets at a temperature colder than 21:49.800 --> 21:53.770 zero degrees Celsius. 21:53.767 --> 21:55.127 It wants to freeze. 21:55.133 --> 21:56.603 It's cold enough to freeze. 21:56.600 --> 21:59.600 But it needs something to trigger the freezing. 21:59.600 --> 22:02.730 That's called a freezing nucleus. 22:02.733 --> 22:07.403 Some little particle of dust or some little speck of 22:07.400 --> 22:09.170 something or other that would trigger 22:09.167 --> 22:10.267 one of those to freeze. 22:10.267 --> 22:15.167 So imagine you've got a cloud with supercooled liquid water. 22:15.167 --> 22:17.797 And something makes one of those 22:17.800 --> 22:19.500 droplets suddenly freeze. 22:19.500 --> 22:24.230 So I've drawn it now with a six-sided shape indicating 22:24.233 --> 22:25.133 it's a ice crystal. 22:25.133 --> 22:31.033 It turns out, here's the key, that ice at the same 22:31.033 --> 22:35.433 temperature as liquid water has a slightly lower 22:35.433 --> 22:38.803 saturation vapor pressure. 22:38.800 --> 22:46.230 Ice and water at the same temperature, the ice has a 22:46.233 --> 22:49.533 slightly lower saturation water vapor pressure. 22:49.533 --> 22:53.403 That means that the ice is a little more attractive to 22:53.400 --> 22:56.930 water vapor than the liquid is. 22:56.933 --> 23:01.403 So I freeze one of these droplets. 23:01.400 --> 23:04.330 And immediately, it starts to draw water vapor 23:04.333 --> 23:07.973 towards it and grow. 23:07.967 --> 23:11.397 Meanwhile, the other droplets begin to shrink because the 23:11.400 --> 23:16.400 humidity is decreasing around, which starts to evaporate the 23:16.400 --> 23:17.800 cloud droplets that remain. 23:17.800 --> 23:29.070 So before long, this thing has grown to be quite large 23:29.067 --> 23:34.167 through a vapor deposition process, initially at least, a 23:34.167 --> 23:36.367 vapor deposition process. 23:36.367 --> 23:38.167 This is a snowflake. 23:38.167 --> 23:39.567 And now it's large. 23:39.567 --> 23:43.427 It will begin to fall out of the sky and may reach the 23:43.433 --> 23:47.533 ground as a snowflake if the temperature is warm enough. 23:47.533 --> 23:48.673 It may do other things. 23:48.667 --> 23:52.597 Once it starts to fall, it may start to hit. 23:52.600 --> 23:54.800 It may kind of go back to this mechanism. 23:54.800 --> 23:57.430 You might start to hit some of these 23:57.433 --> 23:59.073 supercooled liquid droplets. 23:59.067 --> 24:02.027 And they will freeze on impact. 24:02.033 --> 24:07.203 In which case, this will grow further by riming, by having 24:07.200 --> 24:09.500 supercooled droplets hit, and stick, and 24:09.500 --> 24:11.470 freeze when they hit. 24:11.467 --> 24:16.667 Eventually, that could lead, for example, to a hail stone, 24:16.667 --> 24:18.697 where you first form a snowflake. 24:18.700 --> 24:22.670 And then as it falls out, you would collect by riming more 24:22.667 --> 24:24.597 of these supercooled water droplets. 24:24.600 --> 24:28.730 That would eventually produce a hail stone. 24:28.733 --> 24:33.173 Any questions on this ice phase mechanism? 24:33.167 --> 24:37.667 This is believed to be the most common mechanism for 24:37.667 --> 24:40.527 producing rain and snow around the world. 24:40.533 --> 24:44.803 In fact, the rain we had over the weekend was almost 24:44.800 --> 24:48.200 certainly of this type. 24:48.200 --> 24:50.430 So most of the rain you've experienced unless spent over 24:50.433 --> 24:52.933 the tropical oceans has probably been 24:52.933 --> 24:55.033 formed in this way. 24:55.033 --> 24:55.373 Question? 24:55.367 --> 24:55.697 Yes? 24:55.700 --> 24:59.170 STUDENT: Why does the lower saturation vapor pressure mean 24:59.167 --> 25:01.067 it attracts more water? 25:01.067 --> 25:04.597 PROFESSOR: So if you remember the definition of 25:04.600 --> 25:10.000 saturation vapor pressure, if I have a chamber with a 25:10.000 --> 25:16.900 condensed phase there and the vapor up here, saturation 25:16.900 --> 25:20.170 vapor pressure is a vapor pressure that will exist here 25:20.167 --> 25:24.897 when you're in equilibrium with the condensed vapor. 25:24.900 --> 25:32.570 So if I suddenly change this from water to say ice, the ice 25:32.567 --> 25:36.027 is more attractive to the vapor and will suck a little 25:36.033 --> 25:39.073 bit of that extra water vapor out dropping this until the 25:39.067 --> 25:41.127 new equilibrium is reached. 25:41.133 --> 25:45.233 So I haven't given you a say quantum mechanical explanation 25:45.233 --> 25:47.633 for why the vapor pressure over ice is lower. 25:47.633 --> 25:49.703 But I've explained what that means. 25:49.700 --> 25:52.130 I think that's the best I can do at this stage. 25:54.900 --> 25:56.770 This is a trickier mechanism. 25:56.767 --> 26:00.297 So I'd be happy to take questions on this if I haven't 26:00.300 --> 26:02.670 made this clear. 26:02.667 --> 26:05.227 It's called the ice phase mechanism for 26:05.233 --> 26:07.633 generating rain and snow. 26:07.633 --> 26:12.033 Sometimes it's called the Bergeron mechanism after the 26:12.033 --> 26:17.733 Norwegian scientist that developed the idea. 26:17.733 --> 26:24.633 OK, now with that, then let's talk about some different rain 26:24.633 --> 26:26.173 scenarios-- rain or snow scenarios. 26:28.700 --> 26:34.070 If you have a shallow cloud, it's warm at the surface. 26:34.067 --> 26:36.467 And you know somehow from a balloon sounding or an 26:36.467 --> 26:42.467 aircraft pattern that the top of that cloud is below the 26:42.467 --> 26:44.327 freezing level. 26:44.333 --> 26:46.833 That is the temperature is greater than zero degrees 26:46.833 --> 26:50.333 Celsius below, less above, but the cloud never 26:50.333 --> 26:51.703 reaches that height. 26:51.700 --> 26:52.930 And it's raining. 26:55.333 --> 26:59.203 A simple process of elimination tells you that 26:59.200 --> 27:01.070 this must be the mechanism. 27:01.067 --> 27:02.697 It must be the collision-coalescence 27:02.700 --> 27:05.230 mechanism, because there's no supercooled 27:05.233 --> 27:06.173 liquid water in this. 27:06.167 --> 27:08.867 All the liquid water in the cloud is at a temperature 27:08.867 --> 27:10.697 above zero degrees Celsius. 27:10.700 --> 27:14.300 So this mechanism is excluded. 27:14.300 --> 27:15.830 It must be a warm rain mechanism. 27:18.833 --> 27:19.833 Let's go on to the next phase. 27:19.833 --> 27:23.403 Let's say this cloud is taller now. 27:23.400 --> 27:25.730 And the zero degrees Celsius line is there, meaning it's 27:25.733 --> 27:29.873 colder above and warmer below. 27:29.867 --> 27:33.367 There's a good section of this cloud then that will have 27:33.367 --> 27:36.267 supercooled liquid water in it. 27:36.267 --> 27:38.727 Probably what's happening, if it's raining out the bottom of 27:38.733 --> 27:41.703 this cloud, it's probably because the ice phase 27:41.700 --> 27:44.630 mechanism is working, producing snowflakes up here. 27:44.633 --> 27:47.733 I've drawn my crude little snowflake. 27:47.733 --> 27:52.033 I made the cardinal sin of making it a five-pointed star 27:52.033 --> 27:54.773 when ice always has a six-fold symmetry. 27:54.767 --> 27:57.067 So I hope you can do better than that. 27:57.067 --> 27:58.467 You should put a Star of David there. 27:58.467 --> 28:00.767 That would at least have a six-fold symmetry. 28:03.333 --> 28:07.733 So snow is being produced via this mechanism. 28:07.733 --> 28:09.003 Then it's falling out. 28:09.000 --> 28:12.100 As soon as it falls and reaches the zero degrees 28:12.100 --> 28:18.270 Celsius line, it is going to melt and become a raindrop. 28:18.267 --> 28:20.927 And then it'll fall as rain all the way to the ground. 28:20.933 --> 28:24.233 So again, the rain we had this weekend was almost certainly 28:24.233 --> 28:27.573 of that type formed as snow higher in the atmosphere. 28:27.567 --> 28:30.497 And as it fell, it melted and became a raindrop. 28:30.500 --> 28:32.370 And we felt it as rain. 28:32.367 --> 28:34.967 That's the most common situation of all, by the way. 28:34.967 --> 28:39.027 This is extremely common, at least here in New Haven. 28:39.033 --> 28:42.773 In the winter time, if you had temperatures less than zero 28:42.767 --> 28:46.397 degrees Celsius everywhere, if it was cold at the surface, 28:46.400 --> 28:50.730 and of course, even colder aloft in the troposphere, you 28:50.733 --> 28:54.273 could have the ice phase process forming snowflakes. 28:54.267 --> 28:57.597 And then they would simply fall all the way to the ground 28:57.600 --> 28:59.470 as a snowflake. 28:59.467 --> 29:00.797 That's fine. 29:03.233 --> 29:06.033 Occasionally, you may have encountered situations, or you 29:06.033 --> 29:10.833 will this winter if you keep your eyes open, where the 29:10.833 --> 29:14.233 temperature at the surface is almost exactly zero Celsius, 29:14.233 --> 29:16.803 within two or three degrees of zero Celsius. 29:16.800 --> 29:21.670 So that melting level is right about at the surface. 29:21.667 --> 29:29.367 What you'll find then is that the snow that's falling is wet 29:29.367 --> 29:31.467 and sticky. 29:31.467 --> 29:35.297 Because it's actually begun its melting process in just 29:35.300 --> 29:39.370 the last few seconds as it's fallen to Earth. 29:39.367 --> 29:44.327 But that'd be the special case somewhere just between these 29:44.333 --> 29:48.603 two where the zero line-- zero degrees Celsius is just about 29:48.600 --> 29:51.970 at the surface of the Earth. 29:51.967 --> 29:53.927 Questions on these scenarios so far? 29:56.600 --> 29:57.030 OK. 29:57.033 --> 30:01.103 Now here's one that's a little less common but can be quite 30:01.100 --> 30:02.900 important when it happens. 30:02.900 --> 30:05.300 It's called an ice storm. 30:05.300 --> 30:07.770 And you have to imagine a temperature profile like I've 30:07.767 --> 30:08.697 drawn here. 30:08.700 --> 30:11.130 The vertical line for reference is the zero degrees 30:11.133 --> 30:13.703 Celsius line. 30:13.700 --> 30:19.130 So there's a zero line aloft but another zero line close to 30:19.133 --> 30:19.703 the surface. 30:19.700 --> 30:23.330 In other words, cold, warm, and cold 30:23.333 --> 30:24.533 again near the surface. 30:24.533 --> 30:25.573 So what's going to happen? 30:25.567 --> 30:29.097 Well, up here it's going to be just like this one. 30:29.100 --> 30:31.430 You're going to have the ice phase mechanism producing 30:31.433 --> 30:33.903 snowflakes. 30:33.900 --> 30:35.100 They're going to melt and become 30:35.100 --> 30:38.630 raindrops below that line. 30:38.633 --> 30:43.973 Now they are falling to Earth as a raindrop, a liquid drop. 30:43.967 --> 30:47.467 But then in the last few hundred meters before they hit 30:47.467 --> 30:52.967 the surface, they enter a cold layer of air. 30:52.967 --> 30:57.767 That air is going to cool down the droplet, probably even 30:57.767 --> 30:59.797 below the freezing mark. 30:59.800 --> 31:04.630 If that air temperature is say minus five Celsius, then it's 31:04.633 --> 31:06.833 going to cool down that droplet to about that same 31:06.833 --> 31:09.173 temperature, minus five Celsius. 31:09.167 --> 31:10.827 So now what do you have? 31:10.833 --> 31:12.803 The first time I mention this, but you now have 31:12.800 --> 31:15.900 a supercooled raindrop. 31:15.900 --> 31:18.770 Before I was talking about supercooled cloud droplets. 31:18.767 --> 31:20.097 Now I'm talking about a supercooled raindrop. 31:20.100 --> 31:22.900 It's a millimeter in size. 31:22.900 --> 31:25.230 But it's supercooled. 31:25.233 --> 31:28.533 That is going to freeze upon impact and coat 31:28.533 --> 31:30.733 everything with ice. 31:30.733 --> 31:33.873 The power lines are going to be drooping with the weight. 31:33.867 --> 31:36.197 The branches on the trees are going to be leaning and 31:36.200 --> 31:44.170 breaking over the weight of that ice freezing on impact as 31:44.167 --> 31:46.727 those supercooled raindrops hit the-- 31:46.733 --> 31:48.073 STUDENT: So they're raindrops? 31:48.067 --> 31:49.827 PROFESSOR: Supercooled raindrops. 31:49.833 --> 31:50.633 Right. 31:50.633 --> 31:53.633 This is called freezing rain or an ice storm. 31:53.633 --> 31:56.433 And the damage is caused normally-- 31:56.433 --> 31:58.373 well, of course, the roads are going to be dangerous too 31:58.367 --> 32:00.197 because the roads would be icy. 32:00.200 --> 32:04.130 But most of the damage is going to come, I think, from 32:04.133 --> 32:09.533 the weight of this on trees, and power lines, and so on, 32:09.533 --> 32:10.803 occasionally breaking them, making 32:10.800 --> 32:14.030 them come to the surface. 32:14.033 --> 32:17.833 But for that, you need this special behavior of the air 32:17.833 --> 32:19.603 temperature. 32:19.600 --> 32:24.470 Which around New England, you typically get once or twice 32:24.467 --> 32:29.527 each winter, you'll get an ice storm that has this particular 32:29.533 --> 32:32.333 behavior to it. 32:32.333 --> 32:33.803 Any questions on that? 32:38.567 --> 32:42.997 Well, I can probably then say a few 32:43.000 --> 32:47.930 words about cloud seeding. 32:47.933 --> 32:51.473 Cloud seeding is an attempt to get clouds that are not 32:51.467 --> 32:52.667 raining to rain. 32:52.667 --> 32:57.997 You can imagine the frustration of a farmer who 32:58.000 --> 33:02.400 finds his crops withering for lack of rain. 33:02.400 --> 33:06.670 And yet all these clouds are passing by overhead with lots 33:06.667 --> 33:07.627 of condensed water. 33:07.633 --> 33:11.303 But it's all in the cloud droplet form. 33:11.300 --> 33:13.230 Raindrops are not forming. 33:13.233 --> 33:18.473 He would give anything if he could just get this process-- 33:18.467 --> 33:21.197 one of these two processes to work, the 33:21.200 --> 33:23.030 collision-coalescence or the ice phase process. 33:25.533 --> 33:32.173 If the cloud has supercooled water, he might have a chance. 33:32.167 --> 33:33.397 And this is the way you do it. 33:33.400 --> 33:37.500 You inject into the cloud some freezing nuclei. 33:37.500 --> 33:41.900 You want to get a few of these droplets to freeze. 33:41.900 --> 33:46.330 And a good freezing nuclei would be a compound called 33:46.333 --> 33:48.273 silver iodide. 33:48.267 --> 33:51.197 AgI is the chemical formula for it. 33:51.200 --> 33:53.600 Because it has a crystal structure very much 33:53.600 --> 33:55.600 like that of ice. 33:55.600 --> 34:01.100 And so if a particle of silver iodide hits a supercooled 34:01.100 --> 34:04.400 cloud droplet, it is likely to cause it to freeze. 34:04.400 --> 34:07.100 And then this process could start. 34:07.100 --> 34:13.130 So there is a big industry today in cloud 34:13.133 --> 34:16.103 seeding of cold clouds. 34:16.100 --> 34:20.330 I don't know if I forgot to mention it, but a cloud which 34:20.333 --> 34:23.973 has no part of it at a temperature below freezing is 34:23.967 --> 34:27.827 often called a warm cloud. 34:27.833 --> 34:30.003 Well, that's not going to work for silver 34:30.000 --> 34:30.900 iodide cloud seeding. 34:30.900 --> 34:33.670 You need to have supercooled water in that 34:33.667 --> 34:34.767 cloud for it to work. 34:34.767 --> 34:37.767 It would have to be something like this. 34:37.767 --> 34:40.527 In that case, you inject the silver iodide either from the 34:40.533 --> 34:44.033 ground, or from an aircraft, or from a rocket. 34:44.033 --> 34:47.603 And you hope that you're putting in just 34:47.600 --> 34:48.870 enough and not too much. 34:48.867 --> 34:50.597 What happens if you put in too much? 34:50.600 --> 34:54.700 If you put in too much, you're going to freeze a large number 34:54.700 --> 34:57.370 of cloud droplets. 34:57.367 --> 35:00.767 And they're all going to be competing for the same water. 35:00.767 --> 35:02.427 So they're not going to grow. 35:02.433 --> 35:07.933 So on average, you have to freeze about one in a million. 35:07.933 --> 35:10.833 To make this most effective, you want to freeze about one 35:10.833 --> 35:13.573 in a million of the cloud droplets. 35:13.567 --> 35:16.267 Then you would get a million cloud droplets contributing to 35:16.267 --> 35:17.327 one snowflake. 35:17.333 --> 35:19.633 That would be ideal. 35:19.633 --> 35:25.303 This is a bit of a dirty subject though because there 35:25.300 --> 35:28.170 is a lot of people doing this, a lot of cash being handed 35:28.167 --> 35:31.467 around and hopeful farmers paying for this. 35:31.467 --> 35:34.927 But the scientific, and I should say statistical 35:34.933 --> 35:39.803 evidence that this works is not so clear. 35:39.800 --> 35:44.100 And the National Science Foundation has stopped funding 35:44.100 --> 35:48.930 most research on cloud seeding because the people who were 35:48.933 --> 35:51.733 doing that work were not maintaining the highest 35:51.733 --> 35:58.833 standards of statistical verification of their work. 35:58.833 --> 36:04.103 The problem is if you seed a cloud, and it rains, how do 36:04.100 --> 36:07.830 you know that it wasn't going to rain anyway? 36:07.833 --> 36:09.333 That's the big problem. 36:09.333 --> 36:12.103 So in order to do this properly, you would have to 36:12.100 --> 36:21.800 seed perhaps 1,000 clouds, half with a placebo. 36:21.800 --> 36:25.430 You do a blind test. Because if you allow the people that 36:25.433 --> 36:28.573 are doing the seeding to pick the clouds they're going to 36:28.567 --> 36:31.167 seed, they may pick the clouds that look most 36:31.167 --> 36:33.327 likely to rain anyway. 36:33.333 --> 36:35.573 So you really need to maintain very high standards of 36:35.567 --> 36:38.397 statistical verification on this. 36:38.400 --> 36:40.830 And in the early days, this was not done. 36:40.833 --> 36:43.103 The field is beginning to come back now with a little higher 36:43.100 --> 36:47.670 level of statistical verification and beginning to 36:47.667 --> 36:50.397 become acceptable again scientifically. 36:50.400 --> 36:53.430 But as I say, there's still a big industry. 36:53.433 --> 36:56.433 For example, every year, seeding all along the Sierra 36:56.433 --> 36:59.403 Nevada range in California is done, and the claim is that 36:59.400 --> 37:04.570 has been increasing the snow pack every year. 37:04.567 --> 37:11.097 Even in the tropics, there is an industry claiming that they 37:11.100 --> 37:14.170 can seed warm clouds. 37:14.167 --> 37:18.397 This is even less likely to be the case. 37:18.400 --> 37:22.430 First of all, silver iodide wouldn't work. 37:22.433 --> 37:23.273 You'd have to have something else-- in this case, you'd be 37:23.267 --> 37:25.867 trying to enhance this mechanism because it's the 37:25.867 --> 37:28.527 only one that could work for a warm cloud. 37:28.533 --> 37:34.133 And the way you would do that would be perhaps to drop some 37:34.133 --> 37:39.503 kind of hydroscopic nuclei into it, like a salt particle, 37:39.500 --> 37:40.570 for example. 37:40.567 --> 37:44.027 Salt likes to pick up water. 37:44.033 --> 37:47.273 And if you could drop some salt crystals in there, you 37:47.267 --> 37:49.667 might be able to produce some droplets that are a little bit 37:49.667 --> 37:51.797 larger than the others. 37:51.800 --> 37:53.100 And that would enhance the 37:53.100 --> 37:55.700 collision-coalescence mechanism. 37:55.700 --> 37:58.700 But as I say, this is even less further along as a 37:58.700 --> 38:02.770 scientific development than is the seeding of cold clouds. 38:02.767 --> 38:03.197 Question? 38:03.200 --> 38:03.600 Yes? 38:03.600 --> 38:05.030 STUDENT: Are there any environmental concerns, 38:05.033 --> 38:07.373 putting silver iodide in-- ? 38:07.367 --> 38:08.667 PROFESSOR: The question is are there are 38:08.667 --> 38:09.727 environmental concerns? 38:09.733 --> 38:12.373 Well, yeah, that's been discussed widely. 38:12.367 --> 38:16.567 Silver iodide, I think the saving thing here is that you 38:16.567 --> 38:18.127 don't want to put very much in anyway. 38:18.133 --> 38:21.103 So you only put a small amount of silver iodide in. 38:21.100 --> 38:23.130 And there has been though-- 38:23.133 --> 38:27.373 people have been able to find the remains of this in the 38:27.367 --> 38:28.627 environment. 38:28.633 --> 38:31.303 But as far as I know, silver iodide is not a particularly 38:31.300 --> 38:33.070 dangerous compound. 38:33.067 --> 38:34.297 But do a check for yourself. 38:34.300 --> 38:36.130 Just Google silver iodide. 38:36.133 --> 38:38.403 AgI is the chemical formula. 38:42.233 --> 38:43.933 And see if it has any-- as far as I know, except for recent 38:43.933 --> 38:46.133 literature, I haven't followed, there been no 38:46.133 --> 38:47.973 environmental problems detected for that. 38:47.967 --> 38:52.697 But that's a good question worth looking into. 38:52.700 --> 38:56.200 Other questions on rain? 38:56.200 --> 38:56.600 Yes? 38:56.600 --> 38:57.170 STUDENT: I guess the semi-legendary old belief that 38:57.167 --> 38:57.427 if you fired canons it would cause rain, does that have any 38:57.433 --> 38:58.673 basis in reality? 39:05.500 --> 39:06.570 PROFESSOR: Not that I know of. 39:06.567 --> 39:08.927 I've heard that as well. 39:08.933 --> 39:11.403 Firing cannons was hoping to trigger this. 39:11.400 --> 39:13.000 I don't think there's any scientific 39:13.000 --> 39:14.800 evidence that that works. 39:18.967 --> 39:21.267 Anything else on this? 39:21.267 --> 39:21.597 OK. 39:21.600 --> 39:24.930 So what are we looking for in terms of climatology? 39:24.933 --> 39:28.203 A given region of the Earth will have certain water vapor 39:28.200 --> 39:30.400 sources, nearby bodies of water. 39:30.400 --> 39:32.330 It will have certain characteristics that might 39:32.333 --> 39:34.873 make air rise. 39:34.867 --> 39:36.697 If you get the right conditions, you might get 39:36.700 --> 39:38.070 precipitation. 39:38.067 --> 39:40.427 But this will vary from place to place around the world. 39:40.433 --> 39:43.903 We'll talk about the global climatology of rainfall in 39:43.900 --> 39:44.730 just a few days. 39:44.733 --> 39:47.273 But there are some regions of the Earth 39:47.267 --> 39:50.027 where it never rains. 39:50.033 --> 39:51.033 Why would that be? 39:51.033 --> 39:54.073 Why would there be a place on Earth where it never rains? 39:54.067 --> 39:55.297 Anybody have an idea? 39:59.200 --> 40:01.170 STUDENT: If it's like right after a big mountain range, 40:01.167 --> 40:03.167 that like a cloud would have to lose all its 40:03.167 --> 40:04.767 water to pass over? 40:04.767 --> 40:07.697 PROFESSOR: Yeah, that would be possible. 40:07.700 --> 40:10.630 But I think, even in that scenario, I would offer a 40:10.633 --> 40:12.503 slightly different explanation. 40:12.500 --> 40:14.170 And that is if you have a mountain range, and the wind 40:14.167 --> 40:17.567 is always from one side towards the other, you'd have 40:17.567 --> 40:19.397 rising motion on one side which would give you 40:19.400 --> 40:22.500 precipitation, but then always sinking motion 40:22.500 --> 40:23.430 on the other side. 40:23.433 --> 40:26.873 So even if you hadn't rained out all the water, once you 40:26.867 --> 40:29.827 start the air descending, then you see you 40:29.833 --> 40:31.973 clear out the clouds. 40:31.967 --> 40:36.397 So I would like to take that option because I think it's 40:36.400 --> 40:37.430 more universal. 40:37.433 --> 40:40.903 Wherever you have on the globe, a place where the air 40:40.900 --> 40:45.430 is usually descending for whatever reason, you're 40:45.433 --> 40:49.773 unlikely to form either clouds or precipitation. 40:49.767 --> 40:52.227 Wherever you have regions where the air is often 40:52.233 --> 40:58.803 ascending, then you are much more likely to have clouds or 40:58.800 --> 40:59.800 precipitation. 40:59.800 --> 41:03.200 There are people that say, well, we could make the 41:03.200 --> 41:08.000 deserts bloom by just adding more water vapor. 41:08.000 --> 41:11.500 And that will not work, because I don't care how much 41:11.500 --> 41:13.170 water vapor you add to the atmosphere. 41:13.167 --> 41:16.427 If the air is always going to be descending, for example, in 41:16.433 --> 41:19.273 the Sahara Desert, what makes the Sahara Desert a desert is 41:19.267 --> 41:21.727 that the air there is usually descending. 41:21.733 --> 41:24.573 Adding water vapor, that's not going to change it. 41:24.567 --> 41:25.797 The air is still going to be descending. 41:25.800 --> 41:27.870 You're not going to make clouds. 41:27.867 --> 41:32.997 So precipitation climatology usually has to do with where's 41:33.000 --> 41:35.800 the air going up and where's the air going down. 41:35.800 --> 41:37.100 It's complicated in many ways. 41:37.100 --> 41:41.070 But in that sense, it's very, very simple, rising air versus 41:41.067 --> 41:41.627 descending air. 41:41.633 --> 41:48.503 Now a few numbers just to put this in your mind. 41:48.500 --> 41:51.530 We don't know exactly what part of the Earth gets the 41:51.533 --> 41:53.233 absolute most rainfall. 41:53.233 --> 41:58.473 But we suspect that there are places on Earth that get as 41:58.467 --> 42:03.367 much as say 10 meters of rain per year. 42:03.367 --> 42:06.397 I did a project in the Caribbean last spring. 42:06.400 --> 42:09.000 And at the top of a mountainous island down there, 42:09.000 --> 42:11.330 we had a rain gauge installed for several years. 42:11.333 --> 42:15.403 And at that point, we got six meters of rainfall per year, 42:15.400 --> 42:17.800 which is a lot. 42:17.800 --> 42:23.130 Here in New haven, the average annual rainfall is about 1.5 42:23.133 --> 42:28.433 meters, about that much rain per year on average. 42:28.433 --> 42:34.733 In order to do rain-fed agriculture, not irrigated 42:34.733 --> 42:38.773 agriculture, but rain-fed agriculture, you need about 20 42:38.767 --> 42:43.227 centimeters of rain per year, about that much rain per year. 42:43.233 --> 42:45.603 This will give you some idea what kind of numbers we are 42:45.600 --> 42:48.430 looking for when we try to understand rainfall around the 42:48.433 --> 42:51.073 globe, 20 centimeters about what you need to do. 42:51.067 --> 42:53.527 And it's marginal. 42:53.533 --> 42:56.933 But that will allow you to do a little bit of rain-fed 42:56.933 --> 42:59.433 agriculture. 42:59.433 --> 43:00.733 Questions on this? 43:04.833 --> 43:05.133 OK. 43:05.133 --> 43:09.403 We have just about enough time for me to mention the other 43:09.400 --> 43:13.500 side of this, and that is evaporation. 43:13.500 --> 43:17.870 If we now understand what makes it rain, then we have to 43:17.867 --> 43:21.427 understand how that water gets back into the atmosphere to 43:21.433 --> 43:24.933 balance the water budget of the atmosphere, so just a word 43:24.933 --> 43:26.133 about evaporation. 43:26.133 --> 43:31.803 For the most part, the rate of evaporation, well, it may 43:31.800 --> 43:35.470 depend on many things, like wind speed over the surface 43:35.467 --> 43:40.227 and the humidity of the air. 43:40.233 --> 43:42.433 But the most important thing it depends on is the 43:42.433 --> 43:45.833 availability of heat. 43:45.833 --> 43:48.133 If you don't have enough heat available, you're not going to 43:48.133 --> 43:49.373 be able to evaporate water. 43:49.367 --> 43:50.227 Why is that? 43:50.233 --> 43:53.233 Because remember, the latent heat of condensation 43:53.233 --> 43:58.073 evaporation is a very large number, more than a million 43:58.067 --> 44:03.667 joules of heat required to evaporate 44:03.667 --> 44:07.827 every kilogram of water. 44:07.833 --> 44:13.933 So if you tried to evaporate water without plenty of heat 44:13.933 --> 44:17.433 available, you'll quickly cool that water down and the 44:17.433 --> 44:19.103 evaporation will cease. 44:19.100 --> 44:23.570 In order to sustain hour after hour evaporation, you've got 44:23.567 --> 44:24.467 to have a supply of heat. 44:24.467 --> 44:27.297 And that largely depends on the air temperature. 44:27.300 --> 44:31.170 So of all the possible controls that might be going 44:31.167 --> 44:33.467 on with evaporation, air temperature is the most 44:33.467 --> 44:35.867 important one. 44:35.867 --> 44:41.467 I've put together a crude empirical formula. 44:41.467 --> 44:45.697 I wouldn't use this if I had to do a very accurate 44:45.700 --> 44:46.430 calculation. 44:46.433 --> 44:50.873 But I use it all the time for quick back-of-the-envelope 44:50.867 --> 44:53.197 estimates of how much evaporation 44:53.200 --> 44:54.430 is likely to occur. 44:54.433 --> 44:55.473 And here's the formula. 44:55.467 --> 44:59.797 Now PET means potential evapotranspiration. 45:03.033 --> 45:06.633 Evapotranspiration combines the words evaporation and 45:06.633 --> 45:11.003 transpiration, which means it includes both evaporation from 45:11.000 --> 45:12.830 water surfaces and 45:12.833 --> 45:16.573 transpiration from leaf surfaces. 45:16.567 --> 45:20.927 Leaves are very effective evaporating agents. 45:20.933 --> 45:24.103 Trees bring water up from the ground in their roots. 45:24.100 --> 45:28.330 And then in the leaves, there are small pores that allow 45:28.333 --> 45:31.033 that water to escape into the atmosphere. 45:31.033 --> 45:34.573 So we combine that, evaporation and transpiration, 45:34.567 --> 45:35.967 and call it ET. 45:35.967 --> 45:38.327 But now this is the potential evapotranspiration. 45:38.333 --> 45:41.173 Because I want to remind you that if you don't have water 45:41.167 --> 45:44.967 there to begin with, you can't evaporate it. 45:44.967 --> 45:47.567 So potential evapotranspiration is the 45:47.567 --> 45:51.597 evaporation rate you will have if there is water present. 45:55.933 --> 45:56.073 If there is water present. 45:56.067 --> 46:01.797 And here's the formula, 5.7, a constant that I've developed, 46:01.800 --> 46:05.800 times the temperature expressed in degrees Celsius, 46:05.800 --> 46:08.470 if you want millimeters per month. 46:08.467 --> 46:11.397 If you want millimeters per day, just divide that by 30. 46:11.400 --> 46:16.500 And you get 0.17 millimeters per day per degrees Celsius 46:16.500 --> 46:19.830 multiplied by the temperature in Celsius. 46:19.833 --> 46:20.803 Let's do a quick example. 46:20.800 --> 46:26.800 Let's say the average temperature today is going to 46:26.800 --> 46:30.330 be 20 degrees Celsius. 46:30.333 --> 46:36.703 So 20 degrees Celsius there, multiply that times 20. 46:36.700 --> 46:37.800 And what do you get? 46:37.800 --> 46:44.900 You get about four millimeters of evaporation. 46:44.900 --> 46:47.670 So my prediction today if the average temperature is 20 46:47.667 --> 46:50.597 degrees, that you would get four millimeters of 46:50.600 --> 46:54.430 evaporation from the New Haven region. 46:54.433 --> 46:57.603 Part of it would come from puddles of water, part of it 46:57.600 --> 47:00.470 would come from leaves, part from grass. 47:00.467 --> 47:03.397 But on average with a temperature of 20 degrees 47:03.400 --> 47:08.000 Celsius, you get about two millimeters of evaporated 47:08.000 --> 47:12.770 water, water going from liquid into vapor during the day. 47:12.767 --> 47:15.327 Are there any questions on that? 47:15.333 --> 47:15.833 Yeah? 47:15.833 --> 47:19.773 STUDENT: Does that considered the relative humidity of-- 47:19.767 --> 47:20.527 PROFESSOR: No. 47:20.533 --> 47:26.233 So I've neglected a few effects that are rather 47:26.233 --> 47:29.133 important as well, such as relative humidity. 47:29.133 --> 47:32.933 If the air is drier, it'll evaporate more quickly. 47:32.933 --> 47:38.403 If the wind is blowing strong, it'll evaporate more quickly. 47:38.400 --> 47:40.170 So I have neglected that. 47:40.167 --> 47:44.597 And you should make a note about that, that this is not a 47:44.600 --> 47:48.530 very accurate method for doing it because it's ignored such 47:48.533 --> 47:50.373 factors as the one that was just mentioned. 47:53.233 --> 47:54.773 But I often use it-- when I know what the precipitation is 47:54.767 --> 47:59.127 for some part of the world, and I want to know if that's 47:59.133 --> 48:02.533 matched by evaporation, I'll do a quick estimate of this, 48:02.533 --> 48:04.073 compare with what I know about that. 48:04.067 --> 48:07.497 And that's usually the most important aridity index. 48:07.500 --> 48:10.370 To know how much it rains is not sufficient. 48:10.367 --> 48:13.527 To know whether a climate is going to be wet or dry, you 48:13.533 --> 48:16.133 need to compare the precipitation with the 48:16.133 --> 48:18.803 potential evapotranspiration. 48:18.800 --> 48:22.000 It's only in that comparison that you get 48:22.000 --> 48:22.700 a reasonable number. 48:22.700 --> 48:27.070 For example, in the high latitudes, up in Northern 48:27.067 --> 48:31.127 Canada, for example, it rains very little. 48:31.133 --> 48:33.103 But yet, it's a very wet climate-- 48:33.100 --> 48:36.930 mud, puddles of water everywhere. 48:36.933 --> 48:38.533 How can that be if it rains so little? 48:38.533 --> 48:41.833 Well, it's cold, and therefore, the 48:41.833 --> 48:43.433 evaporation is even less. 48:43.433 --> 48:48.733 So a wet climate is one that has more precipitation than 48:48.733 --> 48:49.903 evapotranspiration. 48:49.900 --> 48:52.070 A dry climate is the reverse of that. 48:52.067 --> 48:54.967 You always must be comparing the two when you're deciding 48:54.967 --> 48:57.667 whether a climate is wet or dry. 48:57.667 --> 48:58.627 We're out of time. 48:58.633 --> 49:00.703 We'll continue this on Wednesday.