WEBVTT 00:01.000 --> 00:02.670 RONALD SMITH: Well, I'm talking about storms, but 00:02.667 --> 00:06.297 someone asked me a question the other day about why the 00:06.300 --> 00:08.930 Coriolis force is opposite in the two hemispheres. 00:08.933 --> 00:12.233 And I can't give you a complete explanation, but I 00:12.233 --> 00:16.773 can give you a partial explanation. 00:16.767 --> 00:19.927 Here's the globe, and of course, it spins toward the 00:19.933 --> 00:23.673 east. The sun rises in the east, sets in the west. So 00:23.667 --> 00:26.067 that's the way the Earth spins. 00:26.067 --> 00:29.497 If I keep spinning it in that direction and I face this 00:29.500 --> 00:32.730 towards you, is that clockwise or counterclockwise? 00:35.300 --> 00:36.600 Counterclockwise. 00:36.600 --> 00:40.600 I'm going to keep spinning it in that direction. 00:40.600 --> 00:43.800 Is that clockwise or counterclockwise? 00:43.800 --> 00:45.070 Clockwise. 00:45.067 --> 00:52.597 So in a sense, while the globe spins eastward, the sense of 00:52.600 --> 00:57.030 rotation is opposite in the two hemispheres. 00:57.033 --> 01:00.603 And that is one way to understand why the Coriolis 01:00.600 --> 01:03.970 force is opposite in the two hemispheres. 01:03.967 --> 01:07.327 So like I said, not a complete explanation, but it's true, 01:07.333 --> 01:12.373 and I think it helps to understand that tricky point 01:12.367 --> 01:15.167 about the Coriolis force. 01:15.167 --> 01:16.427 Any questions on that? 01:20.133 --> 01:23.633 So we're going through storms today. 01:23.633 --> 01:27.173 This is not very quantitative material. 01:27.167 --> 01:29.967 This stuff will be posted after the fact, and your 01:29.967 --> 01:31.167 textbook is very good. 01:31.167 --> 01:33.797 These chapters that I've assigned, a number of them 01:33.800 --> 01:38.300 have to do with storm types and so on. 01:38.300 --> 01:39.470 So you'll find a good-- of course, I will explain things 01:39.467 --> 01:42.597 somewhat differently, but you'll find a good way to get 01:42.600 --> 01:45.970 competing views of this by listening to what I say and 01:45.967 --> 01:48.867 then reading the textbook, and between the two, you should 01:48.867 --> 01:52.997 come out pretty well on this subject. 01:53.000 --> 02:04.170 So we were talking about convective storms. And that is 02:04.167 --> 02:08.367 the class of storms that get their energy from the release 02:08.367 --> 02:09.827 of latent heat. 02:09.833 --> 02:11.733 I mentioned this last time. 02:11.733 --> 02:13.603 That's a number that I carry around in my head. 02:13.600 --> 02:17.170 It's rounded off a little bit, but I use that number so often 02:17.167 --> 02:18.427 that I carry it around in my head. 02:18.433 --> 02:21.673 The latent heat of condensation, or if you like, 02:21.667 --> 02:23.527 the latent heat of evaporation-- 02:23.533 --> 02:27.403 it's the same magnitude, independent of whether you're 02:27.400 --> 02:30.800 condensing or evaporating water-- is about 2 and 1/2 02:30.800 --> 02:33.600 million joules per kilogram. 02:33.600 --> 02:37.830 And so when you condense water, you're then providing 02:37.833 --> 02:42.373 extra heat to the air, which makes it warmer, more buoyant. 02:42.367 --> 02:48.027 It's likely to rise then, and that's what's going on in all 02:48.033 --> 02:50.403 of these storms. It's the basic 02:50.400 --> 02:52.170 underlying energy source. 02:52.167 --> 02:54.297 We talked about air-mass thunderstorms last time and 02:54.300 --> 02:57.500 how they go through a life cycle. 02:57.500 --> 02:59.000 And after-- 02:59.000 --> 03:03.970 they take about 20 minutes to 30 minutes to develop from a 03:03.967 --> 03:08.027 fair-weather cumulus into a cumulonimbus with rain coming 03:08.033 --> 03:09.403 out the bottom. 03:09.400 --> 03:13.330 And then as the rain begins, you begin to get some 03:13.333 --> 03:16.403 evaporation of falling rainwater 03:16.400 --> 03:18.530 just below cloud base. 03:18.533 --> 03:23.333 That produces cool air, which then sinks and spreads out. 03:23.333 --> 03:27.533 It's like you open your freezer door, and that cold 03:27.533 --> 03:30.573 air from the freezer falls down to the kitchen floor and 03:30.567 --> 03:32.327 then spreads out. 03:32.333 --> 03:35.803 That's what's happening, and that goes by various names. 03:35.800 --> 03:39.930 Sometimes we call it a downburst. Sometimes it's 03:39.933 --> 03:42.003 called a gust front. 03:42.000 --> 03:43.500 The front of it is called the gust front. 03:43.500 --> 03:46.570 But anyway, the air is now spreading out away from the 03:46.567 --> 03:51.427 cloud, and that shuts off the warm, moist inflow. 03:51.433 --> 03:54.703 So it kills itself off, basically. 03:54.700 --> 03:56.000 It has a-- after about two hours, it's gone. 03:56.000 --> 04:00.600 Now, that outflow might well trigger another thunderstorm. 04:00.600 --> 04:02.900 So that doesn't mean, for example, over the state of 04:02.900 --> 04:05.800 Connecticut on a day when there are thunderstorms, it 04:05.800 --> 04:09.130 doesn't mean there's just going to be one and then none. 04:09.133 --> 04:11.373 It means there'll be one, and then as it dies, it may 04:11.367 --> 04:13.567 trigger two others, and then as they die, 04:13.567 --> 04:15.467 they may trigger others. 04:15.467 --> 04:18.697 So this may go on throughout the afternoon, but the 04:18.700 --> 04:23.670 individual cells have a short lifetime. 04:23.667 --> 04:25.297 That's an important characteristic. 04:25.300 --> 04:31.170 As opposed to these severe storms, which have the 04:31.167 --> 04:37.127 additional environmental factor of having wind shear 04:37.133 --> 04:41.333 through the troposphere, usually a jet stream aloft. 04:41.333 --> 04:43.733 Strong winds near the tropopause, weak winds near 04:43.733 --> 04:47.573 the surface of the Earth, that tends to bend the storm over 04:47.567 --> 04:53.067 and change its structure into one in which the inflow is at 04:53.067 --> 04:56.267 a different location than the outflow caused by that 04:56.267 --> 04:57.467 evaporative cooling. 04:57.467 --> 05:01.327 In fact, the cold air generated by evaporative 05:01.333 --> 05:05.333 cooling spreads out and actually forms a gust front 05:05.333 --> 05:07.933 over which the warm air then lifts. 05:07.933 --> 05:11.503 So it actually assists in the lifting of the warm air up 05:11.500 --> 05:12.430 into the cloud. 05:12.433 --> 05:16.473 And this structure is more or less stable and can persist 05:16.467 --> 05:18.497 for a number of hours and in some cases 05:18.500 --> 05:21.130 even for even overnight. 05:21.133 --> 05:23.073 And these are the ones that have-- 05:23.067 --> 05:25.427 they're larger and they're severe in 05:25.433 --> 05:27.073 every way you can imagine. 05:27.067 --> 05:29.827 I think I showed these last time. 05:29.833 --> 05:31.973 But heavy rains and flooding, hail, 05:31.967 --> 05:34.367 lightning, tornadic winds. 05:34.367 --> 05:38.267 I don't think the low pressure in the tornado is much of an 05:38.267 --> 05:41.667 issue, but the gust-front winds coming from that cool 05:41.667 --> 05:45.897 air is another source of wind damage. 05:45.900 --> 05:49.200 In many-- in some cases, that causes more damage than even a 05:49.200 --> 05:50.170 tornado could. 05:50.167 --> 05:53.767 Usually, the tornadic winds are stronger when there is a 05:53.767 --> 05:58.667 tornado, but the gust-front winds may cover a wider area 05:58.667 --> 06:00.667 and in some cases will cause more damage 06:00.667 --> 06:03.567 than a possible tornado. 06:06.833 --> 06:09.733 I don't know if I got this far, but here's 06:09.733 --> 06:10.573 a picture of one. 06:10.567 --> 06:13.427 And earlier in the course, we talked about the visual 06:13.433 --> 06:17.173 appearance of a funnel. 06:17.167 --> 06:19.367 And this is called the condensation funnel. 06:19.367 --> 06:26.967 That is basically a cloud composed of tiny liquid water 06:26.967 --> 06:33.997 droplets that's produced when air from outside moves in and 06:34.000 --> 06:35.030 drops its pressure. 06:35.033 --> 06:40.073 The pressure inside a tornado is much lower than in the 06:40.067 --> 06:42.597 surrounding atmosphere, so that air that moves in there 06:42.600 --> 06:46.530 is going to expand, not by moving vertically, but by 06:46.533 --> 06:48.673 moving into the tornado. 06:48.667 --> 06:51.897 It'll expand, cool, and you can form a cloud. 06:51.900 --> 06:52.830 And that's what you're seeing. 06:52.833 --> 06:55.803 In fact, you often see this descending from cloud base. 06:55.800 --> 06:59.170 The cloud base will appear to be flat, and then as the 06:59.167 --> 07:01.867 tornado develops, that funnel cloud will 07:01.867 --> 07:03.397 descend to the ground. 07:03.400 --> 07:06.630 When you see that, it's more natural to understand that 07:06.633 --> 07:09.233 it's really like a cloud, because it's actually an 07:09.233 --> 07:12.833 extension of that cloud reaching down to the surface. 07:12.833 --> 07:15.203 Whereas this other thing here is the debris funnel. 07:15.200 --> 07:19.400 That's stuff that's been kicked up by the high winds at 07:19.400 --> 07:21.370 the surface, and that's not water. 07:21.367 --> 07:26.067 That's usually dirt or soil or cows or cars or houses and 07:26.067 --> 07:27.297 things like that. 07:29.433 --> 07:33.003 And here again, you see the condensation funnel, and you 07:33.000 --> 07:35.970 see a debris funnel around that. 07:39.300 --> 07:41.030 Lightning comes along with that. 07:41.033 --> 07:45.673 And of course, these sparks of electricity is what you hear 07:45.667 --> 07:46.897 as thunder. 07:49.300 --> 07:51.630 And I think you know that-- how many know how to compute 07:51.633 --> 07:53.633 in your mind how far a storm is? 07:53.633 --> 07:56.133 Do you know that trick? 07:56.133 --> 07:59.133 It's pretty simple, because you assume that the light that 07:59.133 --> 08:04.233 comes from the lightning strike travels to your eye at 08:04.233 --> 08:07.003 the speed of light, which is, you might say, 08:07.000 --> 08:09.670 instantaneous, right? 08:09.667 --> 08:15.597 Whereas the sound that reaches your ear moves at the speed of 08:15.600 --> 08:19.600 sound, which is something like 300 meters per second. 08:19.600 --> 08:23.730 And so if you simply-- when you see a lightning flash, if 08:23.733 --> 08:27.373 you then count out the number of seconds-- 08:27.367 --> 08:29.097 one second, two seconds-- 08:29.100 --> 08:33.530 so for every second delay between the two, that's 300 08:33.533 --> 08:34.573 meters of distance. 08:34.567 --> 08:38.767 So if it was three seconds delayed, that would be 900 08:38.767 --> 08:41.397 meters or about a kilometer away from you. 08:41.400 --> 08:45.030 So that's an easy calculation to do, realizing that the 08:45.033 --> 08:48.903 thunder is produced by the lightning itself. 08:48.900 --> 08:52.830 You suddenly heat that air with the electrical discharge, 08:52.833 --> 08:57.473 it expands suddenly, produces an acoustic signal that then 08:57.467 --> 09:03.667 travels to your ear, and you recognize that as thunder. 09:03.667 --> 09:04.897 Questions on that? 09:07.800 --> 09:12.570 Hailstones, because there's a lot of supercooled water above 09:12.567 --> 09:16.267 the freezing line in those storms and very strong updraft 09:16.267 --> 09:19.527 motions that can suspend particles while 09:19.533 --> 09:22.373 they grow by riming. 09:22.367 --> 09:26.197 And eventually, when either that updraft weakens or the 09:26.200 --> 09:28.400 object becomes too large, it'll then 09:28.400 --> 09:29.570 fall out of the sky. 09:29.567 --> 09:32.667 And it's too large to melt. 09:32.667 --> 09:36.027 I told you many times that a snowflake will melt as it 09:36.033 --> 09:37.903 passes down through the freezing level 09:37.900 --> 09:39.300 and become a raindrop. 09:39.300 --> 09:43.530 But if I've got a chunk of ice that's five centimeters across 09:43.533 --> 09:45.903 and it falls down, it's too big. 09:45.900 --> 09:48.730 It's going to take minutes to melt. 09:48.733 --> 09:51.373 And by that time, it'll be at the surface. 09:51.367 --> 09:55.827 It'll be wet, it'll have a wet outer surface, but it'll still 09:55.833 --> 09:59.773 be a solid chunk of ice when it falls to Earth and hits. 10:05.467 --> 10:08.967 This is thunderstorm frequency across the United States, 10:08.967 --> 10:12.127 number of thunderstorms per year. 10:12.133 --> 10:13.673 Now, I've told you that these are called-- 10:13.667 --> 10:16.397 most of these are just air-mass thunderstorms. 10:16.400 --> 10:18.870 They're not so damaging. 10:18.867 --> 10:24.067 And it looks like Florida is the capital for air-mass 10:24.067 --> 10:28.427 thunderstorms. You get 100 of them per year. 10:28.433 --> 10:31.803 Other areas get far less. 10:31.800 --> 10:33.270 But these aren't the severe ones. 10:33.267 --> 10:35.667 I'll show you the severe ones in just a moment. 10:35.667 --> 10:37.827 You don't get many thunderstorms in the West 10:37.833 --> 10:43.473 Coast, because there's too strong an inversion there. 10:43.467 --> 10:46.527 Theres a temp-- normally, with the cold water off the coast 10:46.533 --> 10:49.633 from the California Current, the atmosphere over here is 10:49.633 --> 10:54.573 just too stable, too many inversions to get 10:54.567 --> 10:57.697 thunderstorms forming in that part of the world. 10:57.700 --> 11:00.130 So it really is kind of an East Coast phenomenon. 11:00.133 --> 11:03.003 But it starts right at the Rocky Mountain Front here in 11:03.000 --> 11:07.270 Colorado, and you find storms then pretty much all the way 11:07.267 --> 11:09.727 to the east, including all the way up into Canada you get 11:09.733 --> 11:12.473 quite a few. 11:12.467 --> 11:17.427 Now, I don't often in this course talk about the research 11:17.433 --> 11:21.173 that goes on here at Yale or in my group, but I wanted to 11:21.167 --> 11:22.927 show you one thing. 11:22.933 --> 11:26.233 I had a Ph.D. Student who finished a couple of years ago 11:26.233 --> 11:30.403 who tried to understand the timing of thunderstorms from 11:30.400 --> 11:32.100 west to east across-- 11:32.100 --> 11:34.830 in a region generally like this. 11:34.833 --> 11:38.603 And so she looked at a lot of climate-- 11:38.600 --> 11:44.200 of rainfall data from the summer months and came up with 11:44.200 --> 11:45.230 a diagram like this. 11:45.233 --> 11:46.333 Now, I'm going to explain this. 11:46.333 --> 11:48.573 It's going to take a minute just to figure out what in the 11:48.567 --> 11:49.867 heck is plotted here. 11:49.867 --> 11:52.297 But on this axis is longitude. 11:52.300 --> 11:57.270 So it's distance east-west basically through a region 11:57.267 --> 11:58.197 something like this. 11:58.200 --> 12:01.230 So it's the distance east-west in a latitude belt 12:01.233 --> 12:03.403 something like that. 12:03.400 --> 12:07.370 And on this axis is the local solar time. 12:07.367 --> 12:10.867 So instead of actually using time zones, which are kind of 12:10.867 --> 12:12.497 an artificial construct-- 12:12.500 --> 12:15.530 they make arbitrary choices about where to switch from 12:15.533 --> 12:18.203 Eastern Time to Central Time to Mountain Time-- 12:18.200 --> 12:19.430 this is just local time. 12:19.433 --> 12:23.003 Basically, how high is the sun in the sky, that determines 12:23.000 --> 12:25.570 what this clock would be. 12:25.567 --> 12:29.397 And then contoured is the precipitation rate, the hourly 12:29.400 --> 12:30.970 precipitation rate. 12:30.967 --> 12:34.567 So from this diagram, for any given latitude, if you scan 12:34.567 --> 12:37.227 up, you'll find how much it rains at different 12:37.233 --> 12:38.173 hours of the day. 12:38.167 --> 12:40.267 Now, we did repeat. 12:40.267 --> 12:41.927 We plotted everything twice. 12:41.933 --> 12:45.373 So you've got zero to 24 and then 24 to 48. 12:45.367 --> 12:48.367 But if you notice, the plot just repeats as well. 12:48.367 --> 12:51.467 So we've just put the same data on there twice, which 12:51.467 --> 12:53.627 makes it a little bit easier to go through 12:53.633 --> 12:55.833 a full day's cycle. 12:55.833 --> 12:59.403 You can start at any hour you want and go 24 hours ahead. 12:59.400 --> 13:01.570 So this is the Rocky Mountain Front. 13:01.567 --> 13:06.197 This is where a lot of thunderstorms form. 13:06.200 --> 13:08.170 It's over in here. 13:08.167 --> 13:14.497 And those storms tend to form between 1200 and 1800. 13:14.500 --> 13:17.530 So that's in the afternoon, local solar time. 13:17.533 --> 13:18.933 Now, that's not a surprise, right? 13:18.933 --> 13:21.503 Because one of the most important triggers for 13:21.500 --> 13:26.370 thunderstorms is the heating of the Earth by the Sun. 13:26.367 --> 13:30.467 So it's in the afternoon when you've built up this deep 13:30.467 --> 13:32.867 convective boundary layer by heating the 13:32.867 --> 13:34.067 surface of the Earth. 13:34.067 --> 13:37.367 And finally, that heating is going to generate cumulus 13:37.367 --> 13:40.027 clouds, and they'll then grow to form cumulonimbus. 13:40.033 --> 13:42.573 So it's not surprising that you find 13:42.567 --> 13:43.967 storms beginning there. 13:43.967 --> 13:46.927 And over the East Coast-- here in Connecticut, for example-- 13:46.933 --> 13:53.673 you also find that the storms form in the afternoon, due to 13:53.667 --> 13:55.267 the heating of the Earth by the Sun. 13:55.267 --> 14:00.697 But look what happens in this middle territory, which is 14:00.700 --> 14:04.800 basically this very large region through here. 14:04.800 --> 14:07.370 That has some other kind of a trigger, because, look, that 14:07.367 --> 14:09.367 is advancing in time. 14:09.367 --> 14:13.997 As I move eastward, the time of day when thunderstorms 14:14.000 --> 14:18.930 occur is 6 o'clock, 9 o'clock in the evening, midnight, 3 14:18.933 --> 14:21.633 o'clock in the morning, 6 o'clock the next 14:21.633 --> 14:22.573 morning, and so on. 14:22.567 --> 14:25.767 So you've got something that is departing from the simple 14:25.767 --> 14:29.467 idea of the Sun as the driving force. 14:29.467 --> 14:33.097 What this turns out to be is a disturbance that's generated 14:33.100 --> 14:36.600 each day over the Rocky Mountains and then is-- 14:36.600 --> 14:38.470 whoops, wrong direction-- 14:38.467 --> 14:42.927 and then is pushed eastwards by the westerly winds. 14:42.933 --> 14:46.103 And it's a warm air disturbance 14:46.100 --> 14:47.370 generated over the mountains. 14:47.367 --> 14:50.997 And as it drifts eastward aloft, it triggers 14:51.000 --> 14:54.230 thunderstorms as it moves along. 14:54.233 --> 14:56.803 I had somebody in my office a couple of weeks ago, a 14:56.800 --> 15:01.430 scientist who grew up in one of these states and always 15:01.433 --> 15:05.203 wondered why there they had a maximum thunderstorm frequency 15:05.200 --> 15:06.070 in the middle of the night. 15:06.067 --> 15:07.097 Who would expect that? 15:07.100 --> 15:10.700 It doesn't seem to fit with the standard thermal solar 15:10.700 --> 15:11.470 forcing idea. 15:11.467 --> 15:12.327 Well, this is why. 15:12.333 --> 15:15.073 In this case, my student discovered that it's a 15:15.067 --> 15:17.927 disturbance generated over the Rockies that moves eastward 15:17.933 --> 15:21.873 and gives that progressive timing to the thunderstorms. 15:21.867 --> 15:25.227 Then you get east of the Appalachian Mountains, and you 15:25.233 --> 15:28.773 get back to a more normal mid-afternoon 15:28.767 --> 15:30.967 thunderstorm maxima. 15:30.967 --> 15:32.667 Any questions on that? 15:32.667 --> 15:34.897 STUDENT: What are those dotted lines? 15:34.900 --> 15:36.800 PROFESSOR: I think they're reference lines 15:36.800 --> 15:41.100 for some other figure in the paper that's not shown here. 15:41.100 --> 15:44.000 So there are sloping lines that are drawn in here at 15:44.000 --> 15:45.100 different speeds. 15:45.100 --> 15:49.800 Since this is distance versus time, a slope represents a 15:49.800 --> 15:51.500 propagation speed-- 15:51.500 --> 15:54.500 30 meters per second, 14 meters per second, seven 15:54.500 --> 15:55.470 meters per second. 15:55.467 --> 15:58.927 These disturbances move eastward at about 14 meters 15:58.933 --> 16:03.103 per second, bringing those thunderstorms progressively 16:03.100 --> 16:04.100 later as you move 16:04.100 --> 16:05.970 towards the east. Yes. 16:05.967 --> 16:09.467 STUDENT: Was it already known that there was a warm weather 16:09.467 --> 16:13.127 air disturbance over the Rockies, or did 16:13.133 --> 16:13.703 she figure that out? 16:13.700 --> 16:15.970 PROFESSOR: No, that's not shown in this 16:15.967 --> 16:17.267 figure, but that was actually-- 16:17.267 --> 16:21.267 there was some hint about this progression already from radar 16:21.267 --> 16:24.667 data, but she confirmed it using surface data. 16:24.667 --> 16:27.667 And then she discovered this warm air pulse aloft that was 16:27.667 --> 16:29.497 doing the triggering. 16:29.500 --> 16:32.830 That was the point of her thesis, yeah. 16:32.833 --> 16:34.403 Anything else on that? 16:37.733 --> 16:40.703 So now let's get to the severe thunderstorms. And the way 16:40.700 --> 16:43.570 I'll get at that is just to show you this tornado 16:43.567 --> 16:46.667 frequency map. 16:46.667 --> 16:48.097 And of course, it looks a bit different. 16:48.100 --> 16:52.070 There is a maximum in Florida, but it's smaller. 16:52.067 --> 16:55.727 And the big maximum is over Texas, 16:55.733 --> 16:58.303 Oklahoma, Kansas, Nebraska. 16:58.300 --> 17:01.800 And of course, this is what's called Tornado 17:01.800 --> 17:03.170 Alley by some people. 17:03.167 --> 17:04.467 You've heard that expression. 17:04.467 --> 17:05.227 It's kind of the region. 17:05.233 --> 17:08.033 But what's special about this region apparently-- and by the 17:08.033 --> 17:11.433 way, this maximizes in the spring. 17:11.433 --> 17:16.803 So typically, this is an April-May phenomenon. 17:16.800 --> 17:20.100 If you have watched any of these shows about chasing 17:20.100 --> 17:23.770 tornadoes and if you want to do that, then schedule your 17:23.767 --> 17:27.697 trip in April and May, because that's when things get 17:27.700 --> 17:29.730 exciting out there. 17:29.733 --> 17:33.473 And what's special about this region is that you get some 17:33.467 --> 17:36.697 warm, moist air moving in from the Gulf of Mexico at low 17:36.700 --> 17:40.800 levels, and then you get the jet stream coming across the 17:40.800 --> 17:42.730 Rocky Mountains at high levels. 17:42.733 --> 17:46.303 And that gives you the shear that you need. 17:46.300 --> 17:48.800 And you've got the moisture, because you've got the high 17:48.800 --> 17:51.430 temperatures and high humidities in the low levels. 17:51.433 --> 17:54.273 So all the ingredients are kind of there. 17:54.267 --> 17:57.067 Not every day-- it fluctuates from day to day-- but 17:57.067 --> 18:00.397 generally, that's the reason why this area is so special. 18:00.400 --> 18:04.730 And as far as I know, there's no other place on Earth that 18:04.733 --> 18:08.103 has as many-- as high a tornado frequency as this 18:08.100 --> 18:08.700 region does. 18:08.700 --> 18:11.630 So this is special even if you look globally. 18:11.633 --> 18:15.403 There are a few other places in Asia and in Australia where 18:15.400 --> 18:18.700 they get this phenomenon, but this is kind of the most 18:18.700 --> 18:23.470 frequent place for it to occur. 18:23.467 --> 18:25.967 Questions there? 18:25.967 --> 18:28.367 Anybody from Tornado Alley? 18:28.367 --> 18:30.767 Anybody live in these states? 18:30.767 --> 18:32.627 Not at all? 18:32.633 --> 18:35.803 You're all East Coast or West Coast people? 18:35.800 --> 18:36.270 Yeah. 18:36.267 --> 18:38.397 STUDENT: The little spots in the Southeast, is there-- is 18:38.400 --> 18:39.630 that a coincidence? 18:41.933 --> 18:43.803 PROFESSOR: Yeah, that's a good question. 18:43.800 --> 18:47.430 For example, I read a paper once about this one, who tried 18:47.433 --> 18:51.303 to claim there was something special about the terrain in 18:51.300 --> 18:52.130 that region. 18:52.133 --> 18:54.603 But to be honest with you, I would doubt the statistical 18:54.600 --> 18:57.670 significance of something like that, unless I really had seen 18:57.667 --> 19:00.867 a very rigorous statistical analysis. 19:00.867 --> 19:04.427 I would wonder whether these spots are really realistic or 19:04.433 --> 19:07.073 whether they're just a fluke of what-- and he had one or 19:07.067 --> 19:10.197 two events there, and by random chance, that happened 19:10.200 --> 19:12.300 to look like a little maximum or something like that. 19:12.300 --> 19:16.800 So I wouldn't try to put too much significance on those. 19:16.800 --> 19:18.430 Whether that-- that may be distinct from this one. 19:18.433 --> 19:20.803 I think the case could be made there. 19:20.800 --> 19:22.270 But I'm not sure why. 19:22.267 --> 19:27.267 I'm not sure why there would be two maxima there. 19:27.267 --> 19:31.197 We do get them in Connecticut as well. 19:31.200 --> 19:33.970 Not frequently, but every few years, we get a tornado that 19:33.967 --> 19:35.197 comes down through the state. 19:35.200 --> 19:38.170 It can cause quite a bit of damage. 19:38.167 --> 19:42.667 But it's like a ratio of 7:1 or 9:1 in terms of the 19:42.667 --> 19:47.697 relative frequency of those two things. 19:47.700 --> 19:50.230 So before we leave the subject, I want to give you a 19:50.233 --> 19:54.933 little bit of a sense for what a tornado is like. 19:54.933 --> 19:59.333 And here's a picture from May 2007, the devastation in 19:59.333 --> 20:00.703 Greensburg, Kansas. 20:00.700 --> 20:02.330 Really a whole town was-- 20:02.333 --> 20:03.333 isn't that horrible? 20:03.333 --> 20:04.873 It's just everything wiped out. 20:04.867 --> 20:07.827 And the only way I could think of-- and this is going to take 20:07.833 --> 20:11.773 a couple minutes and may be a little bit boring-- we'll 20:11.767 --> 20:15.667 see-- but I want to show you what the news was saying at 20:15.667 --> 20:16.927 this point. 20:20.467 --> 20:21.727 So this is-- 20:41.633 --> 20:44.473 this is the radar, the Doppler radar-- 20:44.467 --> 20:46.567 I'll explain why that's important in just a minute-- 20:46.567 --> 20:47.797 blue-red. 20:53.533 --> 20:54.003 [VIDEO PLAYBACK] 20:54.000 --> 20:55.870 -So keep in mind, as far as looking through the 20:55.867 --> 20:58.497 thunderstorm and seeing the tornado appear in Greensburg, 20:58.500 --> 21:00.770 you're not going to, because it's going to be obscured by 21:00.767 --> 21:04.427 the very heavy rain and also the hail between you and that 21:04.433 --> 21:05.303 circulation. 21:05.300 --> 21:07.600 Greenburg's up here at the top of the screen. 21:07.600 --> 21:08.730 -And when we switch over to the 21:08.733 --> 21:10.803 which-way-the-wind's-blowing mode to the 21:10.800 --> 21:12.900 how-hard-it's-raining mode, this is what we have now. 21:12.900 --> 21:14.930 And you can see that very high reflectivity. 21:14.933 --> 21:16.673 What you see is that red. 21:16.667 --> 21:19.027 That is the heavy rain and hail. 21:19.033 --> 21:21.433 So you're not going to be able to visualize the thunderstorm 21:21.433 --> 21:23.173 if it's still producing a tornado. 21:23.167 --> 21:26.027 -Now, contact with Lanny Dean, we temporarily lost it because 21:26.033 --> 21:28.673 of the cell service in this part of Kansas, but once 21:28.667 --> 21:30.827 again, you're seeing a very strong, very well-pronounced 21:30.833 --> 21:33.903 hook echo in addition to the very strong velocity returns-- 21:33.900 --> 21:34.330 [END VIDEO PLAYBACK] 21:34.333 --> 21:37.073 PROFESSOR: That's the hook echo right there. 21:37.067 --> 21:37.367 [VIDEO PLAYBACK] 21:37.367 --> 21:39.227 -The city of Greensburg and to the south, I really suggest 21:39.233 --> 21:41.273 you go down to your storm shelters. 21:41.267 --> 21:43.197 I said I'm not an alarmist kind of guy, but if the only 21:43.200 --> 21:45.270 thing-- this storm is very scary-- 21:45.267 --> 21:47.727 in the worst case, I'm just going to waste a little bit of 21:47.733 --> 21:50.173 your time tonight if you go down to your shelters and it 21:50.167 --> 21:52.197 turns out not to be producing a tornado. 21:52.200 --> 21:54.000 But just what we're seeing here is 21:54.000 --> 21:55.600 a very scary signature. 21:58.533 --> 22:00.373 -Lanny, how close are you to the county line? 22:00.367 --> 22:03.427 -We have passed the county line already, Jay. 22:03.433 --> 22:05.633 -So you're north of the county line, and the tornado, from 22:05.633 --> 22:08.833 your vantage point, is to the east of US 183, correct? 22:08.833 --> 22:11.003 -Jay, a very large tornado. 22:11.000 --> 22:13.430 OK, I don't know who reported it in the west. We have two 22:13.433 --> 22:15.203 tornadoes on the ground, Jay. 22:15.200 --> 22:18.530 We have a small-- well, excuse me it's a medium, if you will, 22:18.533 --> 22:24.333 to my north-northeast. We have a very large wedge tornado on 22:24.333 --> 22:25.903 the ground that's going to be almost 22:25.900 --> 22:27.470 due north of my location. 22:27.467 --> 22:31.167 I'm going to put it about five miles, Jay, about five miles, 22:31.167 --> 22:34.497 I'm guessing, north, maybe five to eight miles north of 22:34.500 --> 22:35.930 my location right now, Jay. 22:35.933 --> 22:37.203 -All right, Landy, try to get the first cam 22:37.200 --> 22:39.000 fired up if you can. 22:39.000 --> 22:39.800 And now let's go over to Darren. 22:39.800 --> 22:42.200 And, Darren, what do you see from your vantage point? 22:42.200 --> 22:44.930 -Right now, we're just about ready to get on the Highway 22:44.933 --> 22:48.573 183 to go north towards Greensburg. 22:48.567 --> 22:51.767 We still have a large wedge tornado on the ground to our 22:51.767 --> 22:54.497 north with possible satellite tornadoes. 22:54.500 --> 22:59.670 We just saw a lightning flash that still probably-- 22:59.667 --> 23:02.827 I would say probably five miles to our north is where 23:02.833 --> 23:03.873 the tornado would be located. 23:03.867 --> 23:07.997 And it has been on the ground for probably 25 minutes, maybe 23:08.000 --> 23:08.730 even longer. 23:08.733 --> 23:10.233 [END VIDEO PLAYBCK] 23:10.233 --> 23:11.603 PROFESSOR: It's moving in that direction. 23:11.600 --> 23:12.670 [VIDEO PLAYBACK] 23:12.667 --> 23:12.997 -What cross-street? 23:13.000 --> 23:15.030 How far north of the Kiowa-Comanche 23:15.033 --> 23:16.633 county line are you? 23:16.633 --> 23:18.433 -We're right on the line. 23:18.433 --> 23:19.933 -You're right on the line currently. 23:19.933 --> 23:21.373 And you're about to proceed northbound 23:21.367 --> 23:22.827 back behind the tornado? 23:22.833 --> 23:23.833 -That's correct. 23:23.833 --> 23:25.473 -And you've been watching this for quite a little while. 23:25.467 --> 23:28.797 And what did you estimate its maximum width was? 23:28.800 --> 23:32.030 -I would say easily a mile, and that's kind of being 23:32.033 --> 23:33.433 conservative. 23:33.433 --> 23:38.003 Before that, we had two separate mesocyclones with-- 23:38.000 --> 23:41.370 it looked to be a cone tornado, and then off to our 23:41.367 --> 23:44.297 east probably a quarter to a half mile was a stovepipe 23:44.300 --> 23:47.600 tornado, a lot of power flashes with it. 23:47.600 --> 23:48.670 -All right, Darren. 23:48.667 --> 23:49.467 And I know you've-- 23:49.467 --> 23:49.897 [END VIDEO PLAYBACK] 23:49.900 --> 23:50.630 PROFESSOR: I'm going to stop 23:50.633 --> 23:53.173 that there for a minute. 23:53.167 --> 23:55.227 Well, I guess I'll stop it there anyway. 23:55.233 --> 23:57.903 But there are two things I wanted to point out to you. 23:57.900 --> 23:59.870 So he's got two things he's looking at. 23:59.867 --> 24:04.627 He's got the radar, and he's looking at the reflectivity, 24:04.633 --> 24:06.803 which is telling you where the rain is falling. 24:06.800 --> 24:11.230 And in that, he's finding this shape, which is 24:11.233 --> 24:12.473 called the hook echo. 24:15.967 --> 24:18.867 So there's rain falling through here, and the spin up 24:18.867 --> 24:23.097 in the cloud is wrapping this rain shaft around 24:23.100 --> 24:25.400 in a kind of spiral. 24:25.400 --> 24:28.000 And that's a very clear signature of a very strong 24:28.000 --> 24:30.000 cyclone aloft. 24:30.000 --> 24:31.700 Then he's also got the Doppler. 24:31.700 --> 24:37.930 For the same instrument, the same radar, it measures the 24:37.933 --> 24:41.573 frequency of the reflected radar beam. 24:41.567 --> 24:44.167 And they know what frequency they sent out. 24:44.167 --> 24:47.197 If it comes back a higher frequency, that means the air 24:47.200 --> 24:49.700 at that location is moving towards you. 24:49.700 --> 24:51.370 If it comes back at a lower frequency, it's 24:51.367 --> 24:52.297 moving away from you. 24:52.300 --> 24:54.500 This is the Doppler effect. 24:54.500 --> 24:58.030 When a car goes by or a train goes by, you hear that 24:58.033 --> 24:59.803 [ZOOMING NOISE] 24:59.800 --> 25:02.400 as it's coming towards you and then going away from you. 25:02.400 --> 25:03.870 There's that frequency shift. 25:03.867 --> 25:07.097 And what he was showing with these two regions, one was red 25:07.100 --> 25:11.370 and one was blue, indicating in one region the wind is away 25:11.367 --> 25:14.067 from you, in the other region it's toward you. 25:14.067 --> 25:19.627 So it probably was like this if the radar was down here, 25:19.633 --> 25:23.503 which means there is a cyclone. 25:23.500 --> 25:24.000 On this side its--you're not 25:24.000 --> 25:25.470 measuring this other component. 25:25.467 --> 25:27.267 You're just measuring the component towards and away 25:27.267 --> 25:28.067 from the radar. 25:28.067 --> 25:33.097 But when you see that doublet, that velocity away from you 25:33.100 --> 25:36.200 and velocity towards you, there's really only one 25:36.200 --> 25:37.000 explanation for that. 25:37.000 --> 25:40.930 There's a tight vortex right in there up in the cloud, and 25:40.933 --> 25:45.503 very possibly that's causing a tornado at the ground. 25:45.500 --> 25:49.330 So what's nice about that film loop is that's 25:49.333 --> 25:50.333 not after the fact. 25:50.333 --> 25:51.333 That's during the fact. 25:51.333 --> 25:54.503 So he's putting himself on the line and saying, listen, I 25:54.500 --> 25:58.000 know sometimes we have false alarms about these things, but 25:58.000 --> 26:01.470 this is one of the strongest signatures I've ever seen. 26:01.467 --> 26:06.367 So he's putting out that kind of warning to the people. 26:06.367 --> 26:08.667 It's pretty-- it takes some guts to do that, because 26:08.667 --> 26:12.567 occasionally they do get this wrong. 26:12.567 --> 26:16.997 So we're going to continue, and I think that's-- 26:17.000 --> 26:20.100 yeah, so that's all we have on severe thunderstorms. Are 26:20.100 --> 26:27.270 there any questions on severe thunderstorms before we go on 26:27.267 --> 26:28.997 to another subject? 26:32.667 --> 26:35.327 So you can read about it in this book. 26:35.333 --> 26:37.003 It's fascinating stuff. 26:37.000 --> 26:39.430 And we'll turn to hurricanes. 26:42.000 --> 26:45.400 Now, a few basic bullet points about hurricanes. 26:45.400 --> 26:48.700 As with other convective storms, they get their energy 26:48.700 --> 26:51.370 from latent heat release. 26:51.367 --> 26:54.897 In this case, they get that water vapor 26:54.900 --> 26:57.130 directly from the ocean. 26:57.133 --> 27:03.773 So it doesn't-- you don't use up the water vapor in the 27:03.767 --> 27:09.667 atmosphere as a tornado begins to draw on it, because with a 27:09.667 --> 27:13.227 warm ocean, you can evaporate water from the ocean just as 27:13.233 --> 27:15.873 fast as you're drawing it up in the storm. 27:15.867 --> 27:19.667 So in one way, it's better to say that really it's the warm 27:19.667 --> 27:24.867 ocean that provides the fuel for this, but yet it's also 27:24.867 --> 27:25.497 water vapor. 27:25.500 --> 27:29.800 The warm water evaporates, and then you get the water vapor 27:29.800 --> 27:30.930 that goes up into the cloud. 27:30.933 --> 27:34.003 And as it rises, you get the latent heat release and the 27:34.000 --> 27:36.570 buoyancy and so on and so forth. 27:36.567 --> 27:39.727 Now, if you have too much wind shear-- 27:39.733 --> 27:43.103 this is just the opposite of the severe thunderstorm. 27:43.100 --> 27:46.900 Severe thunderstorms require that wind shear to distort it 27:46.900 --> 27:50.870 and give it that severe condition, whereas hurricanes 27:50.867 --> 27:53.227 out over the ocean, if you have a lot of wind shear, they 27:53.233 --> 27:54.703 will not form. 27:54.700 --> 27:58.970 They're a symmetric kind of deal, and if you try to shear 27:58.967 --> 28:00.827 that off, you'll just destroy it. 28:00.833 --> 28:04.173 So you need a lot of warm ocean water and not very much 28:04.167 --> 28:08.297 shear, and then you can, under some conditions, you can 28:08.300 --> 28:10.930 generate a hurricane. 28:10.933 --> 28:12.973 They cannot form at the Equator, because they have to 28:12.967 --> 28:17.027 spin one way or the other, and they get that cue from the 28:17.033 --> 28:18.803 Coriolis force. 28:18.800 --> 28:20.970 And so if there's no Coriolis force, you 28:20.967 --> 28:23.127 cannot form a hurricane. 28:23.133 --> 28:24.673 So they cannot form at the Equator. 28:24.667 --> 28:27.067 And as I mentioned the other day, they cannot even cross 28:27.067 --> 28:28.127 the Equator. 28:28.133 --> 28:30.133 Once they've formed in one hemisphere, they've got to 28:30.133 --> 28:31.003 stay in that hemisphere. 28:31.000 --> 28:34.370 They can't suddenly decide to wander across to the other 28:34.367 --> 28:34.797 hemisphere. 28:34.800 --> 28:39.800 They couldn't exist with the spin that they have. They do 28:39.800 --> 28:43.430 spin oppositely in the two hemispheres. 28:43.433 --> 28:46.733 Just like in the tornado, the pressure is very low in the 28:46.733 --> 28:51.573 center, and that balances the Coriolis force and the 28:51.567 --> 28:54.927 centripetal force of the air spinning around the outside. 28:54.933 --> 28:58.533 So this is a cyclone in the same sense that I defined 28:58.533 --> 29:00.973 cyclone for you the other day. 29:00.967 --> 29:05.597 It's a low-pressure center with air currents moving 29:05.600 --> 29:07.600 around it in a cyclonic direction-- 29:07.600 --> 29:09.870 that is, counterclockwise in the Northern Hemisphere, 29:09.867 --> 29:13.297 clockwise in the Southern Hemisphere. 29:13.300 --> 29:15.870 They occur in the late summer and early fall in both 29:15.867 --> 29:17.667 hemispheres. 29:17.667 --> 29:24.667 Be careful how you use those terms. On land, the highest 29:24.667 --> 29:29.367 temperatures normally occur four to six weeks after the 29:29.367 --> 29:30.997 summer solstice. 29:31.000 --> 29:34.800 The summer solstice is about June 21. 29:34.800 --> 29:40.230 The warmest days of summer are usually mid-July for air over 29:40.233 --> 29:41.673 the continents. 29:41.667 --> 29:45.367 But oceans have more heat capacity, and they take a 29:45.367 --> 29:47.767 longer time to warm up. 29:47.767 --> 29:51.397 So the warmer ocean temperatures occur almost 29:51.400 --> 29:55.470 three months after the summer solstice. 29:55.467 --> 29:58.827 The warmest ocean temperatures typically are in August, 29:58.833 --> 30:00.373 September, and October. 30:00.367 --> 30:04.427 And since the hurricanes need that, they need warm ocean 30:04.433 --> 30:09.773 temperatures, the hurricane season is then delayed that 30:09.767 --> 30:10.267 three months. 30:10.267 --> 30:12.597 So it's not midsummer or early summer. 30:12.600 --> 30:15.270 It's late summer and early fall, because that's when the 30:15.267 --> 30:16.597 ocean's sea surface temperature-- 30:16.600 --> 30:20.970 SST is the abbreviation for sea surface temperature-- 30:20.967 --> 30:24.267 is the highest. That defines the 30:24.267 --> 30:25.627 so-called hurricane season. 30:25.633 --> 30:28.303 Now remember, in the Northern Hemisphere, that is going to 30:28.300 --> 30:33.870 be July, August, September, and maybe a little bit in 30:33.867 --> 30:39.427 October, whereas in the Southern Hemisphere, that is 30:39.433 --> 30:43.973 going to be, well, January, February, March, and April in 30:43.967 --> 30:45.097 the Southern Hemisphere. 30:45.100 --> 30:47.900 That's their summertime, and that's when hurricanes are 30:47.900 --> 30:52.100 going to be most frequent in the Southern Hemisphere. 30:52.100 --> 30:56.070 They tend to drift westward, because they occur in the 30:56.067 --> 31:01.167 tropics under the influence of the trade winds. 31:01.167 --> 31:04.467 It's like you had a stream of water and you dip your hand in 31:04.467 --> 31:07.227 and made a little vortex, and you would watch and see that 31:07.233 --> 31:10.873 that vortex is just carried along by the stream. 31:10.867 --> 31:13.597 Whatever direction the stream is going in, that vortex would 31:13.600 --> 31:14.570 move as well. 31:14.567 --> 31:20.597 And that's primarily what's going on when tornado is-- 31:20.600 --> 31:22.200 when a hurricane is moving westward. 31:22.200 --> 31:26.030 However, it also has a poleward drift that can't be 31:26.033 --> 31:29.973 explained as simply as that, and that has something to do 31:29.967 --> 31:31.227 with the curvature of the Earth. 31:31.233 --> 31:33.173 I've published on this, but I don't have time 31:33.167 --> 31:33.727 to go through it. 31:33.733 --> 31:40.633 But there is a drift of hurricanes toward the pole in 31:40.633 --> 31:42.133 both hemispheres-- 31:42.133 --> 31:44.933 the North Pole in the Northern Hemisphere, the South Pole in 31:44.933 --> 31:46.603 the Southern Hemisphere-- 31:46.600 --> 31:51.270 that is added to that general westward drift due to being in 31:51.267 --> 31:53.167 the belt of the easterly trade winds. 31:56.233 --> 32:01.733 So these are the global hurricane tracks. 32:01.733 --> 32:04.973 It's interesting for what is in this diagram and what's not 32:04.967 --> 32:05.697 in this diagram. 32:05.700 --> 32:09.670 For example, in the North Atlantic Ocean-- 32:09.667 --> 32:12.797 the Equator is about here-- 32:12.800 --> 32:16.970 generally, hurricanes form in the central and sometimes even 32:16.967 --> 32:20.967 eastern part of the tropical North Atlantic, and then they 32:20.967 --> 32:22.697 move westward and northward. 32:22.700 --> 32:26.570 So generally, they follow a looping trajectory like this, 32:26.567 --> 32:29.927 or maybe they go straight into the Gulf of Mexico or through 32:29.933 --> 32:34.303 the Caribbean into the Gulf of Mexico like that. 32:34.300 --> 32:41.030 In the eastern tropical North Pacific, you have a very high 32:41.033 --> 32:44.373 density of hurricanes. 32:44.367 --> 32:47.827 Most of these don't threaten land, because they're going 32:47.833 --> 32:51.403 out to sea, but some of them that move up along the coast 32:51.400 --> 32:53.400 can threaten land. 32:53.400 --> 32:55.430 They don't go very far north, because there's a cold 32:55.433 --> 32:59.603 California Current that comes down here. 32:59.600 --> 33:01.530 I didn't put this in the notes, but you should write it 33:01.533 --> 33:04.973 down that many people think there's a critical ocean 33:04.967 --> 33:08.897 temperature below which you cannot have hurricanes. 33:08.900 --> 33:11.970 And some people will tell you that's 27 degrees Celsius. 33:11.967 --> 33:14.367 Some people say it's 28 degrees Celsius. 33:14.367 --> 33:18.267 But somewhere about there is a critical ocean temperature. 33:18.267 --> 33:21.197 And so the ocean has to be warmer than, let's say, 28 33:21.200 --> 33:25.530 Celsius before it can support a hurricane. 33:25.533 --> 33:28.903 And you don't have that up here, because the cold 33:28.900 --> 33:31.030 California Current comes down, but closer to 33:31.033 --> 33:32.133 the Equator you do. 33:32.133 --> 33:35.833 Notice, right at the Equator, no hurricanes. 33:35.833 --> 33:40.303 Plenty of warm water, but no Coriolis force, so no 33:40.300 --> 33:42.500 hurricanes. 33:42.500 --> 33:48.330 The western tropical North Pacific is a hotbed-- the 33:48.333 --> 33:52.473 world's biggest hotbed of hurricanes. 33:52.467 --> 33:54.497 Be careful, though. 33:54.500 --> 33:58.970 The word "hurricane" traditionally is not used in 33:58.967 --> 34:00.227 that part of the world. 34:00.233 --> 34:04.903 These are normally referred to as typhoons, after a Japanese 34:04.900 --> 34:09.070 word for "hurricane." So it wouldn't be wrong to call 34:09.067 --> 34:12.767 these hurricanes, but it wouldn't be the vernacular. 34:12.767 --> 34:15.027 The vernacular would be typhoon. 34:15.033 --> 34:19.173 And so if I say there's a typhoon in the Western 34:19.167 --> 34:20.967 Pacific, you'll know I'm talking 34:20.967 --> 34:22.927 about a hurricane there. 34:22.933 --> 34:25.703 And they've got the same kind of pattern as in the Atlantic. 34:25.700 --> 34:29.170 They go westward, but they also arc northwards. 34:29.167 --> 34:33.027 In the Indian Ocean, you get some in the Arabian Sea and 34:33.033 --> 34:35.073 the Bay of Bengal. 34:35.067 --> 34:37.097 Not many, but you do get some, and they 34:37.100 --> 34:38.770 can be quite damaging. 34:38.767 --> 34:40.397 In the Southern Hemisphere, look at this. 34:40.400 --> 34:41.800 You get them in the Indian Ocean. 34:41.800 --> 34:44.570 You get them in the west tropical Pacific. 34:44.567 --> 34:49.627 You don't get them in the east at all. 34:49.633 --> 34:55.873 And I would've said you don't get them in the Atlantic, but 34:55.867 --> 34:57.327 there was one about four years ago. 34:57.333 --> 35:01.703 It was the first-ever reported hurricane in the tropical 35:01.700 --> 35:04.600 South Pacific, and they've got that one marked in here. 35:04.600 --> 35:06.370 Now, this is pretty easy to explain. 35:06.367 --> 35:08.297 There's a cold current-- 35:08.300 --> 35:10.570 it's called the Humboldt Current, 35:10.567 --> 35:12.597 after a German explorer-- 35:12.600 --> 35:14.970 a cold current that comes up along the coast of South 35:14.967 --> 35:22.167 America and floods this part of the southern eastward 35:22.167 --> 35:24.097 tropical Pacific. 35:24.100 --> 35:25.770 And therefore, the temperatures are not warm 35:25.767 --> 35:27.967 enough to support hurricanes. 35:27.967 --> 35:30.267 And pretty much the same thing is true here. 35:30.267 --> 35:33.727 There's a cold current that comes up along here. 35:33.733 --> 35:37.503 And while there is some warm air right-- some water right 35:37.500 --> 35:41.200 along the Equator, there you don't have the Coriolis force. 35:41.200 --> 35:43.530 And here you've got the Coriolis force, but the 35:43.533 --> 35:44.603 temperatures are too low. 35:44.600 --> 35:47.670 So you're not getting those two ingredients together, the 35:47.667 --> 35:51.367 warm water and the Coriolis force. 35:51.367 --> 35:54.297 Questions on this diagram? 35:54.300 --> 35:58.130 Remember, these are not occurring simultaneously. 35:58.133 --> 36:01.203 This is in Northern Hemisphere hurricane season. 36:01.200 --> 36:04.000 These are in Southern Hemisphere hurricane season. 36:04.000 --> 36:09.670 We've just put them all together on one diagram there. 36:09.667 --> 36:15.667 So here's a zoom in of the Atlantic Ocean. 36:15.667 --> 36:19.427 And you can see that many of them come-- 36:19.433 --> 36:22.503 so once a hurricane gets started, it's going to 36:22.500 --> 36:26.970 persist. It's a good, stable storm system. 36:26.967 --> 36:28.897 It'll persist until the conditions no 36:28.900 --> 36:31.030 longer make it possible. 36:31.033 --> 36:34.003 One of two things will happen. 36:34.000 --> 36:37.470 It'll move over land, in which case it no longer has that 36:37.467 --> 36:42.067 water supply, or it'll move over cold water, in which case 36:42.067 --> 36:44.397 it no longer has enough water supply. 36:44.400 --> 36:49.870 Because the water can't evaporate fast enough from a 36:49.867 --> 36:51.097 cold ocean. 36:53.967 --> 36:57.897 Typically, from the satellite, they look like this. 36:57.900 --> 37:01.870 They have a well-developed, large cloud shield. 37:01.867 --> 37:04.867 It's like a thunderstorm anvil, except it's kind of 37:04.867 --> 37:05.567 axisymmetric. 37:05.567 --> 37:09.767 That means moving out in all directions. 37:09.767 --> 37:13.267 And very often, but not always, there's an eye, which 37:13.267 --> 37:16.067 is a very peculiar characteristic. 37:16.067 --> 37:20.227 The air everywhere else is rising, but right there, the 37:20.233 --> 37:21.903 air is sinking. 37:21.900 --> 37:24.030 I know that because it's clear of clouds. 37:24.033 --> 37:28.473 Clear of clouds means sinking air, thermodynamically. 37:28.467 --> 37:34.497 Underneath this, you can't see it, but the winds are strong 37:34.500 --> 37:37.200 in the cyclonic direction. 37:37.200 --> 37:41.870 So here's kind of a cartoonish cross-section. 37:41.867 --> 37:43.997 You've got these rain bands. 37:44.000 --> 37:47.070 The rain is not uniform in a hurricane but forms in these 37:47.067 --> 37:51.027 spiral, heavy-raining bands. 37:51.033 --> 37:54.803 The air rises in this main eyewall and then spreads out 37:54.800 --> 37:56.670 in this giant anvil. 37:56.667 --> 37:59.327 First, it's still moving in the cyclonic direction. 37:59.333 --> 38:02.173 When that air moves far enough out, the Coriolis force acts 38:02.167 --> 38:05.727 on it and actually reverses its sense of direction. 38:05.733 --> 38:06.433 So be careful. 38:06.433 --> 38:09.933 If you're looking at movies of hurricanes, you may find that 38:09.933 --> 38:14.433 the outer clouds are actually moving anticyclonically. 38:14.433 --> 38:15.733 Don't be confused. 38:15.733 --> 38:20.303 All the rest of the storm is still a tight, cyclonically 38:20.300 --> 38:22.900 spinning vortex. 38:22.900 --> 38:26.300 And then a little bit of that air that rises in the eyewall 38:26.300 --> 38:30.800 sinks right down in the eye and keeps that little section 38:30.800 --> 38:32.570 clear of clouds. 38:32.567 --> 38:36.497 Most of it, however, spreads out in this giant anvil, whose 38:36.500 --> 38:41.230 diameter is several hundred kilometers across. 38:41.233 --> 38:44.103 So hurricane damage, of course, wind impact on 38:44.100 --> 38:45.330 buildings and vegetation. 38:45.333 --> 38:48.703 Remember that the pressure that a wind exerts on a 38:48.700 --> 38:53.930 surface is proportional to the square of the wind speed. 38:53.933 --> 38:57.673 So if you have a 10 meter per second wind hitting my house, 38:57.667 --> 38:59.397 that produces a certain force. 38:59.400 --> 39:02.900 If you have a 20 meters per second, that's four times-- 39:02.900 --> 39:06.230 twice the speed, but four times the pressure force 39:06.233 --> 39:08.033 pushing on the house, and so on. 39:08.033 --> 39:12.273 So if you have a hurricane that's 60 meters per second or 39:12.267 --> 39:17.627 120 meters per second, the destructive forces of that 39:17.633 --> 39:24.503 push really go up very, very steeply and can destroy most 39:24.500 --> 39:27.300 buildings, unless they're specially designed to be 39:27.300 --> 39:29.700 hurricane-secure. 39:29.700 --> 39:32.130 Another thing is that storm surge. 39:32.133 --> 39:37.003 The rapid air movement across the surface of the ocean 39:37.000 --> 39:41.270 pushes water towards the shore and will give an apparent rise 39:41.267 --> 39:42.227 in sea level. 39:42.233 --> 39:45.533 We talked about this when we were discussing Irene. 39:45.533 --> 39:48.673 And if you're standing on the beach, the water just suddenly 39:48.667 --> 39:51.127 seems to rise and move inland. 39:51.133 --> 39:53.873 On top of that, there's going to be the waves caused by the 39:53.867 --> 39:57.567 winds as well, but that storm surge is often responsible for 39:57.567 --> 39:59.767 some of the major damage. 39:59.767 --> 40:02.097 Inland flooding as well, and we saw that in Irene. 40:02.100 --> 40:05.970 Poor Vermont, and New Jersey to some extent too, had a lot 40:05.967 --> 40:09.397 of rainfall, which collected quickly into the valleys and 40:09.400 --> 40:12.930 rivers and caused a lot of flooding. 40:12.933 --> 40:16.973 Of course, for many people here in Connecticut, the main 40:16.967 --> 40:19.267 impact of Hurricane Irene was losing their 40:19.267 --> 40:21.797 electrical power for days. 40:21.800 --> 40:26.630 And that's mostly in this part of the world, because the wind 40:26.633 --> 40:30.303 will take down a tree limb and wipe out most-- in this 40:30.300 --> 40:34.530 country, most of our power lines are above ground. 40:34.533 --> 40:38.803 It's cheaper to do that, easy to maintain in most cases, but 40:38.800 --> 40:41.870 they're so vulnerable to tipping trees. 40:41.867 --> 40:44.367 So when you got a high wind event like this, trees come 40:44.367 --> 40:46.927 down, hit the wires. 40:46.933 --> 40:49.333 You lose both communication and power. 40:49.333 --> 40:55.503 In other parts of the world, hurricanes are often followed 40:55.500 --> 41:00.670 a few weeks later by enormous loss of life from disease. 41:00.667 --> 41:03.697 For example, if you have a hurricane hitting Bangladesh 41:03.700 --> 41:07.970 or some of the other undeveloped countries, what 41:07.967 --> 41:09.167 you'll have-- 41:09.167 --> 41:11.397 the hurricane will be finished and it'll be 41:11.400 --> 41:13.400 long gone, and then-- 41:13.400 --> 41:15.800 but people will be displaced from their homes. 41:15.800 --> 41:18.670 And more importantly, water sources get mixed. 41:18.667 --> 41:22.567 So sewage gets into drinking water typically, and then 41:22.567 --> 41:23.867 disease begins. 41:23.867 --> 41:26.197 And in the weeks and months that follow, you could have 41:26.200 --> 41:30.570 hundreds, thousands, even tens of thousands of deaths arising 41:30.567 --> 41:34.897 from the, quote, "hurricane," but it really has to do with 41:34.900 --> 41:38.030 just contamination of water supplies and a lack of a 41:38.033 --> 41:41.303 public health service to be sure that that sort of thing 41:41.300 --> 41:42.070 doesn't happen. 41:42.067 --> 41:46.497 So this is the biggest loss of life from hurricanes by far, 41:46.500 --> 41:51.300 but these other things are important as well. 41:51.300 --> 41:54.500 Any questions on that? 41:54.500 --> 41:59.670 So the most famous hurricane in New England was nicknamed 41:59.667 --> 42:03.067 the Long Island Express in 1938. 42:03.067 --> 42:08.567 It had a track like this and hit Connecticut directly. 42:08.567 --> 42:11.067 A lot of damage along the Connecticut coast, even more 42:11.067 --> 42:15.527 along the Rhode Island coast. The hurricane produces its 42:15.533 --> 42:18.373 biggest damage on the right front quadrant. 42:18.367 --> 42:21.797 So it's got four quadrants based on how it's moving. 42:21.800 --> 42:25.800 It's that right front quadrant that tends to have the 42:25.800 --> 42:27.800 strongest winds and in many cases the heaviest 42:27.800 --> 42:28.900 precipitation. 42:28.900 --> 42:32.300 And in this case, that hit Rhode Island and caused a lot 42:32.300 --> 42:33.530 of damage there. 42:33.533 --> 42:37.473 If you Google "hurricane, 1938," that one will pop up, 42:37.467 --> 42:39.167 and you can read all about it. 42:39.167 --> 42:41.667 It really was a major event and is still talked about 42:41.667 --> 42:43.067 today in Connecticut. 42:43.067 --> 42:45.327 We had our own chance at this just a few weeks ago. 42:45.333 --> 42:49.433 Hurricane Irene had a track similar to that but not 42:49.433 --> 42:50.673 exactly the same. 42:50.667 --> 42:54.067 The difference probably was that it brushed ashore here 42:54.067 --> 42:59.297 near Cape Hatteras and lost a lot of its strength before it 42:59.300 --> 43:03.700 hit New England, partly by its lack of water vapor over the 43:03.700 --> 43:05.830 land and also the frictional-- 43:05.833 --> 43:09.373 with all the trees and so on over land, that friction 43:09.367 --> 43:09.967 weakened it. 43:09.967 --> 43:14.797 So by the time it hit us, it had weakened to a Category 2 43:14.800 --> 43:17.330 or perhaps even 1. 43:17.333 --> 43:22.273 And it still caused a lot of flooding damage but not so 43:22.267 --> 43:26.327 much wind damage and other things. 43:26.333 --> 43:29.133 And then of course, the most damaging hurricane in our 43:29.133 --> 43:32.273 country's history was Hurricane Katrina 43:32.267 --> 43:34.627 in August of 2005. 43:34.633 --> 43:35.803 And it had a different track. 43:35.800 --> 43:37.800 It started here in the eastern-- 43:37.800 --> 43:41.500 well, just north of Cuba, crossed over the southern tip 43:41.500 --> 43:45.200 of Florida, regained its strength, and 43:45.200 --> 43:48.600 then hit New Orleans. 43:48.600 --> 43:52.600 And you can read all about that in the history books 43:52.600 --> 43:53.030 now-- in history books now too. 43:53.033 --> 43:56.473 And there's a picture of Katrina just as it was about 43:56.467 --> 44:01.497 to hit New Orleans, a beautifully formed storm with 44:01.500 --> 44:05.800 a beautiful central eye and the cloud shield 44:05.800 --> 44:07.630 and the whole business. 44:07.633 --> 44:15.733 Very strong storm that hit New Orleans. 44:15.733 --> 44:17.733 This is what I mentioned before. 44:17.733 --> 44:21.033 There's a well-defined hurricane season. 44:21.033 --> 44:25.803 And here it is for Atlantic hurricanes. 44:25.800 --> 44:28.200 And of course, when I say "Atlantic," I'm referring to 44:28.200 --> 44:29.000 North Atlantic. 44:29.000 --> 44:30.800 Because, remember, there are no hurricanes in the South 44:30.800 --> 44:34.730 Atlantic, so this is the Northern Hemisphere hurricane 44:34.733 --> 44:36.873 frequencies for the Atlantic Ocean. 44:36.867 --> 44:41.127 And typically, the frequency is non-negligible even in 44:41.133 --> 44:44.473 June, but it begins to rise quickly at the first of 44:44.467 --> 44:48.867 August, peaks in early, mid-September, and then you're 44:48.867 --> 44:52.597 kind of out of the woods by the time you get to the 44:52.600 --> 44:55.070 beginning or the middle of November. 44:55.067 --> 45:01.097 And this all has to do with ocean surface temperatures. 45:01.100 --> 45:02.370 Questions on that? 45:08.567 --> 45:09.127 Yes. 45:09.133 --> 45:13.533 Well, it is-- we have five minutes to go, but rather than 45:13.533 --> 45:16.933 starting a new subject, why don't we call it quits today? 45:16.933 --> 45:21.733 And we'll do mid-latitude frontal cyclones on Friday.