GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 15

 - Convective Storms


There are three main types of convective storms: airmass thunderstorms, severe thunderstorms and hurricanes. These storms are all driven by the release of latent heat into the atmosphere during condensation of water vapor. Severe thunderstorms include both squall line thunderstorms and tornados. They acquire energy from water vapor in the atmosphere over land and therefore typically require warm air temperatures and high humidity. Hurricanes gain energy from water vapor evaporated from the ocean surface. This requires warm ocean temperatures, and is the reason hurricanes weaken over land. Hurricanes are cyclonic and therefore also require a non-zero Coriolis force to form and maintain their structure. For this reason they cannot form over the equator and cannot cross the equator.

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The Atmosphere, the Ocean, and Environmental Change

GG 140 - Lecture 15 - Convective Storms

Chapter 1: Coriolis Force Sign Reversal [00:00:00]

Professor Ron Smith: Well, I’m talking about storms, but someone asked me a question the other day about why the Coriolis force is opposite in the two hemispheres. And I can’t give you a complete explanation, but I can give you a partial explanation. Here’s the globe, and of course, it spins toward the east. The sun rises in the east, sets in the west. So that’s the way the Earth spins.

If I keep spinning it in that direction and I face this towards you, is that clockwise or counterclockwise? Counterclockwise. I’m going to keep spinning it in that direction. Is that clockwise or counterclockwise? Clockwise. So in a sense, while the globe spins eastward, the sense of rotation is opposite in the two hemispheres. And that is one way to understand why the Coriolis force is opposite in the two hemispheres.

So like I said, not a complete explanation, but it’s true, and I think it helps to understand that tricky point about the Coriolis force. Any questions on that?

Chapter 2: Convective Storms [00:01:47]

So we’re going through storms today. This is not very quantitative material. This stuff will be posted after the fact, and your textbook is very good. These chapters that I’ve assigned, a number of them have to do with storm types and so on. So you’ll find a good–of course, I will explain things somewhat differently, but you’ll find a good way to get competing views of this by listening to what I say and then reading the textbook, and between the two, you should come out pretty well on this subject.

So we were talking about convective storms. And that is the class of storms that get their energy from the release of latent heat. I mentioned this last time. That’s a number that I carry around in my head. It’s rounded off a little bit, but I use that number so often that I carry it around in my head. The latent heat of condensation, or if you like, the latent heat of evaporation–it’s the same magnitude, independent of whether you’re condensing or evaporating water–is about 2.5 million joules per kilogram.

And so when you condense water, you’re then providing extra heat to the air, which makes it warmer, more buoyant. It’s likely to rise then, and that’s what’s going on in all of these storms. It’s the basic underlying energy source.

Chapter 3: Airmass Thunderstorms [00:03:19]

We talked about air-mass thunderstorms last time and how they go through a life cycle. And after–they take about 20 minutes to 30 minutes to develop from a fair-weather cumulus into a cumulonimbus with rain coming out the bottom. And then as the rain begins, you begin to get some evaporation of falling rainwater just below cloud base. That produces cool air, which then sinks and spreads out.

It’s like you open your freezer door, and that cold air from the freezer falls down to the kitchen floor and then spreads out. That’s what’s happening, and that goes by various names. Sometimes we call it a downburst. Sometimes it’s called a gust front. The front of it is called the gust front.

But anyway, the air is now spreading out away from the cloud, and that shuts off the warm, moist inflow. So it kills itself off, basically. It has a–after about two hours, it’s gone.

Now, that outflow might well trigger another thunderstorm. So that doesn’t mean, for example, over the state of Connecticut on a day when there are thunderstorms, it doesn’t mean there’s just going to be one and then none. It means there’ll be one, and then as it dies, it may trigger two others, and then as they die, they may trigger others. So this may go on throughout the afternoon, but the individual cells have a short lifetime. That’s an important characteristic.

Chapter 4: Severe Thunderstorms [00:04:52]

As opposed to these severe storms, which have the additional environmental factor of having wind shear through the troposphere, usually a jet stream aloft. Strong winds near the tropopause, weak winds near the surface of the Earth, that tends to bend the storm over and change its structure into one in which the inflow is at a different location than the outflow caused by that evaporative cooling.

In fact, the cold air generated by evaporative cooling spreads out and actually forms a gust front over which the warm air then lifts. So it actually assists in the lifting of the warm air up into the cloud. And this structure is more or less stable and can persist for a number of hours and in some cases even for even overnight. And these are the ones that have–they’re larger and they’re severe in every way you can imagine.

I think I showed these last time. But heavy rains and flooding, hail, lightning, tornadic winds. I don’t think the low pressure in the tornado is much of an issue, but the gust-front winds coming from that cool air is another source of wind damage. In many—in some cases, that causes more damage than even a tornado could. Usually, the tornadic winds are stronger when there is a tornado, but the gust-front winds may cover a wider area and in some cases will cause more damage than a possible tornado.

I don’t know if I got this far, but here’s a picture of one. And earlier in the course, we talked about the visual appearance of a funnel. And this is called the condensation funnel. That is basically a cloud composed of tiny liquid water droplets that’s produced when air from outside moves in and drops its pressure. The pressure inside a tornado is much lower than in the surrounding atmosphere, so that air that moves in there is going to expand, not by moving vertically, but by moving into the tornado. It’ll expand, cool, and you can form a cloud.

And that’s what you’re seeing. In fact, you often see this descending from cloud base. The cloud base will appear to be flat, and then as the tornado develops, that funnel cloud will descend to the ground. When you see that, it’s more natural to understand that it’s really like a cloud, because it’s actually an extension of that cloud reaching down to the surface.

Whereas this other thing here is the debris funnel. That’s stuff that’s been kicked up by the high winds at the surface, and that’s not water. That’s usually dirt or soil or cows or cars or houses and things like that.

And here again, you see the condensation funnel, and you see a debris funnel around that. Lightning comes along with that. And of course, these sparks of electricity is what you hear as thunder. And I think you know that–how many know how to compute in your mind how far a storm is? Do you know that trick?

It’s pretty simple, because you assume that the light that comes from the lightning strike travels to your eye at the speed of light, which is, you might say, instantaneous, right? Whereas the sound that reaches your ear moves at the speed of sound, which is something like 300 meters per second. And so if you simply–when you see a lightning flash, if you then count out the number of seconds–one second, two seconds–so for every second delay between the two, that’s 300 meters of distance. So if it was three seconds delayed, that would be 900 meters or about a kilometer away from you.

So that’s an easy calculation to do, realizing that the thunder is produced by the lightning itself. You suddenly heat that air with the electrical discharge, it expands suddenly, produces an acoustic signal that then travels to your ear, and you recognize that as thunder.

Questions on that?

Hailstones, because there’s a lot of supercooled water above the freezing line in those storms and very strong updraft motions that can suspend particles while they grow by riming. And eventually, when either that updraft weakens or the object becomes too large, it’ll then fall out of the sky.

And it’s too large to melt. I told you many times that a snowflake will melt as it passes down through the freezing level and become a raindrop. But if I’ve got a chunk of ice that’s five centimeters across and it falls down, it’s too big. It’s going to take minutes to melt. And by that time, it’ll be at the surface. It’ll be wet, it’ll have a wet outer surface, but it’ll still be a solid chunk of ice when it falls to Earth and hits.

This is thunderstorm frequency across the United States, number of thunderstorms per year. Now, I’ve told you that these are called–most of these are just air-mass thunderstorms. They’re not so damaging. And it looks like Florida is the capital for air-mass thunderstorms. You get 100 of them per year. Other areas get far less. But these aren’t the severe ones. I’ll show you the severe ones in just a moment.

You don’t get many thunderstorms in the West Coast, because there’s too strong an inversion there. There’s a temp–normally, with the cold water off the coast from the California Current, the atmosphere over here is just too stable, too many inversions to get thunderstorms forming in that part of the world. So it really is kind of an East Coast phenomenon. But it starts right at the Rocky Mountain Front here in Colorado, and you find storms then pretty much all the way to the east, including all the way up into Canada you get quite a few.

Now, I don’t often in this course talk about the research that goes on here at Yale or in my group, but I wanted to show you one thing. I had a Ph.D. Student who finished a couple of years ago who tried to understand the timing of thunderstorms from west to east across–in a region generally like this. And so she looked at a lot of climate—of rainfall data from the summer months and came up with a diagram like this.

Now, I’m going to explain this. It’s going to take a minute just to figure out what in the heck is plotted here. But on this axis is longitude. So it’s distance east-west basically through a region something like this. So it’s the distance east-west in a latitude belt something like that.

And on this axis is the local solar time. So instead of actually using time zones, which are kind of an artificial construct–they make arbitrary choices about where to switch from Eastern Time to Central Time to Mountain Time–this is just local time. Basically, how high is the sun in the sky, that determines what this clock would be.

And then contoured is the precipitation rate, the hourly precipitation rate. So from this diagram, for any given latitude, if you scan up, you’ll find how much it rains at different hours of the day. Now, we did repeat. We plotted everything twice. So you’ve got zero to 24 and then 24 to 48. But if you notice, the plot just repeats as well. So we’ve just put the same data on there twice, which makes it a little bit easier to go through a full day’s cycle. You can start at any hour you want and go 24 hours ahead.

So this is the Rocky Mountain Front. This is where a lot of thunderstorms form. It’s over in here. And those storms tend to form between 1200 and 1800. So that’s in the afternoon, local solar time. Now, that’s not a surprise, right? Because one of the most important triggers for thunderstorms is the heating of the Earth by the Sun.

So it’s in the afternoon when you’ve built up this deep convective boundary layer by heating the surface of the Earth. And finally, that heating is going to generate cumulus clouds, and they’ll then grow to form cumulonimbus. So it’s not surprising that you find storms beginning there. And over the East Coast–here in Connecticut, for example–you also find that the storms form in the afternoon, due to the heating of the Earth by the Sun.

But look what happens in this middle territory, which is basically this very large region through here. That has some other kind of a trigger, because, look, that is advancing in time. As I move eastward, the time of day when thunderstorms occur is 6 o’clock, 9 o’clock in the evening, midnight, 3 o’clock in the morning, 6 o’clock the next morning, and so on. So you’ve got something that is departing from the simple idea of the Sun as the driving force.

What this turns out to be is a disturbance that’s generated each day over the Rocky Mountains and then is–whoops, wrong direction–and then is pushed eastwards by the westerly winds. And it’s a warm air disturbance generated over the mountains. And as it drifts eastward aloft, it triggers thunderstorms as it moves along.

I had somebody in my office a couple of weeks ago, a scientist who grew up in one of these states and always wondered why there they had a maximum thunderstorm frequency in the middle of the night. Who would expect that? It doesn’t seem to fit with the standard thermal solar forcing idea. Well, this is why. In this case, my student discovered that it’s a disturbance generated over the Rockies that moves eastward and gives that progressive timing to the thunderstorms.

Then you get east of the Appalachian Mountains, and you get back to a more normal mid-afternoon thunderstorm maxima. Any questions on that?

Student: What are those dotted lines?

Professor Ron Smith: I think they’re reference lines for some other figure in the paper that’s not shown here. So there are sloping lines that are drawn in here at different speeds. Since this is distance versus time, a slope represents a propagation speed–30 meters per second, 14 meters per second, seven meters per second. These disturbances move eastward at about 14 meters per second, bringing those thunderstorms progressively later as you move towards the east.


Student: Was it already known that there was a warm weather air disturbance over the Rockies, or did she figure that out?

Professor Ron Smith: No, that’s not shown in this figure, but that was actually–there was some hint about this progression already from radar data, but she confirmed it using surface data. And then she discovered this warm air pulse aloft that was doing the triggering. That was the point of her thesis, yeah.

Anything else on that?

Chapter 5: Tornados [00:17:05]

So now let’s get to the severe thunderstorms. And the way I’ll get at that is just to show you this tornado frequency map. And of course, it looks a bit different. There is a maximum in Florida, but it’s smaller. And the big maximum is over Texas, Oklahoma, Kansas, Nebraska. And of course, this is what’s called Tornado Alley by some people. You’ve heard that expression. It’s kind of the region.

But what’s special about this region apparently–and by the way, this maximizes in the spring. So typically, this is an April-May phenomenon. If you have watched any of these shows about chasing tornadoes and if you want to do that, then schedule your trip in April and May, because that’s when things get exciting out there.

And what’s special about this region is that you get some warm, moist air moving in from the Gulf of Mexico at low levels, and then you get the jet stream coming across the Rocky Mountains at high levels. And that gives you the shear that you need. And you’ve got the moisture, because you’ve got the high temperatures and high humidities in the low levels. So all the ingredients are kind of there. Not every day–it fluctuates from day to day–but generally, that’s the reason why this area is so special.

And as far as I know, there’s no other place on Earth that has as many—as high a tornado frequency as this region does. So this is special even if you look globally. There are a few other places in Asia and in Australia where they get this phenomenon, but this is kind of the most frequent place for it to occur.

Questions there? Anybody from Tornado Alley? Anybody live in these states? Not at all? You’re all East Coast or West Coast people? Yeah.

Student: The little spots in the Southeast, is there—is that a coincidence?

Professor Ron Smith: Yeah, that’s a good question. For example, I read a paper once about this one, who tried to claim there was something special about the terrain in that region. But to be honest with you, I would doubt the statistical significance of something like that, unless I really had seen a very rigorous statistical analysis.

I would wonder whether these spots are really realistic or whether they’re just a fluke of what–and he had one or two events there, and by random chance, that happened to look like a little maximum or something like that. So I wouldn’t try to put too much significance on those. Whether that–that may be distinct from this one. I think the case could be made there. But I’m not sure why. I’m not sure why there would be two maxima there.

We do get them in Connecticut as well. Not frequently, but every few years, we get a tornado that comes down through the state. It can cause quite a bit of damage. But it’s like a ratio of 7:1 or 9:1 in terms of the relative frequency of those two things.

So before we leave the subject, I want to give you a little bit of a sense for what a tornado is like. And here’s a picture from May 2007, the devastation in Greensburg, Kansas. Really a whole town was–isn’t that horrible? It’s just everything wiped out. And the only way I could think of–and this is going to take a couple minutes and may be a little bit boring–we’ll see–but I want to show you what the news was saying at this point.

So this is–this is the radar, the Doppler radar–I’ll explain why that’s important in just a minute–blue-red.


So keep in mind, as far as looking through the thunderstorm and seeing the tornado appear in Greensburg, you’re not going to, because it’s going to be obscured by the very heavy rain and also the hail between you and that circulation. Greenburg’s up here at the top of the screen.

And when we switch over to the which-way-the-wind’s-blowing mode to the how-hard-it’s-raining mode, this is what we have now. And you can see that very high reflectivity. What you see is that red. That is the heavy rain and hail. So you’re not going to be able to visualize the thunderstorm if it’s still producing a tornado.

Now, contact with Lanny Dean, we temporarily lost it because of the cell service in this part of Kansas, but once again, you’re seeing a very strong, very well-pronounced hook echo in addition to the very strong velocity returns–


Professor Ron Smith: That’s the hook echo right there.


The city of Greensburg and to the south, I really suggest you go down to your storm shelters. I said I’m not an alarmist kind of guy, but if the only thing–this storm is very scary–in the worst case, I’m just going to waste a little bit of your time tonight if you go down to your shelters and it turns out not to be producing a tornado. But just what we’re seeing here is a very scary signature.

Lanny, how close are you to the county line?

We have passed the county line already, Jay.

So you’re north of the county line, and the tornado, from your vantage point, is to the east of US 183, correct?

Jay, a very large tornado. OK I don’t know who reported it in the west. We have two tornadoes on the ground, Jay. We have a small–well, excuse me it’s a medium, if you will, to my north-northeast. We have a very large wedge tornado on the ground that’s going to be almost due north of my location. I’m going to put it about five miles, Jay, about five miles, I’m guessing, north, maybe five to eight miles north of my location right now, Jay.

All right, Lanny, try to get the first cam fired up if you can. And now let’s go over to Darren. And, Darren, what do you see from your vantage point?

Right now, we’re just about ready to get on the Highway 183 to go north towards Greensburg. We still have a large wedge tornado on the ground to our north with possible satellite tornadoes. We just saw a lightning flash that still probably–I would say probably five miles to our north is where the tornado would be located. And it has been on the ground for probably 25 minutes, maybe even longer.


Professor Ron Smith: It’s moving in that direction.


What cross-street? How far north of the Kiowa-Comanche county line are you?

We’re right on the line.

You’re right on the line currently. And you’re about to proceed northbound back behind the tornado?

That’s correct.

And you’ve been watching this for quite a little while. And what did you estimate its maximum width was?

I would say easily a mile, and that’s kind of being conservative. Before that, we had two separate mesocyclones with–it looked to be a cone tornado, and then off to our east probably a quarter to a half mile was a stovepipe tornado, a lot of power flashes with it.

All right, Darren. And I know you’ve–


Professor Ron Smith: I’m going to stop that there for a minute. Well, I guess I’ll stop it there anyway. But there are two things I wanted to point out to you. So he’s got two things he’s looking at. He’s got the radar, and he’s looking at the reflectivity, which is telling you where the rain is falling. And in that, he’s finding this shape, which is called the hook echo. So there’s rain falling through here, and the spin up in the cloud is wrapping this rain shaft around in a kind of a spiral. And that’s a very clear signature of a very strong cyclone aloft.

Then he’s also got the Doppler. For the same instrument, the same radar, it measures the frequency of the reflected radar beam. And they know what frequency they sent out. If it comes back a higher frequency, that means the air at that location is moving towards you. If it comes back at a lower frequency, it’s moving away from you.

This is the Doppler effect. When a car goes by or a train goes by, you hear that [zooming noise] as it’s coming towards you and then going away from you. There’s that frequency shift. And what he was showing with these two regions, one was red and one was blue, indicating in one region the wind is away from you, in the other region it’s toward you.

So it probably was like this if the radar was down here, which means there is a cyclone. On this side it’s–you’re not measuring this other component. You’re just measuring the component towards and away from the radar. But when you see that doublet, that velocity away from you and velocity towards you, there’s really only one explanation for that. There’s a tight vortex right in there up in the cloud, and very possibly that’s causing a tornado at the ground.

So what’s nice about that film loop is that’s not after the fact. That’s during the fact. So he’s putting himself on the line and saying, listen, I know sometimes we have false alarms about these things, but this is one of the strongest signatures I’ve ever seen. So he’s putting out that kind of warning to the people. It’s pretty–it takes some guts to do that, because occasionally they do get this wrong.

So we’re going to continue, and I think that’s–yeah, so that’s all we have on severe thunderstorms. Are there any questions on severe thunderstorms before we go on to another subject? So you can read about it in this book. It’s fascinating stuff.

Chapter 6: Hurricanes [00:27:04]

And we’ll turn to hurricanes. Now, a few basic bullet points about hurricanes. As with other convective storms, they get their energy from latent heat release. In this case, they get that water vapor directly from the ocean. So it doesn’t–you don’t use up the water vapor in the atmosphere as a tornado begins to draw on it, because with a warm ocean, you can evaporate water from the ocean just as fast as you’re drawing it up in the storm.

So in one way, it’s better to say that really it’s the warm ocean that provides the fuel for this, but yet it’s also water vapor. The warm water evaporates, and then you get the water vapor that goes up into the cloud. And as it rises, you get the latent heat release and the buoyancy and so on and so forth.

Now, if you have too much wind shear–this is just the opposite of the severe thunderstorm. Severe thunderstorms require that wind shear to distort it and give it that severe condition, whereas hurricanes out over the ocean, if you have a lot of wind shear, they will not form. They’re a symmetric kind of deal, and if you try to shear that off, you’ll just destroy it. So you need a lot of warm ocean water and not very much shear, and then you can, under some conditions, you can generate a hurricane.

They cannot form at the Equator, because they have to spin one way or the other, and they get that cue from the Coriolis force. And so if there’s no Coriolis force, you cannot form a hurricane. So they cannot form at the Equator.

And as I mentioned the other day, they cannot even cross the Equator. Once they’ve formed in one hemisphere, they’ve got to stay in that hemisphere. They can’t suddenly decide to wander across to the other hemisphere. They couldn’t exist with the spin that they have. They do spin oppositely in the two hemispheres.

Just like in the tornado, the pressure is very low in the center, and that balances the Coriolis force and the centripetal force of the air spinning around the outside. So this is a cyclone in the same sense that I defined cyclone for you the other day. It’s a low-pressure center with air currents moving around it in a cyclonic direction–that is, counterclockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.

They occur in the late summer and early fall in both hemispheres. Be careful how you use those terms. On land, the highest temperatures normally occur four to six weeks after the summer solstice. The summer solstice is about June 21. The warmest days of summer are usually mid-July for air over the continents.

But oceans have more heat capacity, and they take a longer time to warm up. So the warmer ocean temperatures occur almost three months after the summer solstice. The warmest ocean temperatures typically are in August, September, and October. And since the hurricanes need that, they need warm ocean temperatures, the hurricane season is then delayed that three months.

So it’s not midsummer or early summer. It’s late summer and early fall, because that’s when the ocean’s sea surface temperature–SST is the abbreviation for sea surface temperature–is the highest. That defines the so-called hurricane season.

Now remember, in the Northern Hemisphere, that is going to be July, August, September, and maybe a little bit in October, whereas in the Southern Hemisphere, that is going to be, well, January, February, March, and April in the Southern Hemisphere. That’s their summertime, and that’s when hurricanes are going to be most frequent in the Southern Hemisphere.

They tend to drift westward, because they occur in the tropics under the influence of the trade winds. It’s like you had a stream of water and you dip your hand in and made a little vortex, and you would watch and see that that vortex is just carried along by the stream. Whatever direction the stream is going in, that vortex would move as well. And that’s primarily what’s going on when a tornado is—when a hurricane is moving westward.

However, it also has a poleward drift that can’t be explained as simply as that, and that has something to do with the curvature of the Earth. I’ve published on this, but I don’t have time to go through it. But there is a drift of hurricanes toward the pole in both hemispheres–the North Pole in the Northern Hemisphere, the South Pole in the Southern Hemisphere–that is added to that general westward drift due to being in the belt of the easterly trade winds.

So these are the global hurricane tracks. It’s interesting for what is in this diagram and what’s not in this diagram. For example, in the North Atlantic Ocean–the Equator is about here–generally, hurricanes form in the central and sometimes even eastern part of the tropical North Atlantic, and then they move westward and northward. So generally, they follow a looping trajectory like this, or maybe they go straight into the Gulf of Mexico or through the Caribbean into the Gulf of Mexico like that.

In the eastern tropical North Pacific, you have a very high density of hurricanes. Most of these don’t threaten land, because they’re going out to sea, but some of them that move up along the coast can threaten land. They don’t go very far north, because there’s a cold California Current that comes down here.

I didn’t put this in the notes, but you should write it down that many people think there’s a critical ocean temperature below which you cannot have hurricanes. And some people will tell you that’s 27 degrees Celsius. Some people say it’s 28 degrees Celsius. But somewhere about there is a critical ocean temperature. And so the ocean has to be warmer than, let’s say, 28 Celsius before it can support a hurricane. And you don’t have that up here, because the cold California Current comes down, but closer to the Equator you do.

Notice, right at the Equator, no hurricanes. Plenty of warm water, but no Coriolis force, so no hurricanes.

The western tropical North Pacific is a hotbed–the world’s biggest hotbed of hurricanes. Be careful, though. The word “hurricane” traditionally is not used in that part of the world. These are normally referred to as typhoons, after a Japanese word for “hurricane.” So it wouldn’t be wrong to call these hurricanes, but it wouldn’t be the vernacular. The vernacular would be typhoon. And so if I say there’s a typhoon in the Western Pacific, you’ll know I’m talking about a hurricane there.

And they’ve got the same kind of pattern as in the Atlantic. They go westward, but they also arc northwards. In the Indian Ocean, you get some in the Arabian Sea and the Bay of Bengal. Not many, but you do get some, and they can be quite damaging.

In the Southern Hemisphere, look at this. You get them in the Indian Ocean. You get them in the west tropical Pacific. You don’t get them in the east at all. And I would’ve said you don’t get them in the Atlantic, but there was one about four years ago. It was the first-ever reported hurricane in the tropical South Pacific, and they’ve got that one marked in here.

Now, this is pretty easy to explain. There’s a cold current–it’s called the Humboldt Current, after a German explorer–a cold current that comes up along the coast of South America and floods this part of the southern eastward tropical Pacific. And therefore, the temperatures are not warm enough to support hurricanes.

And pretty much the same thing is true here. There’s a cold current that comes up along here. And while there is some warm air right—some water right along the Equator, there you don’t have the Coriolis force. And here you’ve got the Coriolis force, but the temperatures are too low. So you’re not getting those two ingredients together, the warm water and the Coriolis force.

Questions on this diagram? Remember, these are not occurring simultaneously. This is in Northern Hemisphere hurricane season. These are in Southern Hemisphere hurricane season. We’ve just put them all together on one diagram there.

So here’s a zoom in of the Atlantic Ocean. And you can see that many of them come–so once a hurricane gets started, it’s going to persist. It’s a good, stable storm system. It’ll persist until the conditions no longer make it possible. One of two things will happen. It’ll move over land, in which case it no longer has that water supply, or it’ll move over cold water, in which case it no longer has enough water supply. Because the water can’t evaporate fast enough from a cold ocean.

Typically, from the satellite, they look like this. They have a well-developed, large cloud shield. It’s like a thunderstorm anvil, except it’s kind of axisymmetric. That means moving out in all directions. And very often, but not always, there’s an eye, which is a very peculiar characteristic. The air everywhere else is rising, but right there, the air is sinking. I know that because it’s clear of clouds. Clear of clouds means sinking air, thermodynamically. Underneath this, you can’t see it, but the winds are strong in the cyclonic direction.

So here’s kind of a cartoonish cross-section. You’ve got these rain bands. The rain is not uniform in a hurricane but forms in these spiral, heavy-raining bands. The air rises in this main eyewall and then spreads out in this giant anvil. First, it’s still moving in the cyclonic direction. When that air moves far enough out, the Coriolis force acts on it and actually reverses its sense of direction.

So be careful. If you’re looking at movies of hurricanes, you may find that the outer clouds are actually moving anticyclonically. Don’t be confused. All the rest of the storm is still a tight, cyclonically spinning vortex. And then a little bit of that air that rises in the eyewall sinks right down in the eye and keeps that little section clear of clouds. Most of it, however, spreads out in this giant anvil, whose diameter is several hundred kilometers across.

So hurricane damage, of course, wind impact on buildings and vegetation. Remember that the pressure that a wind exerts on a surface is proportional to the square of the wind speed. So if you have a 10 meter per second wind hitting my house, that produces a certain force. If you have a 20 meters per second, that’s four times–twice the speed, but four times the pressure force pushing on the house, and so on.

So if you have a hurricane that’s 60 meters per second or 120 meters per second, the destructive forces of that push really go up very, very steeply and can destroy most buildings, unless they’re specially designed to be hurricane-secure.

Another thing is that storm surge. The rapid air movement across the surface of the ocean pushes water towards the shore and will give an apparent rise in sea level. We talked about this when we were discussing Irene. And if you’re standing on the beach, the water just suddenly seems to rise and move inland. On top of that, there’s going to be the waves caused by the winds as well, but that storm surge is often responsible for some of the major damage.

Inland flooding as well, and we saw that in Irene. Poor Vermont, and New Jersey to some extent too, had a lot of rainfall, which collected quickly into the valleys and rivers and caused a lot of flooding.

Of course, for many people here in Connecticut, the main impact of Hurricane Irene was losing their electrical power for days. And that’s mostly in this part of the world, because the wind will take down a tree limb and wipe out most–in this country, most of our power lines are above ground. It’s cheaper to do that, easy to maintain in most cases, but they’re so vulnerable to tipping trees. So when you got a high wind event like this, trees come down, hit the wires. You lose both communication and power.

In other parts of the world, hurricanes are often followed a few weeks later by enormous loss of life from disease. For example, if you have a hurricane hitting Bangladesh or some of the other undeveloped countries, what you’ll have–the hurricane will be finished and it’ll be long gone, and then–but people will be displaced from their homes.

And more importantly, water sources get mixed. So sewage gets into drinking water typically, and then disease begins. And in the weeks and months that follow, you could have hundreds, thousands, even tens of thousands of deaths arising from the, quote, “hurricane,” but it really has to do with just contamination of water supplies and a lack of a public health service to be sure that that sort of thing doesn’t happen. So this is the biggest loss of life from hurricanes by far, but these other things are important as well.

Any questions on that?

So the most famous hurricane in New England was nicknamed the Long Island Express in 1938. It had a track like this and hit Connecticut directly. A lot of damage along the Connecticut coast, even more along the Rhode Island coast. The hurricane produces its biggest damage on the right front quadrant. So it’s got four quadrants based on how it’s moving. It’s that right front quadrant that tends to have the strongest winds and in many cases the heaviest precipitation.

And in this case, that hit Rhode Island and caused a lot of damage there. If you Google “hurricane, 1938,” that one will pop up, and you can read all about it. It really was a major event and is still talked about today in Connecticut.

We had our own chance at this just a few weeks ago. Hurricane Irene had a track similar to that but not exactly the same. The difference probably was that it brushed ashore here near Cape Hatteras and lost a lot of its strength before it hit New England, partly by its lack of water vapor over the land and also the frictional–with all the trees and so on over land, that friction weakened it. So by the time it hit us, it had weakened to a Category 2 or perhaps even 1. And it still caused a lot of flooding damage but not so much wind damage and other things.

And then of course, the most damaging hurricane in our country’s history was Hurricane Katrina in August of 2005. And it had a different track. It started here in the eastern–well, just north of Cuba, crossed over the southern tip of Florida, regained its strength, and then hit New Orleans. And you can read all about that in the history books now—in history books now too. And there’s a picture of Katrina just as it was about to hit New Orleans, a beautifully formed storm with a beautiful central eye and the cloud shield and the whole business. Very strong storm that hit New Orleans.

This is what I mentioned before. There’s a well-defined hurricane season. And here it is for Atlantic hurricanes. And of course, when I say “Atlantic,” I’m referring to North Atlantic. Because, remember, there are no hurricanes in the South Atlantic, so this is the Northern Hemisphere hurricane frequencies for the Atlantic Ocean.

And typically, the frequency is non-negligible even in June, but it begins to rise quickly at the first of August, peaks in early, mid-September, and then you’re kind of out of the woods by the time you get to the beginning or the middle of November. And this all has to do with ocean surface temperatures.

Questions on that?

Yes. Well, it is–we have five minutes to go, but rather than starting a new subject, why don’t we call it quits today? And we’ll do mid-latitude frontal cyclones on Friday.

[end of transcript]

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