GG 140: The Atmosphere, the Ocean, and Environmental Change
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The Atmosphere, the Ocean, and Environmental Change
GG 140 - Lecture 18 - Seasons and Climate Classification
Chapter 1: Sunlight Seasonality in the Arctic Circle [00:00:00]
Professor Ron Smith: I wanted to start out just by reminding you, we’ve already talked about how the tilt of the Earth drives the seasons. But I wanted to be sure you were aware of the standard definitions regarding the tilt of the Earth. The tilt of the Earth’s axis is 23 and 1/2 degrees in the current geologic era. And these things like the Arctic Circle, the Tropic of Capricorn, the Tropic of Cancer, and the Antarctic Circle are based on that tilt, right?
So the Arctic Circle is 23 and 1/2 degrees off the North Pole, The tropic of Cancer—of Capricorn is–both tropical lines are 23 and 1/2 degrees off the Equator, and the Antarctic Circle is 23 and 1/2 degrees off the South Pole. So these really are all based on the tilt of the Earth.
One definition of the Arctic Circle would be that it’s only points north of there that experience 24-hour sunlight in summer and 24-hour hour darkness in winter. Has anybody been north of the Arctic Circle? No one? I think you will someday. Try to get up there.
I lived–I taught in Norway for a year and had a couple of opportunities to travel north from there. In the summertime, I went up to visit my wife’s relatives up in northern Norway, which is about 69 degrees north, just slightly north of the Arctic Circle. And we enjoyed spending a couple of weeks with them, but that’s the period of time in which there is sunlight 24 hours a day. So the difference between night and day is lost a little bit, and the people adapt to that.
I remember the first night we were there, we were up drinking coffee at 1 o’clock in the morning with our relatives, and they suggested that we pop next door to visit their neighbors. And I thought that was a little bit odd, or at least I thought we should call before we dropped in at 1 o’clock in the morning. But they said no, no, no, they’ll be up.
So we just went next door, knocked on the door. They were up drinking coffee, enjoying the night. And so we sat there in their living room for a few hours. And a couple of hours later, 3 o’clock AM, their little 12-year-old came back. He’d been playing soccer with his friends down the street.
So the day just kind of rolls around there, and people catch catnaps off and on during the day. Sometimes they sleep in the middle of the afternoon. But you do lose your sense of rhythm because you don’t have the dark of the night and the light of the day to keep your clock going. So that’s an odd situation.
Several months after that, I was up in–in the month of November, I was on an oceanographic expedition up north of Svalbard. We got to 81 degrees north at the end of November, so that is well north of the Arctic Circle. And that’s only a few weeks from the winter solstice. So in fact, there was perpetual darkness up there.
So we were on the ship doing our work, putting over–taking water samples and measuring things, and the only light we had outside the ship were the spotlights and the floodlights from the ship itself. I was surprised, because I thought that even north of the Arctic Circle in the winter, I thought if I looked south, I’d see a glimmer, some hint that there was a Sun. No. There’s absolutely–even at noon, you’d look south, and the southern horizon–which you couldn’t see, but you’d look south–no glint, no light at all, so just as if the Sun pretty much doesn’t exist at that point.
And again, you lose your cycle. So as long as we had things to do on the ship, we were standing regular four-hour watches, four hours on, four hours off. And that had its own kind of rhythm. You’d catch sleep when you were off. But then our work ended, and we still had four days or five days to sail back to Norway. And then it was really tough, because without your work cycle and without the day-night cycle, you really were lost. And people would kind of be wandering around the ship, eyes glazed over, not knowing if they should be awake or asleep.
And it’s an odd kind of thing to be north of the Arctic Circle, really in either of the extreme seasons, either the winter or the summer. So if you get a chance, get up there and experience that different kind of a climate zone. Any questions on that?
Chapter 2: Seasonal Zone Shifts [00:05:03]
So we’re going to pick up on this seasonal theme. And how do the seasons work on our planet? And I added a little bit to this figure. I want to depict the Earth–there’s the rotation axis, so this is the Northern Hemisphere and the Southern Hemisphere, June, July, and August compared to December, January, February.
And remember, June, July, and August is Northern Hemisphere summer. The Earth is tilted towards the–the North Pole is tilted towards the Sun, so the Northern Hemisphere receives much more heat than the Southern Hemisphere. And all of the belts and zones defining climate shift northwards, both those in the Southern Hemisphere and the Northern Hemisphere.
So the band of frontal cyclones weakens and moves northwards. The ITCZ moves north of the Equator. Frontal storms which were down here before move up there. And I’ve put in five cities or five locations–A, B, C, D, and E–and we’ll talk about those in just a minute.
So six months later–December, January, February–the Sun is hitting the Southern Hemisphere more, so all the belts and zones–which, remember, are driven by the Sun, differential heating by the Sun, so it’s not surprising that they can shift when the source of the heating shifts–they all shift southwards.
So let’s look at these cities A, B, and C. So the thing that brings most of the rain to the world would be either the ITCZ in the tropics, which is that convergence of trade winds and lifting and deep convection, or the frontal storms, the comma clouds, the cold fronts, the warm fronts.
So in this season, city B will be getting rain from the ITCZ having shifted northward to its location, and city E in the Southern Hemisphere will be receiving rain from frontal storms that have shifted northward to its location. So rain in cities B and E in that season of the year.
Six months later, it’s A and D that are receiving the rain, A because the frontal storms have shifted southward to its location and D because the ITCZ has shifted southward to its location. So there’s a rainfall seasonality that can generally be understood with this kind of a cartoon.
Now, as you can imagine, it’s going to be a little more complicated than this because of the continents and the mountains. So here’s an actual depiction of the ITCZ shift. And in the Northern Hemisphere summer, it’s located generally along there. In Northern Hemisphere–in Southern Hemisphere summer, it’s generally located there. And it tends to shift more over the continents than it does over the oceans, at least if you look at South America and Africa.
Notice that the Equator comes right through here. And in both the Eastern Pacific and the Atlantic Oceans, the ITCZ never actually shifts south of the Equator. And I think you know the reason for that, because we talked about this in regards to hurricanes. Remember, there are no hurricanes in those two locations, and that’s because these two continents stretch far enough into the Southern Hemisphere that they draw water, cold currents up into this region.
And by keeping that area cold, it’s no longer possible for that area to be the center of convection, because that’s not where you have a lot–you have a lot of heat from the Sun, OK, but that’s being counteracted by these cold currents coming up in the oceans in those locations.
In the Indian Ocean, however, you have an exaggerated shift in the Intertropical Convergence Zone. And this is called the monsoon effect or the monsoon region or just the monsoon. It is an exaggerated shift in the ITCZ, giving strong rainfall seasonality to that part of the world. The word “monsoon” comes from an Arabic term “mausam”–Sharif, check me on this or look it up–meaning simply “seasons.” But it refers, in climatology, it refers to this exaggerated rainfall seasonality in southeast Asia and the Indian Ocean due to this big shift.
Now, that shift I think is due–most people believe is due to having this large continent up here, which can heat up in the summertime and draw air northwards and move that convergence zone up over the land. And then in wintertime, this cools down. We saw that in the continentality diagram from last time. This mass of continent cools down. When you get cold air over it, that cold air sinks and spreads out. You get northerly winds, and it drives the ITCZ way down to the south here.
So the ocean currents are playing some role in this. The continentality is playing some role in this. Otherwise, you see kind of just the shift that I had in the earlier cartoon for the Intertropical Convergence Zone. This will dominate the control of precipitation in the tropics, and I’ll be going back over this.
For example, let’s look at Rondonia in Brazil, which is south of the Equator. It’s somewhere in here. And what’s plotted here are the monthly precipitation values in millimeters. And notice that it has a January—or a November, December, January, February maximum.
Now, be careful how you use the language here. This is Southern Hemisphere, so this would be a summertime rainfall maxima. And we would explain this by a shift of the Intertropical Convergence Zone coming down to that location in the Southern Hemisphere summer. And notice, it’s a gigantic effect. July and August are very–June and July are very dry, whereas the summer months–be careful, these are the summer months–are quite wet. Question there?
If you look in India, here’s precip and temperature plotted here. Let’s just look at the precip. There’s a big maximum in June, July, and then it starts to decrease in August and September. We’re in the Northern Hemisphere now, and this would be Northern Hemisphere summer. So this location also has a summertime maxima in precipitation due to the northward shift of the Intertropical Convergence Zone.
So on a typical day, you’d have deep convective clouds, heavy precipitation, sometimes flooding. And that would be a typical condition in that season of the year due to the northward shift of the Intertropical Convergence Zone.
Let’s look at this interesting site–I don’t know how to say it–Yaounde, Cameroon. Pretty close to the Equator. And you could probably see this coming, couldn’t you? So we’re here somewhere, and the point is that this doesn’t just appear there and appear there. It actually moves back and forth.
So if you are close to the Equator, you are actually going to get two rainy seasons a year, one when the ITCZ is moving northward past you and the other six months later when the ITCZ is moving back southward past you. So these mark the extremes of the movement, but in between, you’re going to get, in some cases, a double passage of the ITCZ. And that’s what you see here, generally kind of a double rainfall peak due to the double passage of the ITCZ.
You can see this in satellite images too. For example, here’s an image of Africa and the Pacific Ocean. There’s the ITCZ in one season. And you see most of the precip is up in here. Here’s the other season, where there seems to be two. Maybe there is a weak one in the Southern Hemisphere, but most of the precip in Africa has shifted down in here. And this is all cloud-free and not getting any precipitation at that time.
Now, the polar front also moves. Everything moves together. And so in the Northern Hemisphere summertime, the polar front is weak, and it moves northward. Weak, of course, because the pole and the Equator are getting about the same amount of sunlight. Therefore, you don’t have very much of a north-south temperature gradient, and that’s what defines the word “front.”
However, in the winter season, polar latitudes are getting no radiation from the Sun, the Equator is still getting a lot, and therefore you build up a strong polar front, and it moves southward to roughly 30 degrees north. But you see it meanders a little bit around there.
So if you looked, for example, at Los Angeles and plotted up its precipitation, it’ll get precipitation starting in November, then December, January, February, March, decreasing into April. This is Northern Hemisphere, so we would call this a wintertime precipitation. Here’s the temperature plotted up here, so that rain is coming in the cool season, when the temperature is down a little bit. So that is a mark of this kind of a phenomenon, where it’s getting its rain from frontal cyclones that have moved equatorward in the cool season.
Here is Rome in Italy at 42 degrees north, and you have a similar story. There’s the temperature in red, and it’s in the cool season of the year. We’re still in the Northern Hemisphere, so the cool season gives the precipitation due to frontal cyclones.
And here is Perth, Australia, 32 south. Now, be careful. We’ve got to flip everything around here. So the rainy season is in June, July, and August. That is wintertime, as you can tell, because the temperatures are minimum at that time of the year. And so that is a, in this case, a northward shift or an equatorward shift in the polar front, bringing frontal cyclones into Perth in the wintertime.
So this all fits that picture of shifting belts and zones following the Sun. Any questions on that?
Chapter 3: Precipitation Seasonality [00:16:57]
Now, some of this is nicely summarized in a diagram that appeared in a book by Wallace and Hobbs. They took rainfall data from the Northern Hemisphere and plotted it with little clock arrows, little clock hands. The length of each arrow is the degree of seasonal change in precipitation. So the longer the arrow, the bigger the difference there is between the dry season and the wet season. And the orientation of the arrow tells you in what season of the year do you get the rainfall maximum.
And the key is over here. So a poleward-oriented arrow, such as these here, would have a July 1st maximum. That is a summertime maximum. This is Northern Hemisphere, so I can use the word “summer.”
And of course, now we know what that would be. That would be the shift of the ITCZ northward to bring you rainfall in the summertime. And if we look at India, Mumbai here or many other places in southeast Asia, we find those northward-pointing arrows. So every time you see the northward-pointing arrows, including some over here, that is the northward shift of the ITCZ reaching that location in the summertime.
But let’s look at some other locations. For example, in the Mediterranean region, all the arrows are towards the south, which means a wintertime maxima. And that would be, of course, the cyclones, the frontal cyclones. The polar front strengthens and moves southwards, and then throughout the winter, you have storm after storm after storm, frontal activity that’ll bring rain to this area.
And if we go around to California, we find something a little like that. The arrows are a little bit tilted. They’re generally tilted aiming towards the Equator, with some lag. But that would be called, as I’ll describe later–California is often described as having a Mediterranean climate, because it has the same kind of wintertime precipitation that does the Mediterranean.
So that’s a pretty handy diagram. Now, there are some oddballs. There are some things in weird directions. For example, the arrows here pointing towards the east, October 1. Anyone want to make a suggestion of why the islands here have a fall maximum? Hurricanes. And so you can find exceptions like that, where special types of events may be the things that bring precipitation to the region of interest. Questions?
So this is a reminder, to read you something here. These questions we’re asking about what causes the seasons, as you might imagine, these are old questions. People have been asking themselves these questions for years. And Herodotus was very curious about it. He was Greek, perhaps the first historian, some would say. And he spent most of his life in Greece. But once in his life, he traveled down to Egypt. And here’s what he writes about that.
“About why the Nile behaves precisely as it does, I could get no information from the priests or anyone else. What I particularly wished to know was why the water begins to rise at the summer solstice, continues to do so for 100 days, and then falls again at the end of that period so that it remains low throughout the winter”–he’s talking about the water level in the Nile River–“remains low throughout the winter until the summer solstice comes around again in the following year. Nobody in Egypt could give me any explanation of this, in spite of my constant attempts to find out what was the peculiar property which made the Nile behave in the opposite way to other rivers?”
So I’ll leave it there, but can anyone answer Herodotus’ question? Why does the Nile behave the opposite to other rivers? Want to venture a guess? Yeah.
Student: In the summer months it’s getting rain from the shift of the ITCZ, whereas in the north where he knew, the frontal cyclones were contributing more?
Professor Ron Smith: That’s exactly right. So when he says “other rivers,” he’s talking about all the rivers he’s ever seen in his life, which is generally the rivers coming off from the Mediterranean region. And that’s going to be mostly wintertime due to the southward shift of the polar front. The Nile, on the other hand, actually, the water doesn’t originate from here. The water originates further south.
Let’s go back to my little arrow diagram. Yeah, there it is. So here’s Herodotus spending his life here looking at rivers that have a wintertime or a springtime maxima, and now he’s down in Egypt looking at water that actually fell down in this region. The source of the Nile, we now know, is up in the Horn of Africa, over in this region, and that has a summertime precipitation.
So he’s really–the answer to his question then is exactly what you said. It’s the fact that the Nile is controlled by the water that falls here, which is controlled by the northward shift of the Intertropical Convergence Zone. Questions on that?
So now we’re going to–oh, I want to make this one last point about seasons. So you being raised around here or in North America, these latitudes, probably think that seasonality has a lot to do with temperature. And you’d be right, but that’s a narrow, parochial attitude from where you were raised. If you were raised in the tropics, you would think that seasonality has everything to do with rainfall.
If you make a figure like this–and I think you might be able to do this with some of the data that you’re given in the problem set–if you take the 12 monthly precip and 12 monthly temperature points and plot them and connect them with lines that link them up in chronological order, they’ll form some kind of a figure on this temperature, precipitation map.
And in the tropics, for sites in the tropics, that figure looks like this. You get a big precipitation seasonality, but not very much change in temperature. On the other hand, in mid-latitudes, you get a big temperature change but not much change in precipitation.
Now, this wouldn’t apply for every mid-latitude site and every tropical site, but generally this is the rule, that seasonality in the tropics is referring to a precipitation seasonality, a wet season and a dry season, whereas seasonality in mid and high latitudes is referring primarily to a temperature seasonality. So seasons mean different things depending where you were brought up. So don’t get caught in that when you’re talking to someone from a different land.
Chapter 4: Climate Classification [00:24:49]
So now we turn to this question of climate classification. We’d like to codify all of this. And the way forward on this was over 100 years ago by Koppen, and then later others have modified this slightly. So there are a few different versions of this scheme around, but generally it’s called the Koppen climate classification scheme. It uses the 12 monthly temperature and precipitation values with thresholds to assign each point on the Earth to a small set of climate zones.
And the way those thresholds are designed is that it tries to capture those aspects of climatology that would control the vegetation that grows. So ideally, if the Koppen scheme works, in a particular climate zone defined by temperature and precipitation, you would find a particular kind of natural vegetation, because it’s adapted to that particular climate zone.
There are six broad categories–tropical, dry, temperate, continental, polar, and highland climates–and then as you know already because you’re working on the problem set, there are subcategories under those. And well they’re a little bit–let’s face it, they’re a little bit messy to work with.
This is a table I pulled off of a recent published paper on the Koppen classes. And it may be that these definitions are a little bit different than the one you have in the back of the book, but you get the point. That is, you define the broad category, like the A climates, in terms of whether the minimum temperature is higher than a certain value, and then you break it down into subcategories based, in this case, on precipitation.
The arid climates’ precipitation annual total, less than 10 PTH. I don’t know what that is. But then it’s subdivided into two categories. Desert would be the very dry areas, and the steppe would get a little more precipitation, and so on. So using the temperature and precipitation values, you go through and you classify each area by its seasonal cycle and the mean values.
And you end up with something like this. I think this is from your textbook, or you have a diagram like this in your textbook, where the color schemes represent the different climate categories. For example, the moist tropical climates are the A climates, Aw, Af, Am. The dry climates, you find they’re the B climates. You find some of them in the desert areas here and here. And in the desert Southwest, you find some B climates, and so on.
This is the Western Hemisphere, North and South America, and then we do something similar to that for Asia, Africa, and Australia as well using the same scheme. So it’s not zonal, but you can see zonal aspects to it. But all of those factors that I mentioned the other day are working, not only the zonal aspects of the general circulation, but also the mountains, the continentality, and the cool and warm ocean currents that are bringing up water to the coastlines of these continents. They’re all playing some role in this.
North America looks like this. It’s pretty simple east of the Mississippi. West of the Mississippi, it’s more complicated, because you have some mountainous areas and some contrast between wet and dry areas.
Let’s look at three cities in North America. Sacramento, in California, now, that is a classic Mediterranean climate, right? The precip is in the wintertime when the temperature is down. So Sacramento has a Mediterranean climate, and that would be a Csa climate in the Koppen scheme.
New York City, close to us, this might as well be New Haven. New Haven’s very much like this. Pretty much the same rainfall throughout the year, but a very strong temperature seasonality. That’s a Dfa climate.
Miami has a summertime precip mostly from thunderstorms in the summertime, and then Denver is a drier climate in general, also–well, it’s a more of a spring maxima. We talked about why that is. In the springtime, you tend to get these severe thunderstorms that occur in that part of the world, because you get the right jet stream aloft and the moist air coming in from the Gulf of Mexico.
But that extends to some extent throughout the winter—throughout the summer as well. The reason why the winter is so dry is because in the wintertime, those storms coming from the Pacific lose their rain in the Rockies. And by the time it gets to Denver, they’re pretty dry, so you don’t get much precipitation in the wintertime in Denver.
Let’s look at Africa for a second. There’s the Koppen scheme for Africa. And let’s look at the Af climates, which are the tropical rainforest. That’s this deep blue area here, a lot of water and dense tropical forest vegetation.
Then if we look at the BSh climate, which is the steppe climate, that would be up in here, these little golden or yellow areas. They get one pulse of precip every year when the ITCZ moves northward, and then it’s dry for the rest of the year. So a brief rainy season providing enough water for the grass, but you see now it’s browned out–well, still a little bit green up there–and then that rain moves back to the Equator and then back into the Southern Hemisphere later on. So that gives you the steppe climates of Kenya, for example.
By the way, these animals, the wildebeests, are mobile animals, and very often they will travel several hundred miles trying to follow that shift in the ITCZ. So they don’t understand it, but they have evolved to know that if they move southward, they can get a longer season where the grass will be green, and they can get good eating that way.
And then you have the BWh climate, the really dry climates up in here. That’s the red one, the deep red one, and of course that’s a place where it really seldom rains in any season of the year. The ITCZ doesn’t get that far north, and the frontal cyclones don’t get that far south. So you’ve got this zone in between where you’ve got the–it’s the descending branch of the Hadley cell, of course–the air is descending there, keeping it from precipitating–but that’s because the ITCZ never reaches there and the polar front never reaches there. It’s this dry area in between.
India. Let’s take a look at Europe, for example. I want to get back on this issue of the Mediterranean climate. This yellowish is the Csa climate. You see it all along the coast, north and south coast of the Mediterranean, and that’s why that is called a Mediterranean climate. Whoops. And so when you look at Lisbon in Spain [correction: Portugal], it has that characteristic of a cool-season precipitation. If you look at Jerusalem, it has the characteristic of the cool-season precipitation. Cairo, the same thing. So, you know, that’s a pretty common–that’s a uniform climate zone.
And then I’ve already shown you this, but switch over to Sacramento. Well, it looks just the same. So that’s why we say Southern California has a Mediterranean climate, because it fits that general picture from the Mediterranean. And again, I’ve said this enough times, but this one is due to the southward shift of the polar front and the rain that comes in frontal cyclones.
Chapter 5: Examples of Seasonality [00:33:44]
Let’s look at, let me–I think we’re close to the end here. We may finish a few minutes early. That’d be nice. Let’s look at a couple little additional examples of seasonality. Here’s a couple of satellite images taken from North America—from South America, where the Maranon and the Ucayali join to form the Amazon.
And at the end of the rainy season, those rivers are pretty swollen. Every place you see blue, that’s water. But at the end of the dry season, the flow in those rivers is very much reduced. And what the purple color here is are those dry sand banks on the side of the river. Because the river is now lower and moving much more slowly, you have barren, unvegetated areas.
And of course, this has been meandering through time. On the field trip the other day, we were talking about how that meandering river has flattened out the Quinnipiac salt marsh. Well, you see here, everywhere you look, old places where that river used to be. So that’s been meandering back and forth over geologic time, leaving all these little remnants of meanders in the river. And most of that erosion takes place in the wet season, when the flow of the rivers is high.
Here in New England, the Dfa climate, as I mentioned, it’s a temperature seasonality, rather than a precipitation one. And so the summertimes look like this, with the deciduous trees being out. Then they drop their leaves, and it gets very cold and the snow falls, such as you see there.
I put together a little set of satellite images here to make another point about New England’s seasonality. Looking down from a satellite, you can map out the distribution of vegetation using something called NDVI. It’s the Normalized Difference Vegetation Index. It’s the ratio of the reflectivities in two different visible bands. Oh, sorry. It’s the ratio of the reflectivity in the near-infrared to the red. It turns out that vegetation has a very strong signature if you look at those two bands.
So here’s four months, April, June, August, and November. Now, as you first glance at that, you may say, well, they look a lot alike. And I guess they do. But I’m going to take some differences between these plots to try to bring out how things have changed with season, how the vegetation has changed with season. And that’s shown in the next plot here.
In this one, I’m calling it a leaf-out index, because I’ve taken the June NDVI and subtracted it from April. So I’m finding how much the vegetation has changed from April into June. Well now, if you’ve lived in New England at all, you know that the big event between April and June is the leaf-out of the deciduous trees. Usually that happens in mid-May here in Connecticut.
And so when I take this difference, what I’m mostly going to be seeing is the distribution of deciduous trees–oaks, maples, and so on–and that’s what we see. So the seasonality in New England, while it is–at its root it’s a temperature seasonality, it also has a big impact on the vegetation. Leaf-on, leaf-off controlled by that temperature, and you can see it from space.
Now, in areas where there hasn’t been much change, those might be conifers. They look about the same in the winter and summer. Or they might be barren areas, where there’s no vegetation in either season. Is that clear what I’ve done there? I’ve subtracted the NDVI of April–June minus April to get the difference in vegetation. And in this case, it’s going to show me where the deciduous trees have leafed out because of the seasonality.
I’m going to do the same trick now for August minus June to see how vegetation has changed over the summer. And you see these little pockets of red are where there’s a big positive change, where there’s much more vegetation in August than in June.
And I’m calling that the growing season, because what that’s going to do is pick out the agriculture. Farming in this part of the world is usually a summer activity. You plow and plant in the spring, and then your crops grow–your corn or your wheat or whatever grow over the summer months, and you harvest in August, September, October. So this difference between those two months is largely going to be a measure of where agriculture remains.
There’s not much left in New England, but we can see exactly where it is. There’s quite a bit of agriculture in western New York State near the Finger Lakes. There’s some in the very upper Hudson Valley and up into Canada around Montreal. There’s quite a bit down here in eastern Pennsylvania. Those are the Amish farms growing their crops down in eastern Pennsylvania. A little bit in Connecticut up around Springfield in Massachusetts. And some places in Connecticut you find some agriculture.
But again, this is seasonally driven. So human activity here is being driven by the seasonality and ultimately which goes back to the tilt of the Earth’s axis.
Any questions on this?
Now, another aspect of seasonality for New England–and this is particularly timely, because we were just out on the river the other day. So I’ve already told you–and I’ve shown you, in fact–that New England gets about the same precipitation in every month of the year.
Why is it then, if I look at the daily discharge data from the Quinnipiac or most other rivers, that there is a seasonal cycle to it? More discharge in the winter, less in the summer, June, July, and August. Winter, less in the summer, June, July, August. It didn’t drop quite as much in the summer of 2006, but then it dropped even more in the summer of 2007.
This is a log scale, so this is a very significant change. This is an order of magnitude difference. Look, this is 200 to 300 to 400 cubic feet per second down to, well, 60 or 70 cubic feet per second down there. So a gigantic seasonality in the river flow and yet not the rain. What’s going on there? You’ve got to tell me this one. Somebody tell me this.
Student: The snow melting?
Professor Ron Smith: Part of it, but not in the Quinnipiac. That would explain some of the rivers further north in New England, but the Quinnipiac doesn’t get much snow. It’s down here in Connecticut, so that wouldn’t explain it. We need another explanation for this. Yeah.
Professor Ron Smith: Evaporation. So remember, evaporation is–(a), it’s very strongly controlled by temperature, and since New England has a temperature seasonality, you’re going to get a big seasonality in evaporation. Also, from the trees. The deciduous trees evaporate water from their leaves. As soon as the leaves fall off, you don’t get that so-called evapotranspiration. So that’s a big seasonal cycle.
So during the wintertime, most of the rain that falls ends up in the rivers. During the summertime, most of the rain that falls evaporates, and much less flows down the river to the sea.
So here’s the question. Does New England have a wet-dry season? Well, that depends on how you define it, doesn’t it? Because it doesn’t have much of a seasonal variation in precipitation, and yet it’s got a strong seasonality in the amount of water in the rivers because of the evaporation effect.
Questions on that?
And I think that’s the last one, almost the last one. So I took a couple of satellite images from Alaska, one in June and one in November. And June, early summer, there’s still a lot of snow cover from the previous year. The melting has begun there, but it takes several months of warm conditions before you get rid of most of that snow. So you see snow, a little bit of barren land, some water out here.
In November, next month, it looks so different. Well, part of that is because there happened to be more clouds on that day, but I wanted to make a couple of other points. There’s actually less snow then than there was earlier, because the snowfall is just beginning.
But here’s the thing I wanted to show you that intrigued me. I hope you can see that from the back. There’s a curious little line, a little dark-light contrast right there. And that is evidence of the low Sun angle. Remember, in this season of the year in Alaska, high latitudes, the local solar zenith angle is going to be quite large. That Sun is slanting in at a very low angle. And that little line I’m seeing there is the shadow of this mountain range cast across the valley onto the facing slope.
So not too relevant for climatology, I grant you, but a reminder that the Sun angle is very low at that season. And that itself, I guess, is a kind of seasonality. If you lived down in this valley, you wouldn’t see any sunlight in the wintertime, but you might in the summer when the Sun’s higher in the sky. But then you’d have snow at that earlier part of the year.
Chapter 6: Seasonally Controlled Events [00:44:06]
And then just to remind you that it’s not just this smooth cycle of the seasons that matters, but it’s the events that can also matter and would be normally considered to be part of the climatology. For example, hurricanes, which are discrete events, and you may not even get one in any particular year, but still, they occur in the late summer or fall in either hemisphere.
Severe Oklahoma thunderstorms are a springtime phenomenon. Nor’easters in New England are wintertime. California fires are primarily in the fall. Antarctic ozone hole–we’ll discuss that later in the course–occurs in October. And El Niño when it starts up–we’ll discuss this too–usually starts up in December. So it’s not just a smooth cycle of the seasons, but it’s the way the seasons control events that can be quite important as well.
Any questions on this or anything else about seasonality? I think that’s it today.
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