GG 140: The Atmosphere, the Ocean, and Environmental Change
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The Atmosphere, the Ocean, and Environmental Change
GG 140 - Lecture 17 - Seasons and Climate
Chapter 1: Climate Definition [00:00:00]
Professor Ron Smith: We’re starting a new subject and I wanted to just remind you what we’ve done, so you see how this fits into the larger picture. We spent quite a few weeks looking at physical processes in the atmosphere, like why do winds blow, the geostrophic law, hydrostatic law, thermodynamics of lifting air, how clouds form, how rain forms, how radiation causes the layering in the atmosphere. So that was an important section. Now we’re moving into some other sections that are a little more descriptive. We’ll still be doing some quantitative work as we move through the course, but for the moment, we’re kind of putting some of these pieces together to see the big picture.
So we looked at the general circulation of the atmosphere. And then last week we looked at storms, various storm types. Now we’re going to put those two things together and try to look at the distribution of climate around the globe. And I’m going to do this in part–I’m going to start out very briefly ignoring the seasons. But we can’t get very far with describing climate without understanding the seasons, how the seasons work in various places, because that’s such an important part of climate also.
So what’s up here says that we can describe climate pretty well if we know the monthly values of temperature and precipitation. That would be 24 numbers. With 24 numbers, what are the average precip values for each month? What are the average temperature values for each month? That’s pretty much a full description of climate. The way you would normally do that is to collect data for 10 or 20 or 30 years, and then you’d average together all the Januarys to get the average January, all the Februarys to get the average February. And you’d plot that up as the average seasonal cycle. And that’s typically the definition of climate.
Now, storms come into that because very often, storms are controlling the precipitation. But you might want to include some other aspects of climatology not mentioned here. For example, if an area is prone to hurricanes, probably a description of climate should include that as a little separate item, because it’s important for the people that live there and the farmers and the fishermen and so on. So the definition of climate is not quite cut and dried, but I would say that for the most part, you need those 24 numbers in order to describe the climate of any particular place on the Earth.
Chapter 2: Latitudinal Climate Variations [00:02:55]
The factors that control climate are primarily these. Latitude, because of the way that the general circulation tends to give a latitudinal structure to climate–the Hadley cells, the belts of storms, the belts of deserts. So as you go from latitude to latitude, you’re kind of moving through these different zones. So latitude’s very important, but it’s not everything. Mountains play a very big role, something called continentality–which I’ll describe in a minute–and ocean currents also play a role in determining what climate it is. And I’ll be going through these today.
So just to remind you about the general circulation, you’ve got the symmetry about the equator, with a Hadley cell rising near the equator sending air poleward in both hemispheres, descending near the tropics of Cancer and Capricorn, and then the trade winds under the influence of the Coriolis force–these winds don’t move directly back towards the equator. Instead they get deflected to the right and to the left and end up being the northeast and the southeast trade winds. And then you’ve got the belts of storms, the belts of deserts.
And so if this were the full—if you took away the continents–if you either had a uniform ocean-covered planet or a uniform land-covered planet, then you could probably pretty much end with the latitudinal control. That’d be really the dominant factor. But in fact, you’ve got mountain belts, you’ve got continents, you’ve got oceans, you’ve got ocean currents, all of which give some east-west variations in climate. So it’s not just latitude control, as this diagram would have you think, but it also depends where you are because of the mountains, the continents, and the ocean currents. Stop me if you have any questions here.
So when you look at an annual precip map–this is for land only. So this is traditional data collected from rain gauges, summed up over many years, and summed up over all the seasons as well. So this is annual precipitation. You do see the latitudinal control from the general circulation. You see a belt of rainforest near the equator, expressed in South America, in Africa, and in the continent—in the so-called maritime continent off of southeast Asia.
You see the belt of deserts in both hemispheres about here, but it’s best expressed in the Sahara desert and the Arabian peninsula. Then it tends to slide northward a little bit because of the mountains here. And we see it expressed a little bit in the desert southwest, but not so much in Florida, for example. And then the mirror image of that coming down here–you see the belt of deserts especially on the west coast of South America, the west coast of Africa, and most of the interior of Australia.
Now the high latitudes appear as areas with very small precipitation. I hesitate to call them deserts, however, because remember, because they’re cold, the evaporation rate is very slow. And so what little precipitation you get could cause a very moist climate there. So precipitation is not the only factor. Evaporation plays an equally important role in determining the climate of these various regions. So as you’ve seen, then, you can see this latitudinal control, but it’s also quite broken up by these other factors–the mountains, the continents, and the ocean currents.
Probably one of the best examples of that is in South America, where you’ve got a wet west coast and a dry east coast in these mid-latitudes. Well, that’s because this is the belt of westerlies. The winds are coming like this. They have to climb over the mountains there. As the air rises, it cools adiabatically, and you get rain on the west coast. But then the air descends and you get dry conditions on the east coast.
You go up here–it’s just reversed. The dry is on the west coast, the wet is in the interior, but this is the belt of easterlies. So you get this orographic effect, this mountain effect, kind of reversed. You find it elsewhere, but nowhere quite as nicely expressed as in the Andes and the southern Andes there. So that’s a good example of where mountains are screwing up that simple latitudinal distribution of climate.
When you include the oceans–and now this comes not from traditional land-based weather stations, of course. This is a satellite product. We’ve had satellites in orbit that can detect precipitating clouds for a decade or more, and so we can begin to put together a full global picture of precipitation. You still see the maxima over the continents, just where we had them in the other pictures–the belt of rainforest is here and here.
But you also see what’s going on over the oceans. And rather dramatically, you find these east-west elongated structures, especially in the North Pacific. And remember, the equator is right here. And that is an expression of the inter-tropical convergence zone, where the two trade winds are converging. Notice it’s not at the equator. It’s north of the equator. In the Western Pacific, you have a double structure, indicating that the inter-tropical convergence zone spends a few months a year here and a few months a year there. So it tends to move back and forth a little bit in the in the western tropical Pacific. Generally very dry conditions where you see the purples, and wetter conditions where you see the warmer colors there.
Questions on this? Again, this is annual average, so the seasons don’t show up in this yet. I’m going to get to the seasons in a couple of moments.
When you take a diagram like this and average it east to west, it’s called a zonal average. A zonally averaged precipitation would be, at each latitude, just average around the globe to get the total precipitation. And that gives you a diagram like this. Now, this is broken up by season, or by month of the year rather, but they’re not so terribly different. Basically, it’s a three-humped profile. Three peaks. The biggest one is somewhere around the equator, although you see it moves around a little bit with month.
And that’s the inter-tropical convergence zone. Winds converging at low altitudes. The air goes up, deep clouds, heavy precipitation. Then you get the belt of deserts. So even though you have all these interferences due to mountains and ocean currents and continents and so on, still you see that three-banded structure come through that’s based on the general circulation. The ITCZ, the two belts of deserts where air is descending in the Hadley cells. And then these are the belts of mid-latitude storms. This is up in the westerlies, where you’re getting all these comma clouds and cold fronts and warm fronts moving west to east, bringing you precipitation. So three nice peaks, a certain symmetry–that’s what the general circulation is doing.
Questions here? OK. We can zoom in to a particular continent, like North America or our own country. Here is the annual average precipitation. The values are given here in inches. The dry values are the deep reds and the highest precipitations are the blues and the grays. And the highest precipitation in our country generally occurs in the west coast, especially the Pacific Northwest, where you get a lot of moist air masses and frontal storms moving in off the Pacific Ocean, hitting these mountains.
And so you get the combination of cyclonic frontal storms which are already precipitating, then the air has to run up over the mountains, and you get the extra precip there. Most of that water is removed that moves on from the Pacific Ocean by the mountain ranges of the west coast, leaving the interior rather dry. That’s kind of a rain shadow effect.
And a fundamental question would be, OK, if I’ve rained all the water out, or for whatever reason I’m not getting the precipitation downstream of the coastal mountains, how far does that effect extend eastward? And you see it here. By the time you get west of–into Colorado–and then eastward of that you’re gradually starting to pick up your precipitation again. But that’s because you now have got some new water sources. You’ve got moist air moving up in this region off the Gulf of Mexico, and along the east coast, you’ve got some, even though you’re in the belt of westerlies, you occasionally have easterly flow that can bring moist air in off the Atlantic Ocean. So eventually this rain shadow reduces in its effect when you pick up some other water vapor sources.
Student: In about the middle of the country, it has that line–why is it so–instead of being longer, why is it so abrupt a change?
Professor Ron Smith: You’re speaking about this gradient through here? Well, basically, remember you’re also descending. You’re coming down into lower altitudes as you come across the Great Plains. And that, again, helps this moisture to get up in that region. But basically this is just the ending of the–the further you get east, the more you have the influence of that new water vapor source. I’d say that’s the primary thing controlling that gradient region.
Chapter 3: Orographic Precipitation [00:14:10]
So just to zoom in on California, then–and again, the blue colors are high precipitation and the reds are low. So there’s a coastal mountain range that has precip. Then the central valley, which is quite dry, and then the Sierra Nevada range which lifts the air again. So this is a nice example of orographic precipitation. Orographic precipitation is defined as precipitation caused by the lifting of air by a mountain.
And the thermodynamics of it you are already quite familiar with. You lift the air. It’s moving to lower pressure because pressure decreases as you go up in the atmosphere. The air expands, cools adiabatically–you’ve heard this story over and over now. It drops the saturation vapor pressure. The relative humidity finally exceeds 100%. The excess water vapor condenses and, under the right conditions, you can get precipitation coming out of those clouds.
So orographic precipitation is really a big control. And the rivers that provide water to the people and the agriculture in California basically get their water from the mountains. So the water that falls in the mountains by this process pretty much gives California the water it needs for agriculture and for human consumption. It’s a pretty dry state except for the influence of the mountains.
Oregon is a similar thing. Look at the contrast between the eastern and western part of the state. Precipitation in the west can be 140 to 160 inches. That would be this much water. In the eastern part of the state where that air has climbed over the mountain range and now it descends again, having lost most of its water, the precipitation is less than this amount. So that’s a big contrast. And if you drive across there, boy, you see it immediately in the vegetation. A kind of rainforest here on the west coast of Oregon. You begin to see ponderosa pine, then they get sparser and sparser. And by the time you get to eastern Oregon, there’s mostly grasses and just a scattered pine tree or so.
Alaska is another good example of this. There’s a coastal mountain range. Here’s precipitation. The purples are the high precip. And you get that moist air coming off the Pacific Ocean, giving you heavy rainfalls in the mountains. And then north of that and east of that, the precipitation dies out quite quickly. In the north, you’re getting very low precipitation, because as I mentioned, the air that gets to those very high polar latitudes is cold. If it’s cold, it means that it’s gone through a cooling process as it moved from low latitudes. And the cooling has probably already removed most of its water vapor. So cold air, remember, can’t hold very much water vapor. And therefore you can’t wring out very much rain from cold air, because it doesn’t have very much water to begin with. So finding these seemingly desert-like precipitations–200 millimeters, 20 centimeters–is a result of the air being too cold to provide much water vapor to that region.
Arizona is complicated because it’s got mountains in certain areas and low-lying areas, but this is the mountainous area, and pretty much the high precipitation is controlled by the mountains. But generally, it’s a pretty dry state. Notice the highest precipitation here. What’s that say? Yeah, the highest precipitation would be 32 inches, about this much, up in the highest mountains.
I wanted to just mention this briefly. Occasionally I like to give you some idea of what we do in our department. I’ve had an interest in orographic precipitation for a number of years, and I noticed a few years ago that there’d been a lot of studies of it in mid-latitudes but none in the tropics. So my group put together a field experiment last May. I took a leave from Yale University last April and May to do a project down on the island of Dominica in the eastern Caribbean, where we were looking at orographic precipitation in a tropical location where you have the steady trade winds.
This is a precipitation map for the island of Dominica. Upstream over the ocean you get maybe two millimeters per day on average. But yet, over the high mountains, you get 12 to 14 millimeters per day. That’s about that much rain each day, on average, over the year. And then to the west, almost absolutely dry. So a very strong example of orographic precipitation.
And we went down there for five weeks and flew research aircraft back and forth over the island through the clouds, trying to understand the details of how the clouds are formed, how the precipitation is formed, and how this rain shadow in the lee comes to be. I don’t have time to tell you about the results of that. But that’s the kind of research that one can do to understand in more detail many of these features that I’m just glossing over and saying, well, look, here’s an interesting factor, here’s an interesting process, here’s an interesting part of the Earth. They’ve all got their own physics, only a small fraction of which is really understood. So one can chip away at this and try to understand more and more of the details of how the atmosphere works.
Chapter 4: Continentality [00:20:18]
So continentality. What is that? Well, it primarily has to do with the fact that oceans store heat very effectively, whereas land surfaces do not. So it’s easy to change the temperature on land. You don’t have to provide very much heat or remove very much heat to warm up or cool down a land surface, because remember, all you’re doing is heating the first few centimeters of the soil. And that’s not very much heat capacity. So you put in a little bit of heat, the temperature rises. You take out some heat, the temperature drops.
You try to do that over the ocean and two things get in the way. First of all, water has a very high heat capacity. That is to say, per kilogram, it’s got a large ability to store heat. Second of all, water is a liquid, which means it’s not just the first few centimeters you have got to cool down or heat up. It’s probably the first several hundred meters, because that water is constantly being stirred. So you can’t just heat or cool the top few centimeters. You’ve got to heat or cool the top few hundred meters of the ocean, which is an enormous heat capacity.
So generally temperatures don’t change very much over the ocean, either between day and night–a typical temperature difference between day and night over the ocean is about a half a degree Celsius, whereas on land it could be 5, 10, even in some places 15 degrees Celsius or higher. And between the seasons, between the warm season and the cold season, you’ve got a similar difference. In the ocean you might only be able to have a three or four degree temperature difference. Over land, you might have 15, 20, 30, 40 degrees Celsius difference between winter and summer. So continentality is this contrast in the heat storage capability of land versus ocean.
So here is a plot of the annual range of temperature. It’s the typical maximum temperature versus the minimum temperature. First of all, you don’t see much of a range in the tropics anyway, over land or sea, because there’s not that much difference in how the sun hits the tropics between winter and summer. I’ll say more about that later.
So let’s shift our attention more to mid-latitudes, especially to the northern latitudes where you have a couple of very large continents and then oceans in between. Look, the oceans barely change their temperature between winter and summer, but the continents change their temperature by a great deal. For example, up in the northern part of eastern Eurasia, you could have an annual range of 55–I think that’s in Celsius–of 55 degrees between winter and summer. That’s an enormous temperature difference. That’s sweltering in the summer and so cold it’s hard to describe how cold it is in the wintertime. So in the middle of continents, you get this enormous temperature range.
Continentality. Any questions on that? It’s a big player in climate, as you can imagine. You see it in the southern hemisphere too, but not so much, because by the time you get to midlatitudes–you know, the big difference between northern and southern hemisphere in this regard is that the northern hemisphere is the hemisphere of continents. The southern hemisphere is more the hemisphere of oceans. The percent land mass is much greater in the northern hemisphere that it is in the southern hemisphere, especially if you’re excluding the equatorial region. So by the time you get down to the mid-latitudes, you really only have Australia, New Zealand, a little bit of Africa, and the tip of South America. Whereas in the same latitude belt in the northern hemisphere, you have massive continents. So there’s this asymmetry between the northern and southern part of our planet, climate-wise, because of the way continentality works differently in the two hemispheres.
You notice, if you did a zonal average of this quantity, you’d find a much bigger annual range in the northern hemisphere than in the southern hemisphere. Well that has nothing to do with how the sun hits the Earth. Over a year that’s pretty much symmetric between the northern and southern hemispheres. But it has everything to do with the fact that there’s more continents in the northern hemisphere than there are in the southern hemisphere.
Here it is over the United States, the annual range of temperature. It’s largest in this interior region of the west, where it’s dry and high. If you have higher elevation, you tend to have a larger annual range, because there’s less air above you. You can cool down more at night. And it’s drier. There are fewer clouds, more sunlight. It can heat up more during the day. So you see the continentality here expressed in a little bit of a confusing way. Because you’d say, well, that point’s just as much in the middle of the continent as that is. Yeah, but there are other things having to do with elevation, clouds, that will cause some change in that annual range, even within the same continent.
Alaska has a very small annual range near the ocean. Of course that’s because it’s close to the ocean, which is buffering the temperature. And then up in the interior, especially in high latitudes where there’s a great deal of difference between summer and winter insolation–solar radiation–you get a very big annual range in temperature.
Chapter 5: Ocean Currents and Climate [00:26:39]
And finally, I’ll mention ocean currents. I’ve shown this diagram before. But generally in each of the ocean basins, you have a gyre, a wind-driven gyre. But it carries, generally, cold water near the equator—sorry, cold water towards the equator and warm water away from the equator, or we could say, poleward. So if you lived on this side, along this coast, you’d have a cold current off of your beach, whereas here, you’d have a warm current off of it.
So for example, if you’re a Californian, you’ve got the cold California current coming down along the coast. That’s why it’s always exciting when you go swimming in southern California. Beautiful nice warm climate, beautiful beaches, but be prepared for a shock when you jump in the water, because the water’s always cold there, because you’ve got the California current. Whereas when I go vacationing down on Cape Hatteras, for example, in the summertime, you can walk in the water and it’s very, very pleasant and warm, because that is warm water being brought northwards in the Gulf Stream. So that’s a real difference. And notice that’s going to work oppositely, typically, on the two sides of a continent. You’re going to have cold water or warm water, depending where you fit into that current scheme.
Student: Why is the cold water always on the left side of the continent?
Professor Ron Smith: The question is, why is the cold water always over here? Well, these gyres always go–notice in the southern hemisphere, they’re always going counterclockwise. In the northern hemisphere, they’re going clockwise. We’re going to explain that later, in terms of how the winds drive those currents. But you can see that what they’re doing is bringing cold water from the pole northward. So once you know the sense of the gyre, then the sense of cold water or warm water follows exactly. So it’s only a matter of what direction are the gyres going. And I’ll explain that when I get to the ocean currents section.
Chapter 6: Biome [00:28:39]
Now I still haven’t mentioned seasons very much. What can you do with just the annual average values? This is put together by a couple of authors that have tried to show the relationship between climate and vegetation based on an annual average values only. It’s kind of useful, but I’m going to argue it’s not complete, because it doesn’t include the seasonality. But look what they’ve done. They’ve made good progress.
So if you take the annual average temperature plotted on this axis, the annual average precipitation plotted on the x axis–come along, and then they have tried to illustrate what kind of vegetation you would get for each of those climate zones. Oh, notice the temperature scale is reversed here, by the way, It gets colder to the right. So tundra, taiga, temperate deciduous rainforest, tropical seasonal forest, all these things.
So I enjoy looking at this diagram, but then I get confused because it doesn’t include the seasons in there. And the seasons really are needed to do a good job of this. So be aware of this. The concept is called a biome. The relationship between climate and vegetation is called the biome concept. It’s a very good concept, but it doesn’t work that well unless you include the seasons, which this diagram does not.
Chapter 7: Seasonality [00:30:13]
So we are ready to start this discussion of seasons, which will spill over into next time a little bit. I find this to be a very useful part of the course, a very useful subject to consider. When I travel around the world, I like to know, I like to, first of all, experience the fact that traveling to a different part of the world is going to give me a different climate than that that I grew up in. And I want to understand–every place I visit around the world, I want to understand–why is this climate different and how do the seasons work differently?
In kind of a silly application–I find it useful at the beginning of the year, when I’m coming back to school seeing my colleagues. Somebody will say, well, I’ve just come back from Ghana, or I’ve just come back from Melbourne, Australia. And I ask them, what month were you there? And they tell me and I say something like, oh, that was the dry season, or, oh, that was the middle of the rainy season. Because once you get–I want to give this to you in the next few minutes–once you get this basic picture of how seasons work, which is not too tough, you can immediately know what’s going to happen as you travel to various parts of the world in various months of the year. It’s a pretty easy thing once you get it categorized and worked out. So that’s the understanding we’re striving for here. How do the seasons work in different parts of the world?
Chapter 8: Effects of the Earth’s Rotation around the Sun [00:31:46]
So just five bullet points concerning seasons. What causes the seasons on Earth is the tilt of the Earth’s axis, 23 and a half degrees currently. So that when the–and that tilt is measured–what’s it measured from? It’s measured from the plane of the ecliptic. The plane of the ecliptic is that plane in which the Earth moves in its orbit. That is our reference plane, and relative to that plane, the tilt is measured, the tilt of the Earth spinning on its axis.
For this reason, because it’s a tilt effect, during part of the year the northern hemisphere is tilted towards the sun, and part of the Earth is tilted away from the sun. So the seasons are opposite in the two hemispheres. When it’s winter in the north, it’s summer in the south, and vice versa. That’s the primary effect of it being a tilt effect, is you get those opposite seasonalities.
Now there is another effect. The Earth’s orbit is slightly elliptical, which means for a few months a year, we’re a little bit closer to the sun. For a few months of the year we’re a little bit further away from the sun. That in and of itself would cause a slight seasonality on planet Earth. It’d be warmer when we’re closer to the sun, a little cooler when we’re further away. (A) that’s a pretty small effect. And (b) note that if that were the dominant effect, that would cause simultaneous seasons in both hemispheres. It’d be summer in both hemispheres at the same time and winter in both hemispheres at the same time. So this does not explain the basic thing we know about seasons, in that it’s primarily reversed in the two hemispheres.
Any questions on that?
OK, now before I go any further, I just want to illustrate the basic idea here about how the tilt of the Earth does it. I’m going to darken things off just for a minute, and we’ll do a little globe experiment here. Can I set this right here?
So here’s the Earth. And the sun’s over there of course. So as it goes around the sun, the orientation of that tilt remains fixed. So as it goes around its orbit, notice the orientation of my pencil stays fixed. That’s important. That arises from the principle of conservation of angular momentum. Angular momentum is the quantity that the Earth has when it spins. It’s a vector quantity pointing in a certain direction. And that doesn’t change.
You might have thought that as the Earth goes around, maybe it somehow does something like this. But no, no. That orientation stays fixed as it goes around. So when it’s on one side of the sun, it’s like this, and when it’s on the other side of the sun, it’s like this. So remember, it’s that constant orientation of the rotation axis that is important in understanding the seasons. So let’s start with an orientation like this where the tilt is oriented such that it’s across the direction of the sun’s rays.
Now, September 21st, just two weeks ago–I think it was on the day you took your exam actually–on that day, that was the autumnal equinox. That’s when the Earth had this orientation. That tilt was, in a way perpendicular to a line between the sun and the Earth, so that as the Earth spun on its axis, it had equal day and night everywhere around the globe.
Now we are moving towards December 21st, which will be the winter solstice, and on that day the orientation of the Earth will be like that relative to the sun. Can you all see that? So when it spins on its axis again towards the east, the southern hemisphere is receiving a lot of heat, the northern hemisphere is receiving less, and the north pole is getting none. Notice this is completely in shadow back here, because of that direction.
Three months after that at the spring equinox, it’ll be like this again. And then on June 21st, the summer solstice, that will be the orientation of the Earth relative to the sun. And the north pole will have sunlight 24 hours a day. The south pole will have none. A lot of radiation in the northern hemisphere. Less in the southern hemisphere, because it’s tilted away. And that is the cycle we go through. Remember, it all rises again, because as the Earth goes around in its orbit, that orientation of the axis stays fixed as it goes around the sun. That’s the key ingredient there.
Any questions on that? You’ve probably had that in high school, but there was a survey done about 15 years ago of graduating Harvard seniors. And they were asked, what makes the seasons. And a shockingly small percentage–this was done, actually, as they marched into their graduation ceremony–a shockingly small percentage could explain that it was the tilt of the Earth and so on and so forth. I don’t want to that to happen at Yale. So you’ve got the message, and spread it around, because how can you live on this planet, I ask you, how can you live on this planet without understanding that most basic concept of how the seasons work?
OK. Now, a few other points. What this does–this shift of the radiation from hemisphere to hemisphere moves all of these belts and zones along with them. So we’ve talked about the ITCZ, the belt of deserts to the north to south of that, the belts of storms to the north and south of that. Imagine that whole set of systems shifting northwards in the summertime for the northern hemisphere, and shifting southwards for the summer in the southern hemisphere.
That’s probably the easiest way to understand how climate works on our planet, is to envision that the general circulation is driven by the sun, and therefore if, because of the tilt of the Earth’s axis, you move most of the radiation into the northern hemisphere, everything’s going to move with it. If you move it into the southern hemisphere, everything’s going to move southward into it. Now I will try to describe that in various ways. But you’ve got to understand it well enough to sketch it into your notes there.
So again this is just a picture of the sun going around the Earth–no, the Earth going around the sun. Going back to the old geocentric view of the universe here briefly. The Earth going around the sun, thank you, and keeping that orientation of that axis fixed as it does so. So if you’re looking sideways at the plane of the ecliptic, the Earth here in grey is moving in and out of the board, going around in its orbit staying in that ecliptic plane and with a fixed axis—a fixed tilt to its axis of rotation.
If you’re on the Earth, in kind of an Earth-fixed coordinate system, it seems to you like the sun is actually moving. In the summertime it seems to be up high in the sky. In the winter time it’s lower in the sky. Of course that’s not true. It’s just the fact that the axis of the Earth is tilted.
For example, if you’re at 41 degrees north latitude, as we are here in New Haven, in the summertime at the summer solstice, the sun at noon would be 23 and a half degrees above the equator, which means it’s never overhead here in New Haven. But that solar zenith angle is not very great. You’d have to subtract 23 from 41 to get the solar zenith angle at noon on the 21st of June. On the 21st of December at noon, the sun is down here. We’re still in New Haven. The solar zenith angle would then be 41 degrees plus 23. What’s that? 64 degrees. The sun at noon is 64 degrees below your zenith point.
And that has big influences, of course, on the amount of radiation received because of that cosine factor. It’s the cosine of the zenith angle that determines how much radiation is being received per unit area on the Earth. And that’s shown here. So when the sun is above, you get the full solar radiance. When it’s at some solar zenith angle, it’s the solar constant times the cosine of the solar zenith angle. So the greater that off-zenith angle is, the smaller is the cosine and the less radiation you receive. So that cosine effect is another important way to understand how the seasons work.
I did mention, however, this other effect, this non-circular aspect to the Earth’s orbit. And it’s shown a little bit exaggerated in this diagram. But at the present time, the perihelion–the day of the year when we’re closest to the sun–is the third of January. The aphelion–when we’re furthest away from the sun–is the fourth of July. And notice what that would do, and in fact does do. If this were the only effect, it would make January a slightly warmer month than July. So in the northern hemisphere, when January is a cold month because of the tilt of the Earth, this would slightly reduce the seasonality. In the southern hemisphere, when July is normally a cold month, this effect would strengthen the seasonality slightly.
Now, this is a small effect, but if you want to think through what it’s going to do, with the current perihelion being in January, it’s going to weaken northern hemisphere seasons a little bit and strengthen southern hemisphere seasons a little bit. We’re going to revisit this when we talk about the theory of the ice ages, where it’s believed that changes in this situation may have triggered the coming and going of the ice ages. But at the current time I want to emphasize that this is a pretty small effect. You can do a pretty good job of understanding the seasons on Earth without even knowing about this effect. You’d get it about 90% right.
So if you’re at a particular latitude and you go through the months, how does the insolation vary? So what’s shown here are four different latitudes–the equator, and then three latitudes in the northern hemisphere. And what’s plotted is the solar insolation averaged over a day in watts per square meter.
Let’s start with the north pole. Near the summer solstice, you’re getting about 500 watts per square meter averaged over a day. It drops very rapidly as you go later in the year, and at a certain point it drops altogether. You get perpetual nighttime. The sun never appears in the sky. As you go to lower latitudes–remember the Arctic Circle, because the tilt of the Earth’s axis is 23 and a half degrees, the Arctic Circle is 66 and a half, right? 66 and a half. So at 60 degrees, we’re already southward of the Arctic Circle. And so we don’t quite get darkness all day long, but the amount of radiation received is very small in the middle of the winter.
And then when you get all the way down to the equator, the insolation received is almost constant with the year. Notice that for a few weeks, averaged over a day, the north pole actually receives more than the equator does, but it’s a very brief thing. And for the most part, if you look at the total area under the blue curve, which is the equator, versus the area under the yellow curve, which is the pole, much more radiation is received at the equator than at the pole. So this is a nice diagram for remembering that it’s the amount of sunlight that’s received that’s the key issue here with the seasonality.
Chapter 9: Seasonal Zone Shifts [00:46:32]
OK. So back on this idea of the belts moving seasonally, quote, “following the sun.” This is an expression which is just meant to help make it easier to remember this. They follow the sun. That is, when there’s more sunlight hitting the northern hemisphere, things move north. South, south–it works the opposite way.
So here’s a little cartoon I put together. So, for June, July, and August, when the northern hemisphere is tilted towards the sun, first of all, the polar front is very weak. And that’s because the amount of radiation is almost the same between the pole and the equator. So there’s not very much differential heating in the southern—sorry in the summer hemisphere. And in June, July, and August, the northern hemisphere is the summer hemisphere. The ITCZ, which we’ve been talking about being located at the equator, actually shifts northwards several degrees. And the frontal storms in the southern hemisphere become very strong. And the polar front moves a little bit northward as well.
Six months later–December, January, February–when the southern hemisphere is receiving most of the radiation, the frontal storms in the northern hemisphere are very strong, because remember, there’s no radiation reaching the north pole, and a lot of radiation reaching the equator. So a lot of differential heating. In the southern hemisphere, the ITCZ shifts southwards, and you have weak frontal storms in the southern hemisphere, because there’s very little difference between the solar insolation here and there.
So that’s the cartoon that’s going to guide us through the next lecture, really. We’re out of time today. But try to make some sense of this. Try to think through it. And then I’m going to show you to what extent this simple idea is applicable to the real world. And that’ll be next time.
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