Good morning or good evening, whatever the case may be. If we're going to understand how the Arctic works, one of the first things we have to come to grips with is the amount of solar energy that the Arctic gets. Of course, solar energy is the driving force of ultimately everything on our planet in terms of climate. And think about Arctic as a very special region. Now, first of all, on an annual basis, the Arctic receives much less solar radiation than do the tropics, lower latitudes. And of course, this is why the Arctic is cold. The sun just never shines as much in the Arctic as it does in the lower latitude. But the other thing about the Arctic, which is very important, is this extreme seasonality in the receipts of solar radiation. At the Arctic Circle, at 23 and a half degrees north, what we see is one day of complete polar darkness, the sun never rises above the horizon. And we see one day of what we call midnight sun, that is, the sun never drops below the horizon. Now, those extremes increase as we move north. At the North Pole, what we're getting is a full six months of polar darkness and a full six months of the midnight sun. So it really is pretty extreme. Now, thinking about this distribution of solar radiation, the first question is why does the Arctic on an annual basis get less solar radiation than the lower latitudes? And the simple answer is that the earth is a sphere. And so as a result of this, what happens is the sun's rays, as they come in, they strike the earth's surface at a more grazing angle in the lower latitudes than in the higher latitude. So that's really why we get less solar radiation in the polar regions, in the Arctic and the Antarctic. But there's also axial tilt. Okay, what are we talking about here? Now, the earth revolves around the sun once every year, once every 365 days, or about 365 and a quarter. And that revolution about the sun, we can define that in terms of a plane, which we call the ecliptic plane if we looked at it from the side. But we also know, of course, that the earth rotates on its axis once every 24 hours, day and night hence. But that axial tilt is inclined with respect to that ecliptic plane, 23 and a half degrees, as a matter of fact. This is why we experience seasons. The issue here is that one time of the year we find the Northern Hemisphere tilted towards the sun. So the Northern Hemisphere is getting summer at that time and the Southern Hemisphere is getting winter. When you're tilted towards the sun, that means the sun's rays don't strike so obliquely, they strike the earth's surface at a more direct angle, more solar radiation. Of course, the other extreme is our Northern Hemisphere winter when the earth is tilted away from the sun and we don't get as much solar radiation, and that's the Southern Hemisphere's summer. So it's just a opposite thing between the two polar regions, between the two hemispheres. Now, this figure I'm showing here is just trying to illustrate this idea that the sun's rays hit the earth's surface at a more oblique or grazing angle in the polar regions than in the equatorial regions. It's just spread out more, so you don't get as much energy per unit area, so it could never really get all that warm in the Arctic. But now let's think about the axial tilt and seasons, right? I mentioned that this axial tilt is 23 and a half degrees. Now, what that does is it changes both the length of the day that we get and how high in the sky the sun gets during the day. Now, this is really true here, right in the middle latitudes, but certainly it's the case in the Arctic. These seasons are extreme in the Arctic. As I mentioned, the day length can vary from zero to polar night, sun never gets above the horizon, up to 24 hours. That is, the sun never drops below the horizon, that's your midnight sun. This figure is trying to illustrate these contrasts. If you look on the left, that is what's happening at the June solstice. And what's happening in that case is that's when the earth is tilted most strongly towards the sun in the Northern Hemisphere, 23 and a half degrees. And what's happening here is it's putting all of the Arctic region into daylight, anything north of the Arctic Circle is getting a 24 hour daylight. Whereas the other extreme on the right hand of that figure is showing our Northern Hemisphere winter or the Southern Hemisphere summer. The entire polar region is in darkness. The other two cases are at what we call the equinoxes. On those cases, neither hemisphere is tilted toward the sun. And what happens is that we have an equal day length of 12 hours everywhere, except for the pole where the sun actually grazes the horizon all day, we'll get to that. Now, this is just looking at this in a little different way or a little more closely. This is the Northern Hemisphere summer solstice, usually around June 21st. And you can see clearly how the polar regions or how the Northern Hemisphere is tilted towards the sun and therefore gets more direct solar radiation. And again, all of the Arctic region is seeing 24 hour daylight on that summer solstice. Here is, again, the opposite case, this is the winter solstice. Of course, it's summer in the Southern Hemisphere, winter in the Northern Hemisphere, and the entire Arctic region is in polar darkness. What's pretty darned extreme, as I mentioned, at the North Pole, six months of polar darkness, six months of midnight sun, so it's pretty darned extreme. Now, let's ask ourselves a question. Let's say, by some cosmic accident, the axial tilt was not 23 and a half degrees, but it was actually 30 degrees. What would happen in terms of our seasons? The answer is both B and C. We'd have warmer summers and colder winters in the Northern Hemisphere, and the same case in the Southern Hemisphere, it's just opposite. And the Arctic Circle would be actually located further south. Why would we have warmer summers and colder winters? Well, with more tilt, you'd be tilted more closely, more strongly towards the sun in summer than we do now. And so the sun's rays would strike the surface even more directly. Also, the Arctic Circle would indeed be further south if this was the case. Instead of 66 and a half degree north, it would actually be at 60 degrees north. So it's actually both B and C here. Now, another thing that's related to all of this is the length of day. If you're in the tropics, what you find is that day length doesn't really vary all that much throughout the year. So if you're looking at these low latitudes, like I'm trying to show by this arrow, day length varies from maybe 11 hours to 13 hours per day, depending on the time of year. It just doesn't vary all that much. But the case in the Arctic is very, very different. This figure here is just showing the number of hours of day length. What you have on the x axis is the day and what you have on the y axis, up and down at latitude, so that second arrow is showing what happens at the North Pole. All those areas in very, very dark red is showing where this constant daylight, where the areas in the white are showing where there is no sun at all, the sun is below the horizon. And it's extreme in the Arctic. So we have a situation of the Arctic where day length varies from zero right to 24 hours, whereas in tropics day length may be 11 to 13 hours, doesn't vary all that much. So it's extreme in the Arctic. Now, this shot here is this one I took from Toluca Lake back some years ago, about 11:00 PM, as I recall, and the sun was just grazing the horizon. This was sometime around April, sometime in April. The sun did set, but it never got that far below the horizon, so there was always a lot of dusk. You could actually read a newspaper, for those who read newspapers, even at midnight in this situation. We're getting towards that period of 24 hour sun here. And I've been there during this midnight sun. This is actually shot, took a picture of me on Griffith Island near Resolute Bay, 74 degrees north or something like that. And we could play golf at midnight, we would paint our balls orange so we couldn't lose them. It was an interesting case because we were right in the Polar Bear Alley. So while one person was taking the shot there, I'm in pretty good form, I think, for wearing all those clothes, my partner would have to be carrying a shotgun, because we needed the protection from the bears. Another question. At the June solstice, which is usually around June 21st, the daily average solar radiation at the top of the atmosphere at the North Pole is greater than Miami, Florida. Is that true or is that false? Perhaps counterintuitively, it's actually true, and I can show it from this figure here. What this is showing is the daily average solar radiation at the top of the atmosphere by latitude. And so what it's showing on the x axis is day, Julian day, the middle of that chart is right around the June solstice, and latitude is shown on the y axis. And this arrow comes in, it's showing what you'd get in Miami. The daily average, that's averaging day and night, you end up with about 475 watts per square meter, which is a good amount of solar radiation. But it turns out, if we looked at the North Pole at higher, about 550 watts per square meter. Well, why is that? Well, at the North Pole The sun never gets that far above the horizon but there's 24-hour daylight. Whereas, in Miami, all the sun gets high in the sky during the day. But there's also night and when you average it all together, it turns out averaging over the 24 hours you actually, get a little bit more, a fair bit more at the North Pole than Miami. Kind of counter-intuitive, but it's quite true. Now the intensity of solar radiation, of course, is a different thing if you look at it through the day. The most solar radiation you get at the top of the atmosphere is always at solar noon. That is when the sun is basically, due South and that is when the sun is highest in the sky. And that's what this figure is trying to show is what is happening in terms of the amount of solar radiation we get during the day. So this figure is showing the hour of the day on the X-axis, the middle of it is right around noon. And then it showing the solar radiation on the Y-axis and these different lines are for different latitudes. Well, if we looked at 30 degrees North where that blue arrow just came in, what we find, of course, is that the amount of solar radiation that you get around solar noon is quite strong. It's around 1300 watts per square meter. But there's also darkness, the sunsets at night. But if we were to look instead at about 80 degrees North, what we find is that at noon we don't get nearly the amount of solar radiation that you get at 30 degrees North at solar noon. But the point is, is the sun never sets, and so you get solar radiation all day. It's really a remarkable radiation environment in the Arctic. And it's something that very very much shapes the climate of the region. Thank you.
