 You are clear for launch. And with that, shut down your visors, O2 on, and prepare for ignition to O2. You can copy that and um... Hey there, it's me, Mr. Ruschak. Now, so far in this physical geography unit, we've examined the functions to create our land farms, focusing mostly on the lithosphere of the rock of the Earth. Now we're going to turn our attention to the atmosphere. Specifically, we're going to learn the reason for the seasons. And it all comes down to the relationship between the Earth and the Sun. The reason that this relationship is so important is that it's the energy from the Sun that drives our climate and, quite frankly, supports all life on Earth. Which might seem remarkable when we think about how far we are from the Sun. The Earth is 94 million miles away from the Sun. Put another way, it takes eight and a half minutes for the Sun's light to be able to reach the Earth. Now, before we start talking about how vital the Sun is to the Earth, we need to understand two terms, rotation and revolution. To rotate means to essentially move around an axis point, essentially rotating means to spin, pretty much how like a top spins. To revolve means to move around another object. Now, the Earth both rotates and revolves. The Earth will rotate on its axis every 24 hours, which gives us day and night. But as the Earth is rotating around on its axis, it is also revolving around the Sun. This is a journey that takes the Earth 365 and one quarter days to complete. By the way, if you ever wondered why every four years we have leap year in February, it is how our calendar accounts for that extra quarter day every year. Now, not only does the Sun give us light, what is most important is the impact that its energy or solar radiation has on the Earth. This solar energy drives the Earth's climate machine. Now, we can see the Sun's energy striking the Earth in this graph from NASA, which shows the effect of the Sun on the Earth in terms of temperature. The blues are the coldest parts of the Earth and the red show where the temperature is greatest. And you'll notice that the Earth tends to have waves of heat that go from east to west, east to west, and east to west. And you probably already figured out that this is caused by the Sun's energy, but as we go from day to night, day to night, as the Earth rotates on its axis. We call the amount of solar energy that strikes any given point, solar insulation. And the word insulation actually comes from the word soul, which means Sun. But you might also notice something else in this visual. It seems that most of the solar energy seems to be near the equator, and this is not by accident. Let's pause the Earth's rotation here and look at how the Sun strikes the Earth. The first thing we should notice is the Sun's energy strikes the center of the Earth near the equator. But the amount of sunlight that strikes the poles of the Earth is only about 40% of the energy that is striking the equator. This chart shows the amount of solar insulation that strikes at each latitude. And as you can see, the closer you are towards the equator, the more solar energy you receive. The further away you're from the equator, the less solar energy you receive. What this really means in real terms is that the further away you are from the equator, the colder it's going to be. And the closer you are to the equator, the warmer it's going to be. There are three major reasons for this. First of all, because the Earth is a sphere, the angle that sunlight strikes the Earth, also known as solar incidence, is different depending upon the latitude of the particular point. Now twice a year, if you live in the tropics, which we're going to learn in a few moments, is a region near the equator, the Sun is going to be exactly over you at noon. In fact, on those two days at noon, your shadow will be exactly underneath you as it is in these pictures. This is because the Sun is exactly 90 degrees perpendicular to the ground it is receiving the most amount of solar energy that is possible. However, let's say you live in Alaska at 70 degrees latitude, the Sun's rays hit the ground at an angle that is going to spread that same amount of sunlight over a greater area. This means that the solar energy is essentially less dense, and the solar insulation of these latitudes is just a fracture of what it does at the equator. But there's more. Where the Sun is striking the Earth at 90 degrees, it only has to go through one atmosphere. However, in latitudes further from the equator, such as our example in Alaska, because of the angle the sunlight has to travel to get to the ground, it actually travels through, really, the equivalent of three atmospheres at 70 degrees latitude. This means that more of the Sun's energy is reflected or reflected by the atmosphere and doesn't actually reach the ground. But then there's the fact that the Earth isn't oriented with the North Pole being the top, and the South Pole being at the bottom. Instead, the Earth is tilted. In fact, it's tilted an entire 23 and a half degrees on its axis. Now, this tilt in the Earth's revolution means that the Sun's energy doesn't sit over the equator all year. For example, on December 21st, the Sun's energy rests at 23 and a half degrees south latitude, a line that is known at the Tropic of Capricorn. But look at what happens to the poles in December. Above 66 and a half degrees north latitude, the Sun actually never rises. This latitude is what is known as the Arctic Circle. But this all day pole or night isn't just one day. This simulation looks straight down in the North Pole and be able to give us what the time of the sunrise and the sunsets are at the northern hemisphere. Here we see that the further you are above the Arctic Circle, the longer you will go without any sunlight. For example, if you're at 75 degrees north latitude, you won't even see a sunrise between the last part of October and the first part of February. But even below the Arctic Circle, the days are shorter as you move towards the North Pole from the equator as the sunsets come earlier and the sunrises come later. But in the southern hemisphere, we see the effects of the sun are exactly opposite. Between 66 and a half degrees in that south pole, we see that the sun actually never sets. This actually marks 66 and a half degrees south latitude, marks the Antarctic Circle. In fact, just when we saw you would never see a sunrise between late October and early February, above 75 degrees north latitude, below 75 degrees south latitude, during the same time you're not going to see a sunset. Additionally, since the center of the sun's energy is focused upon the southern hemisphere, temperatures in the southern hemisphere are going to be greater than at the northern hemisphere. But remember, the earth continues to revolve around the sun, so six months later, which would be in June 21st, the situation has completely changed. Instead of the center of the sun's energy being on the Tropicic Capricorn, at 23 and a half degrees south latitude, we now see that it's at 23 and a half degrees north latitude of what is known as the Tropic of Cancer. Also, if you look at the area above the Arctic Circle now in the northern hemisphere, the sun is actually never setting. And between the Antarctic Circle and the South Pole, the sun now never rises on this day. During this period of time, this temperature is in the northern hemisphere will be warmer than in the southern hemisphere because the focus of the sun's energy is in the northern hemisphere on the Tropic of Cancer. By now, you've already realized that we've been talking about summer and winter, and you've figured out that whatever we have in the northern hemisphere is completely opposite in the southern hemisphere. By the way, the people living in the southern hemisphere countries like Australia and New Zealand will call their warmer season summer and the cooler seasons winter, even though it's opposite of 10 hours, which can be confusing. So for the purposes of this course, whenever I refer to summer or winter, unless I'm actually talking about southern hemisphere countries, I'm going to be talking about it in terms from our own context in the northern hemisphere. Now, notice that I used two dates in our examples, December 21st and June 21st. Remember, on December 21st, the sun is focused on the Tropic of Capricorn in the southern hemisphere, and on June 21st, the sun is focused on the Tropic of Cancer in the northern hemisphere. The dates themselves are the time in which the sun is at the furthest extent north or the furthest extent south. They are called solstices. And the winter solstices on December 21st and the summer solstices on June 21st. Now, between these two dates, the sun will move north and south between the Tropic of Cancer and the Tropic Capricorn. However, between the two solstices, on the 21st of March and the 21st of September, the focus of the sun's energy will be right over the equator. When this happens, the entire earth from the north pole to the south pole will be equally illuminated by the sun. As we move to September 21st, we can see the backside of the earth and realize that the earth is once again split between darkness and light from the north pole all the way down to the south pole. On these two days of the year, when the sun is right over the equator, there will be everywhere in the world, regardless of latitude, exactly 12 hours of daylight and 12 hours of nighttime. The term for this is the equinox, which literally means equal night. The sun's over the equator. You have equal day, equal night, equal knocks. So it is the earth's tilt in revolution that cycles the energy of the sun north and south. In this NASA graphic, we can actually see the solar insulation cycle throughout the year. What you are actually seeing is the earth move through the seasons. Here is summer, here is winter, here is summer, here is winter. You should now be starting to understand how important the latitudes for the season. So what we're going to do is we're going to divide the latitudes into three categories, high, middle, and low. Now to avoid confusion and make this easier, just look at the northern hemisphere first. Now because there is between the equator and the north pole, 90 degrees of latitude, we're going to divide these in threes by 30 degrees each. So we have 0 to 30, 30 to 60 to 60 to 90. Now if we just look at the northern hemisphere, it's very easy to be able to name these bands of latitude. Here are the low latitudes, here is the medium latitudes, and here is the high latitudes. For the southern hemisphere, remember that it is always opposite in the southern hemisphere than the northern hemisphere. So all we have to do is take our latitude bands and just flip them down like this. There we go. So we have now classified all the latitudes. So let's start talking about how the seasons relates to each one of these bands of latitudes. We'll start looking at the low latitudes first. So as you can see, the low latitudes include the tropic of cancer and the tropic capricorns. So it's easy to kind of realize that we also call the low latitudes the tropics. So what's special about the tropics is the sun is always focused over the tropics all year long. So they are warm all year long and there is no winter. In fact, there really isn't even a fall or a spring either. At most, the tropics really has two seasons, a warm dry season and a warm rainy or a warm wet season. In fact, some areas really will only have one season where it's warm and wet all year long. Now at the other end of the spectrum, there are our high latitudes. Here you can see that the arctic and the Antarctic circles run through these latitudes. These latitudes also have the north and south pole. So we can call these latitudes also the polar latitudes. In the polar latitudes, the temperatures are always going to be cool. So there really isn't a summer and just like the tropics, there really isn't even a fall or spring either. Instead, we find two seasons here, polar day and polar night, which corresponds to the times when the days are very, very long or the days are very, very short. Then there are the middle latitudes. Due to the Earth's tilt and revolution, these latitudes go through the full cycle from hot to cold every year. These latitudes are also known as the temperate latitudes because they are on average not as hot as the tropics or not as cold as the high latitudes. These middle or temperate latitudes are now the only places in the world that are going to have all four seasons, spring, summer, fall and winter. Now the solstices and equinoxes mark the seasons. The winter solstice begins winter, the spring equinox begins spring, the summer solstice starts the summer and the fall equinox begins fall. Again, these seasons are all caused by the revolution of the Earth around the Sun and the Earth's tilt of 23 and a half degrees. In our next lesson, we'll add four more factors to the latitude to find out what truly creates this climate that our Sun sets in motion. But until then, keep on learning.