What Makes the Tropics So Tropical?

By now you've seen me use the term "tropical" a number of times. This word is used pretty frequently...we talk about tropical weather, tropical drinks, tropical storms, and tropical diseases (YIKES!). But for Geographers the term "tropical" has a very simple meaning. It refers to the part of the globe that is between 23.5 degrees south and 23.5 degrees north of the equator (1). The equator is the imaginary line that runs east-west around the planet, like a belt. The northern boundary at 23.5 degrees north latitude is called the Tropic of Cancer, and the southern boundary at 23.5 degrees south latitude is the Tropic of Capricorn. If you have a hard time remembering this, you can tell yourself that Capricorn has more letters than Cancer, so it's heavier and therefore sinks. One important thing about the tropics is that there isn't much season variation there; there are not big differences between summer and winter. You know this from living in Hawai'i. Indonesia is like this too.

Anyway, this region is really important for a number of reasons. The tropics is the only place you can find the sun directly overhead. In Hawai'i (2) this happens twice a year. This has to do with the relationship between the Earth and the sun. Let me explain. Look at the diagram to the right. This is a simple picture of the Earth and sun. Notice that the Earth is tilted in relation to the sun. You can see this on almost any globe you look at. This tilt is very important for the seasons; if there was no tilt there wouldn't be any summer, spring, winter or fall! Also notice in the diagram that the tilt is always in the same direction relative to the sun. We call this "axial parallelism". The tilt of the Earth and the direction of the tilt don't change over the course of a human lifetime (3). Now as you can see from the second diagram, this means that different parts of the Earth have more or less exposure depending on their latitude, or how far north or south of the equator they are. This affects how long the day is. Think about it this way. We all know that one day has 24 hours, right? Wherever you go on the planet, even Indonesia, even the North Pole, one day lasts 24 hours. That means the Earth spins on its axis one time in 24 hours. Since you are standing on the Earth that means it takes you 24 hours to go around in a big circle. But the size of that circle depends on where you are standing on the Earth. You can see two circles on the second diagram. One is big and one is small. This means that the surface of the Earth is spinning faster at the big circle than at the small circle. If you are riding on the bigger circle, you cover a greater distance in the same amount of time. That's not all. As you can tell from the circle, different parts of the Earth get different amounts of sunlight. This is due to the tilt.

Now let me use some still shots I took from one of the models I used when I teach about this to show you what this means for seasons. In the first three pictures you notice the northern part of the Earth is tilted towards the sun. You should be able to locate several lines on the pictures. Which lines have the longest sections in the daylight? That's right, the ones towards the top. We already said that this means the days are longer. To illustrate the point, have a look at the Geography Giant standing on the globe. You can tell from the different pictures that when he stands closer to the North Pole, the day is longer (about 20 hours!). However, the day is really short when he's close to the South Pole (about 5 and a half hours). This is a huge difference. But when he stands on the Equator, the day is 12 hours long (and night is 12 hours too). Now we've already learned that all energy comes from the sun. The sun also makes things hot! So it makes sense that the longer the sun is out, the warmer it is. What season do you think it is in the northern part of the planet? How about the southern part?

Now let's turn the situation around and think about what happens when the Earth is tilted the other way relative to the sun. Remember what I said about the lines in the last section? Let's try that again. Which lines are longer? So where are the days longer? That's right, closer to the South Pole. When the Geography Giant stands near the North Pole, the day is short (5 hours), but when he moves near the South Pole the day is much longer (19 hours)! That's a big difference from what we just saw! Now notice how long the day is when the Geography Giant stands at the Equator...it's still 12 hours! So this shows us why there are seasons....it's because the day length, and hence the amount of energy from the sun, changes quite a bit over the course of the year. And it also tells us why there is not much seasonal variation near the tropics: because the days are almost the same length all year round, and so there is a pretty constant level of energy from the sun. In fact, because the sun always passes high in the sky in the tropics, there is a lot of energy. Not only does this energy make it warm, it also helps to evaporate a lot of water (and drive the ITCZ, remember?), which leads to lots of rain in some places. In addition, since plants like sunlight so much, there is a lot of it so lots and lots of plants can grow. The sun doesn't pass as high in the sky outside the tropics. The height also changes with the seasons. You can see an example in the picture to the left. The lower the sun is in the sky, the more of its energy is bounced off the Earth into space, making it still colder in the winter time! I'll let you think about this for a while. It might help you to use a globe. When you feel like you've got it down, try the questions I've included below.

1) "Latitude" refers to how far north or south of the equator a location is. Geographers use a special grid, called the Geographic Coordinate System, to describe places on the Earth's surface. Using just two numbers, the latitude and the "longitude" (this describes how far east or west of the Prime Meridian you are, we can give precise information about the location of anything! Can you find out the coordinates for Honolulu? How about Sungai Penuh?

2) Hawai'i is in the tropics. How can you tell this from the information you just obtained about its coordinates?

3) The tilt of the Earth and its direction do change, but these happen on cycles of about 41,000 and 26,000 years, respectively.

Paru Paru Dunia...

"Paru Paru Dunia" means "lungs of the Earth" in Indonesian. This expression is used sometimes to refer to the forests of Indonesia. This is because of the important role trees play in filtering the air. Trees and other plants are crucial for maintaining balance in the atmosphere because they take in carbon dioxide and release oxygen, and people need oxygen to live. The world is interested in the fate of the forests of Indonesia in part because of this, because this country has among the largest areas of tropical rain forest on the planet. But what exactly is a rain forest? They are areas near the equator (1) that receive very high amounts of rain throughout the year (though there are often seasonal variations). These places receive lots of rain because they are near the inter-tropical convergence zone. That's a pretty big expression, but let's see if we can figure out what it means. First, "inter-". This usually means "between" or "among", and that's what it means here. From earlier posts you should already know what "tropical" means. So we are halfway there...the first part means "between the tropics". Next "convergence". What does "converge" mean? When two things converge, what do they do? That's right, they come together. And the last word is easy. So the ITCZ is a place between the tropics where something comes together. But what comes together? The answer is simple: air. In the atmosphere the air is constantly moving. You've certainly experienced this in Hawai'i...every time you feel the wind blowing you are feeling the air move in the atmosphere. Because of the relationship between the Earth and the Sun, the places near the equator are always the closest places on the planet to the Sun (2). And what do we know about the Sun? That's right, it's hot. So it makes sense that the hottest places on the planet will be those closest to the Sun. And when air heats up, it rises. You've probably experienced this before if you've ever been in your grandparents' attic. It's always hotter in the attic than it is in the house because warm air rises. That's exactly what happens at the equator...the air rises because it is warm. As the air rises, more air has to come in to fill the space left by the rising air. This new air comes from north and south of the equator; thus it converges at the equator. There's one last thing you need to know about this. As air rises, it cools down. You've probably experienced this if you've ever gone on any of the great trails that climb up the Wainae or Ko'olau mountains on O'ahu. Or if you've ever been up on Haleakala or Mauna Kea. It's so cold up there it even snows sometimes! So as the air rises and cools down, it makes clouds. And we all know what clouds mean...RAIN! So you can see that this is a kind of cycle that is always at work, and the result of it is very consistent rainfall. I've included a cross-section diagram of how this works to the right. Geographers call this pattern "the Hadley Cell", but you don't have to remember that unless you are in the 3rd grade or higher :).

Now look at the diagram. Notice that this cycle is right at the equator. Next have a look at the rain forest map I included above. Where are most of the rain forests? Now you can understand why there is so much rain in these places. Rain is very good for plants, so the forests that grow in these places are very rich. So what's the problem? Well, the rain forests of the world are disappearing at an alarming rate. They are being cut down and burned faster than they can grow back! Have a look at the map of Borneo (3) I've included. Forest cover is represented in green on this map.
As you can see, the past 60 years has seen a dramatic loss. The same is true with other important forest "hot spots" in Indonesia, including Sumatra (where I am; see the map) and Papua. Where are the forests going? Well you may recall from a previous post that some people cut down the trees to sell, while others cut them down to grow crops to eat and sell. But there are other pressures as well. In many cases, places that are rich in forests are also rich in valuable minerals. Thus mining is another activity that causes forests to be cut down. All of these activities are good for some people in the short term, but they are bad for all the people in the long term. So why don't we just stop all the logging and mining? This is a very tough question, but part of the answer is because most of the countries where rain forests are found are relatively poor. One of the ways these nations make money is to sell raw materials, resources that are taken from the earth and later made into other things. Everything you wear, own, and want is made from raw materials of some kind, and they have to come from some place. In addition, people that are poor do not have the luxury of thinking years into the future, because they are worried about having enough to eat for dinner. And because of the importance of the inter-tropical convergence zone, these forests can't just grow anywhere. So you can see that this is a pretty difficult problem to solve; it is a uniquely geographical problem. As one local farmer said, "TNKS terkenal sebagai paru-paru dunia, tapi pada kesempatan paru-paru masyarakat yang ada desekitar kawasan TNKS terasa kempes dan sesak. Bagaimana TNKS menyikapi hal ini? Apa kontribusi yang diberikan TNKS untuk masyarakat?" Loosely translated, this means "Kerinci Seblat National park is known as the lungs of the world, but as for the lungs of the people living around the park, we feel deflated. How does the national park feel about that? What benefit does the park have for people living around it?"

Think about that for a while. How would you go about solving it? While you're thinking about it I'm going to get some sleep. You can email me some of your ideas!

1) Can you find the equator on a map or globe?

2) You might be able to make a simple model of the Earth-Sun relationship with a lightbulb and a globe. If you have a globe, you will notice that it is tilted (at an angle of about 23.5 degrees). This is because, in relation to the Sun, the Earth is tilted in space! This is very important for the changing seasons. What else can you notice about the Earth-Sun relationship?

3) Borneo is a large island in the Indonesian archipelago (chain of islands...Hawai'i is an archipelago). Most of Borneo is part of Indonesia, but part of it is part of the nation of Malaysia, and there is another small country called Brunei on Borneo as well.