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Chapter 3: Climatic determinants of global patterns of biodiversity

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Edited by Peter Moyle & Douglas Kelt
By Douglas A. Kelt, September 2004

Why does it rain so much in the Tropics?
Now we are ready to evaluate the consequences of the Earth's tilt and its round shape. On average, the sun spends more time directly above the equator than any other part of the earth. And, when the sun is directly over the equator, there will be more solar radiation striking a given area of the surface at the equator than at higher latitudes (either north or south). This leads to our first important observation – the air immediately over the equator will tend to heat up and, because warm air rises, equatorial air tends to rise. As it rises, however, it also escapes the gravitational pull of the earth, and expands. A basic law of physics holds that expanding gasses will cool (this is the basis of the modern refrigerator and will be explained in detail when you take a physics course – for now, I ask that you accept this as fact). Therefore, this warm and rising air begins to cool. Another basic law of physics states that warm air holds more water than colder air (again, take this on faith for the moment). Of course, water can exist in either a gas or a liquid state, and humid air simply contains much water in gas form, with individual water molecules bouncing around but not grouping as water droplets. When this air is cooled, however, there is a temperature at which these molecules condense to form droplets; the temperature at which this occurs is called the "dew point." As air rises above the equator, it is heavily laden with moisture (we all know that the tropics are humid). However, as this air rises it cools and reaches its dew point, after which the water molecules condense to form droplets, and clouds begin to form. These continue to rise, the droplets coalesce to bigger drops, and soon rain drops are produced. Thus, the tropics are characterized by their high amounts of rainfall.

You have probably heard meteorologists talk about high pressure zones and low pressure zones. High atmospheric pressure is generally associated with good weather, whereas low pressure is associated with cloudy or stormy weather. The reason for this is simple. Air consists of molecules of oxygen, nitrogen, carbon dioxide, and other gases, and thus it has mass. Any mass is subject to the inexorable pull of gravity, so even a mass of air puts some pressure on the surface of the earth. However, when air rises the pressure it exerts is reduced, and we refer to this as a region of low pressure. So, in areas of low pressure, air is rising and it generally follows the pattern we just described for the tropics – air rises, cools, water condenses to form clouds, and we often get rain. Now we'll discuss a portion of the earth that is characterized by high atmospheric pressure.

Why are deserts generally located at about 30° latitude?
Air rising in the tropics can't rise forever. If it did, then it would escape the Earth entirely and we would be left with a barren, lifeless planet. Rather, as it rises it dissipates somewhat, it cools, and as it cools it begins to condense again, and becomes heavier. But, air below continues to rise. The air has to go somewhere, and at a high altitude it diverts towards the north and south (see Fig. 4). Air continues to move north (or south), but it is cooling now, and tends to begin falling back towards the surface of the Earth. This air generally manages to reach about 30° latitude before subsiding towards the earth. As it drops in altitude, this air begins warming again, but remember that it has already lost most of its water before moving north of the tropics. Because warm air holds more moisture than cool air, this warming air acts as a sponge, literally drawing moisture from the environment around it. This is our second important observation – as this air descends at about 30° latitude, it literally pulls moisture from the environment. If you refer back to the map of global biomes you will see that this corresponds to the latitude at which most of the world's deserts are found.

To complete one cycle then, the air that descends at about 30° latitude then moves north or south along the Earth's surface (Fig. 4). That which moves towards the equator again becomes captured in the cycle of rising air that we discussed above. This cycle was initially described by a meteorologist named Hadley, and the "cell" of air movement is now known as the Hadley cell.

There are two other cells, although we won't worry about their names. Air moves towards the poles from the 30° region, and rises again at about 60° latitude. This air rises, cools, forms clouds, and spreads both north and south. Finally, at the poles we have another mass of air that descends towards the surface. Thus, there is a generally predictable pattern of air movement over the earth, and it is largely responsible for the distribution of tropical areas as well as of the world's major deserts. This basic pattern can explain why the tropics are moist and lush whereas regions about 30° N and S are relatively xeric (dry).

What are the “horse latitudes” and the “doldrums”?
Imagine that you are at the north pole – if you stood directly over the axis of the earth's rotation you would slowly turn a complete circle. This would require about 24 hours. If you walked south a little bit, you would find that a 24 hour cycle would move you a certain distance – how great that distance is would depend upon how far from the pole you walked. The circumference of the earth is about 4,000 km at the equator, so that if you were standing on the equator, you would travel about 4,000 km every 24 hours. This seems a little esoteric, and might make a useful question for “Jeopardy.” But now imagine you were in a weather balloon, and you flew north from the equator with the north-bound air in the Hadley cell. You would start your trip with a certain velocity – about 4,000 km/day towards the east – but as you move north, the ground below you is traveling east at a slower and slower rate. Another tenet of physics is referred to as the law of conservation of angular momentum. It is a mouthful, but it means that you don't simply lose this 4,000 km / day velocity as you move north. You keep moving east at this rate.

Fig. 4. Relationship between vertical circulation of the atmosphere and wind patterns on the earth's surface. There are three convective cells of ascending and descending air in each hemisphere. As the winds move across the earth's surface in response to this vertical circulation, they are deflected by the Coriolis effect, producing easterly trade winds in the tropics and westerlies at temperate latitudes.

Fig. 5. The Coriolis effect illustrated using a weather balloon floating from the north pole to the equator. On a nonrotating Earth (top figures), the rocket would travel straight to its target. However, Earth rotates 15° each hour. Thus, although the rocket travels in a straight line, when we plot the patch of the rocket on Earth's surface, it follows a curved path that veers to the right of the target (bottom figures).

However, the Earth beneath you moves east more slowly. Thus, rather than traveling north, you will appear to begin veering towards the east, which is towards your right (Fig. 5). The further north you travel, the more rapidly you will veer east. The same thing happens to the air in the Hadley cell – as it moves north it becomes shifted eastward. The reciprocal situation would involve an air mass moving towards the equator (see Fig. 5). As the air mass approaches the equator, the earth beneath it begins moving faster, and the air mass appears to veer westward, which again is towards the right of the direction the air mass is traveling. This intriguing pattern is called the Coriolis Effect. The important thing to understand is that in the northern hemisphere the Coriolis effect results in air shifting to the right when it moves to either higher or lower latitude. You can probably work out the dynamics to realize that the reverse is true in the southern hemisphere – air masses shift to their left.

Now, recall that air generally rises at the equator, descends at about 30°, and then moves either south towards the equator or north towards the rising air at about 60°. Air moving south from 30° will veer westward, whereas air moving north from 30° will veer eastward (in both cases, the air veers to the right of its line of travel). Now, many global features were given names during the days when sailing ships surveyed the earth, and trading ships carried supplies between Europe and the Americas. When these ships traveled from Europe towards the New World they would travel south to intercept westward flowing winds ("easterlies") – as a result, these became known as the tradewinds to reflect their importance in commerce. These ships would return to Europe by a northern route, capturing eastward winds that were subsequently called westerlies. Of course, the southern hemisphere has a similar set of winds at comparable latitudes.

In the vicinity of the equator, however, air is generally moving up, and there is no lateral movement. Here, winds are poor and many early sailing ships became stranded for weeks or more. These areas are called the equatorial doldrums, and this is where the phrase "in the doldrums" arose. Similarly, at about 30° air is descending but not providing much lateral motion. Many ships transporting soldiers became stranded here as well, and many soldiers were forced to resort to eating their horses as food supplies grew thin – thus arose the term “horse latitudes.” You may have heard that the quickest route from Europe to North America in these times was not the most direct. Because of the Coriolis effect, ships could make this trip most rapidly by sailing south from Europe, then west with the trade winds, and then north along the coast of North America.

Table of Contents

1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
4. Biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife

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