Earth Science
Earth science rarely generates excitement in a room full of students destined to study this subject. As is often the case, looks can be deceiving – earth science is a fascinating area of study. A class in high school earth science involves the exploration of concepts in just about every field of science, and some of those concepts are both subtle and complex. In my experience, many of the teachers who are doing their best to teach this subject do not themselves fully understand some of the topics they are trying to cover. I give those teachers all the credit they deserve in giving all that they know to their less than enthusiastic students.
Typical high school courses in earth science will cover many of the following topics:
- The Earth in Space
- Models of the Earth
- Plate Tectonics
- Earthquakes
- Volcanoes
- Earth Chemistry
- Minerals of the Earth's Crust
- Rocks
- Erosion by Weathering, Water, and Glaciers
- The Rock Record
- The Oceans and Atmosphere
- Weather and Climate
- The Sun, Stars, and Galaxies
- The Solar System
There are two topics in earth science that are rather subtle, both of which I would take the opportunity to discuss here. One involves the formation of atmospheric convection cells around our planet, while the other involves the formation of deserts on the leeward side of mountains.
Let's start with atmospheric convection cells. Take a look at the diagram below, which shows the three convection cells that extend from the equator to the north pole, known as the Hadley Cell, Ferrel Cell, and Polar Cell:
Notice that the Hadley and Polar cells circulate air counterclockwise while the Ferrel cell circulates clockwise. The question is: Why do these cells form? I will answer this question now, with the help of some rather crude diagrams that I put together using Microsoft Paint. I promise to upgrade the diagrams soon.
At the equator, the sun is very intense and creates a large rising air mass, as shown below:
As the air rises, it cools and becomes more dense, eventually slowing its ascent. With the rising air mass still pushing from underneath, it has no choice but to spill out in both directions at the top, as shown below:
We will ignore the air mass moving south of the equator, and concentrate on the one moving north:
As the high altitude air mass moves north, it continues to cool due to the decreasing intensity of the sun at higher latitudes. Eventually, it is dense enough that it begins to sink, and it just so happens that this occurs at 30o latitude, as shown below:
Once this sinking air mass reaches the ground, it has no place to go, and must split into a north-moving and south-moving air mass:
Notice we have already created a counterclockwise convection cell circulating between the equator and 30o north latitude - that is the Hadley Cell you saw in the very first diagram. As can be seen in the figure above, the air mass moving back toward the ground (the smaller blue arrow) together with the north-moving air mass (along the ground) are important because they will actually become part of the clockwise convection cell (the Ferrel Cell) that we saw circulating between 30o and 60o north latitude, so please keep this in the back of your mind as we continue.
Let's head up to the north pole and see what's going on up there. The air on top of the north pole is very cold and dense, and is therefore continuously sinking as shown below:
As before, the air mass must split when reaching the ice cap:
We will focus on the air mass moving to the right, as shown below:
The air mass is warming as it moves toward lower latitude, and at 60o has warmed enough to start rising:
Similar to what happens at the equator, the rising air mass will cool, become more dense, and eventually not be inclined to rise anymore, but as it is still being pushed up from underneath, will again split into two air masses, one moving north and one moving south, as shown below:
Notice we have now formed the counterclockwise convection cell called the Polar Cell, as well as the remaining two sides of the intermediate, clockwise-rotating Ferrel Cell. Putting it all together, we can now understand the figure we looked at in the beginning of this tutorial:
As shown above, a mirror-image of these air movement patterns is formed below the equator for exactly the same reasons, giving a complete picture of these atmospheric convection cells. Finally, the winds generated by these cells are acted upon by the rotation of the earth (Coriolis Effect), giving rise to the curved air flow patterns in the diagram above.
Now that you are an expert in atmospheric convection cells, I would like to discuss the other topic: Why can deserts form on the leeward side of mountains? The leeward side of a mountain is the side protected from the wind. Check out the diagram below:
The leeward side is the right side, where the wind is not hitting directly as it is on the left, or windward side. As the air moves up the windward side, it cools to the point where the water vapor molecules condense into rain, and perhaps farther up, into snow. Having left all it's moisture on the windward side, it's easy to understand why the leeward side is dry.
So far, so good. Let's say the ground air temperature on the left side of the mountain is 70oF. On the right side of the mountain, ground temperatures are likely to be around 100oF or more. The question we are trying to answer here is why does the air on the leeward side reach much higher temperatures than the air on the windward side? The answer has to do with the property of a substance called its specific heat capacity, or simply its heat capacity. Let's understand what this is before we come back to our mountain.
There are many types of energy - mechanical, electrical, magnetic, and thermal just to name a few. We are interested here in thermal energy, and to keep things simple, lets just say that the thermal energy of a system is related to its temperature. If you add thermal energy to a system, its temperature will rise. If you remove thermal energy from a system, its temperature will fall.
The heat capacity of a substance is the amount of thermal energy required to change its temperature. Water, for example, happens to have a large heat capacity relative to most other substances, which means that you have to add (or remove) quite a bit of thermal energy in order to raise (or lower) its temperature.
Now think about moist air and dry air. Moist air contains water vapor, whereas dry air does not. So, we would expect that if we add the same amount of thermal energy to similarly-sized samples of moist and dry air, the moist air will not reach as high a temperature as the dry air because most of the energy we are adding is being absorbed by the water molecules with little change in their temperature as a result of their large heat capacity. The dry air sample will end up at higher temperature since all the energy is absorbed by molecules of nitrogen and oxygen that have lower heat capacities than water. By exactly the same reasoning, if we remove the same amount of thermal energy from moist and dry air, the moist air will not reach as low a temperature as the dry air.
What we realize in the discussion above is that moist air changes temperature much less easily than dry air, due to the large heat capacity of the water molecules in moist air. Since the altitude change is the same on both sides of the mountain, the pressure change is of the same magniture going up and coming down. Pressure changes are the mechanism through which we add or remove thermal energy from air samples, because an increase in pressure causes the molecules to move closer together which in turn produces heat due to friction.
As the air moves up the mountain, the lower pressures act to remove thermal energy from the air; as the air moves down the other side, the higher pressures act to add thermal energy to the air. As mentioned above, the altitude changes are the same, so we are in fact reproducing the experiment above with moist and dry air. The dry air will change temperature more as it heats up moving down the mountain as compared to the moist air cooling off as it moves up. So, we will have much hotter temperatures on the leeward side of the mountain, and the combination of hot and dry is the very definition of a desert ecosystem.
Students should follow the International Journal of Earth Science. A great earth science website for students is Exploring Earth. A large number of good books on earth science are available from Google Books.
To fulfill our mission of educating students, our homework help and online tutoring centers are standing by 24/7, ready to assist students who need extra practice in earth science.
|
|
REQUEST LIVE SESSION
CLOSE
