# Lesson 2, part 2: Density and Latent Heat

By: shauntanner, 1:18 PM GMT on September 25, 2011

Do you know what is more freaky than a smoke detector going off, waking you up in the middle of the night? A carbon monoxide detector going off and waking you up in the middle of the night. This is what happened to me tonight, and is the reason I am sitting here writing a blog at 4:30 a.m. Pacific Time. The carbon monoxide detector nearest to the garage woke us up but turned itself off within 30 seconds. So, rather than going back to bed, I have opened the windows of my house, shut off the furnace at the switch, ordered a carbon monoxide meter (very expense, by the way), and am now writing this blog to you. I am in the same room as the active carbon monoxide detector and have placed a second one (from upstairs) in the room with me just in case. Why is an active carbon monoxide alarm more freaky than an active smoke detector? You see, when a smoke dectector goes off, you can get up and check on the house fairly quickly. No smoke, no fire. But, when a carbon monoxide alarm goes off, there is really no way to check on the house because carbon monoxide is an odorless, colorless, tasteless gas that you don't know you are inhaling until you are suffering from carbon monoxide poisoning. The gas comes from combustion, including your car's engine and the furnace in your house. A properly working furnace will not output much carbon monoxide, but the only way to check on this is use a meter to check levels. Hence, I bought a meter. I will also be going to buy a couple more detectors today. Just some food for thought before I start the blog.

Density

Density is defined as mass over volume. In meteorology, the units we use most with density is kg/m3. That is kilograms of mass per cubic meter of a substance (most likely air). Picture a balloon filled with air. There are millions of molecules floating around in that balloon totally a certain mass (kg). Also, that balloon has a measurable volume (this is the cubic meter part of the equation). There are two ways to change the density of that balloon. The is to change the top number (kg). By putting more air into that balloon, we increase the mass, and thus the top number in our density equation will go up, as will the overall density as long as we keep the volume of the balloon the same. The reverse is true if we take air out of the balloon. The second way to change the density of the balloon is to change the bottom number. If we squeeze the balloon to half
its original size while keeping the same amount of air in it, its overall density will increase as well. If we expand the balloon by pulling its sides while keeping the same amount of air inside of it, the density will go down.

How does this relate to meteorology? Well, inside that balloon, again, are gas molecules that have a temperature. Remember, temperature is the average speed of the molecules in a substance. If you apply heat to the balloon, thus causing its molecules to go faster, the temperature of the air inside the balloon will go up. These molecules, energized with heat and moving fast, will impact the sides of the balloon at a faster speed and cause the balloon to expand. Think of a car hitting a barrier at 5 mph and another hitting the same barrier at 50 mph. The faster moving car will be able to push the barrier farther. Thus, is the sides of the balloon are expanding, what is happening to the volume of the balloon? It goes up! What is happening to its density? It goes down!

The opposite is true when you cool the air in the balloon. The molecules in the balloon move slower and are unable to keep the balloon as pumped up as it was before. Thus, the volume goes down and the density of the air in the balloon goes up!

Now, we can come to a very important conclusion. Cold air is more dense than warm air.. Let's say you have two balloons at the surface. One of these balloons is filled with air colder than the surrounding air and the other is filled with air warmer than the surrounding air. Based on our statement about density and air temperature, what will happen to those two balloons. Because nature tries to keep things with similar densities together, the warm air balloon will rise until it reaches a place that is its same density, and the cold air balloon will sink until it reaches air that is closer in density. For cold air, this is probably close to the ground. Now, this isn't quite the whole story, but that is going to have to wait until another lesson.

My analog to this density problem is a bottle of Italian salad dressing. The ingredients of Italian salad dressing and be put into two categories; oil, and seasonings. Left unmixed, these two ingredients will separate naturally with the oil on top and the seasonings are bottom. The seasonings are heavier. If you shake that bottom up, sure the ingredients will mix briefly, but then will separate quickly and go back to their original order once the shaking has stopped. The oil is less dense than the seasonings, thus wants to be on top of the heavier seasonings. The atmospheric analogy is cold is the seasonings, and warm air is the oil. Cold air is "heavier" than warm air.

Now that you know about density, let's move on to the most important part of any of the lessons yet.

Latent Heat

Latent heat can be explained if you can grasp the following idea. For warm liquid water to become cold ice, it has to lose heat. That lost heat does not just vanish, never to be seen again. Rather, that heat is put into the atmosphere. That is called latent heat.

Okay, now let's take a step back. When a substance changes from liquid to solid, that is called a phase change or a change of state. There are several types of phase changes.

Figure 1. Phase change diagram showing all types of phase changes for the atmosphere.

While several of these you are familiar with, some of them you might be seeing for the first time. Two interesting phase changes are sublimination and deposition. When something subliminates, it changes from solid to gas, skipping the liquid phase. Snow can do this, turning from an ice crystal to water vapor when a fairly strong Sun hits it. Deposition is the opposite: turning vapor to ice while skipping the liquid phase.

So, pretend you are stepping out of a shower or swimming pool and head to a towel to dry off. Have you ever noticed that you get cold rather quickly in this situation? If you were to zoom in to your skin, you would see several water droplets resting on it. Inside those droplets are millions of liquid water molecules floating around at various speeds. If you took the average speed of all the molecules, you would get a temperature. Some of these molecules are moving faster than others and will be more likely to jump from the liquid phase to the vapor phase. This is called evaporation. When the fast liquid molecules leave the liquid droplet, what happens to the temperature of the water droplet on your skin? Since the fast liquid molecules have left, the average speed of the molecules in the droplet has decreased, and thus the temperature of the droplet has gone down! Thus, you have a droplet on your skin that is colder than it was just a few seconds ago, and you then get cold. What basically happened was the energy that was in the water droplet was taken out of the droplet and put into the air, thus cooling the droplet. Evaporation is a cooling process.

When we say a warming or a cooling process, keep in mind we are speaking of the environment. In the example above, the environment around the water droplet got colder...your skin. This is an important distinction. So, you can do this exercise with all of the phase changes and your conclusions should be something exactly like Figure 2.

"HEAT ENERGY TAKEN FROM ENVIRONMENT" means the phase change is cooling process and "HEAT ENERGY RELEASED TO ENVIRONMENT" means the phase change is a warming process.

Figure 2. Latent heat diagram for the various phase changes.

There are a couple items that should confuse you in the diagram. The first is freezing of the liquid water. What the diagram is telling you is that the act of freezing is a warming process. How does this makes sense? Remember, when we are talking about latent heat, we are talking about the environment around. Thus, when liquid water wants to become solid, or ice, it has to lose warmth. Where does that warmth go? It goes into the environment, warming it. The opposite is true for melting. If an ice cube wants to melt into liquid water, it has to warm. Where does it get the energy to warm? The environment. It takes heat energy from the environment, leaving the surrounding environment cooler. Thus, it is a cooling process.

I think I have given you enough to chew on for now. I am going to wait for my wife and children to wake up so I can go stock up on batteries and carbon monoxide detectors.

# Lesson 2, part 1: Temperature

By: shauntanner, 2:24 AM GMT on September 18, 2011

For those of you just tuning into this education series, I really recommend going back and reading the three parts of the first lesson. I am doing my best to post a new part once a week so you have all week to get caught up.

Now that you are an expert on the layers of the atmosphere and understand the difference between weather and climate, we need to move onto the next lesson. This part of the lesson we are going to be dealing with something you are very familiar with...temperature. In teaching meteorology, I have found that many of the ideas in the field are already very familiar to a lot of us. There is one really good reason for this. All of us are experts one way or another on weather. We experience it everyday. Thus, a term like temperature is very familiar to us. But, that doesn't mean we understand it any better. So, I have a question for you. What is the definition of temperature? Think about it for a minute before moving on to the next paragraph. If you could give temperature a definition, if you could hold it in your hand, what would it feel like?

Before I give the definition of temperature, we need to lay a bit of foundation. In the world of Physics, there are two main types of energy. The first is called kinetic energy. This energy is the energy associated with motion. So, when you are walking down the street, you are exerting kinetic energy. But, what happens when you stop moving? There is another type of energy set aside specifically for you lazy people. It is called potential energy. This energy is considered stored energy. So, let's say you exert kinetic energy by climbing 10 flights of stairs. By the time you are done climbing, you are tired and decide to take a rest at the top. Since you are resting, you are no longer exerting kinetic energy. However, since you climbed 10 flights of stairs, you have also stored that energy in the form of potential energy. How do you turn that potential energy into kinetic energy? Well, that's simple...jump out the window.

With kinetic and potential energy in mind, we can now define temperature:

Temperature is the measure of the average speed of the atoms and molecules in a substance.

There are two main terms I want to point out in that definition. The first is molecules. We are getting down to the molecular level. Go get yourself a bowl and fill it with water. Or pretend you have such a thing in front of you. In that water are molecules comprised of two hydrogens and an oxygen. Those molecules are floating with a certain speed around the bowl as liquid water.

The second term in the temperature definitiion I want to draw your attention to is average. You see, not all of the water molecules in that bowl of water are moving at the same speed. It is much like a highway where you have many different cars, but none of them are traveling exactly the same speed. One may be traveling 55 mph, another 75 mph, another 105 mph. But you can get an average speed of those cars((55+75+105)/3) that is representative of all the traffic on the highway. If one car speeds up and the others remain at constant speed, then the average speed of all the cars goes up as well. So, in that bowl of water, some of the ambitious water molecules are moving fast, while other more laid back molecules are moving slower. Take an average speed of all of the molecules in the bowl and you get a number representative of temperature.

Now, what happens if you put that bowl of water on your oven and turn the oven on? Intuitively, you know that the temperature of the water will increase. What does that mean in terms of our definition? It means that the average speed of the molecules must be increasing. That is the only way to increase temperature. When you apply heat to a substance, the molecules convert that heat to kinetic energy and move faster.

The next question is more interesting. What happens if you put the bowl of water in the freezer? Well, in terms of our definition, the water molecules will begin to slow down their speeds and eventually the water will freeze (more on that in a minute). But, what happens if you continue to cool the water after it is frozen? When water freezes, the molecules don't stop moving...they just move slower. If you continue to cool the molecules, they will continue to slow down. Let's go crazy and continue to cool the molecules down. They will continue to slow until the molecules simply...stop moving. If you happen to have a thermometer handy at that point, what would it read? What is the temperature of the water at that point?

This point is called absolute zero. Absolute zero is the point at which all molecular motion stops. Your Fahrenheit thermometer would read -459 degrees at absolute zero, which is the coldest possible temperature in the universe. There is nothing colder than absolute zero and humans have tried their best to replicate it in a labratory setting. Scientists have come very, very close to absolute zero, however. Since this is the coldest temperature in the universe, this also makes a perfect spot for the beginning of a temperature scale.

Kelvin Scale
The Kelvin scale was invented by a man with the perfect name of Lord Kelvin. The scale (denoted with a "K") begins at absolute zero and has no negative numbers. So, for those of you who didn't have a good relationship with negative numbers in your algebra career, this temperature scale is for you. Water freezes at 273 K on this scale and there is no upper limit.

Celsius Scale
The Celsius scale (denoted by "°C") is wonderfully simplistic, which makes it even more interesting why we don't use it here in the United States. The freezing point of pure water on the Celsius scale is 0°C. That means that 273 K = 0°C. Why pure water? Well, as it turns out, if you put stuff in water (salt, for instance), it tends to lower the freezing point of the water mixture. That is just one reason why the ocean has a hard time freezing.

The boiling point of pure water at sea level is 100°C. Why sea level? If we have any cooks in the audience, they will know the answer to this question right away. At sea level, water boils at 100°C. But, if you go up to the Mile High City of Denver, CO to boil yourself some water, you will find that the water boils at a temperature less than 100°C. For this reason, cooks often have to heat their meals longer at higher elevations than at lower elevations. It has to do with the pressure differences between sea level and places higher at elevation. As we learned last time, pressure ALWAYS decreases with height. With less pressure pushing down on the water at altitude, the water molecules have less trouble jumping into the vapor phase from the liquid phase as heat is applied to them.

So, this scale is simple. Freezes at 0°C and boils at 100°C. Therefore if I say that the air temperature is 25°C today, you can directly relate that number to the freezing and boiling points.

To convert from K to °C, the simple equation is K = °C+273.

Fahrenheit Scale
For those of you living in the United States, you will know that this is the scale we use. But, it is my hope that, after reading how the Fahrenheit scale (denoted by "°F") is represented, you will petition your government to switch to the Celsius scale. The Fahrenheit scale is much more complicated.

You see, you may know that the freezing point of pure water is 32°F. With that in mind, you may wonder what Gabriel Fahrenheit denoted 0°F as. Well, 0°F was the lowest temperature he could get a mixture of water, ice, and salt before the sustance froze. Thus, for your daily life, 0°F really has no meaning to you.

The boiling point of pure water at sea level is 212°F. Much easier to remember than 100°C, right? Bah!

Knowing the above, we can see that 1°C = 1.8°F. Thus, the conversion equation from C to F is °C=5/9(°F-32).

Okay, I will leave it at that for this lesson. In the next lesson, we will use the definition of temperature and apply it to a mysterious energy force in the universe. No, I am not talking about Star Wars...I am talking about latent heat. See you next time!

Here is how the temperature scales measure up in real life...

Updated: 1:56 PM GMT on September 21, 2011

# Lesson 3, part 3: Layers of the Atmosphere

By: shauntanner, 5:58 AM GMT on September 12, 2011

This is the last part of your first meteorological lesson. By the end of this part of the lesson, you should be able to explain to yourself what is the difference between weather and climate. You should also know what is in the air your breathe and how it gets in there. Finally, by the end of this blog entry, you will know how the atmosphere is structured above your head.

When I was a kid, my family took a yearly trip up to Lake Tahoe where we rented a cabin for a weekend. As our house was on a valley floor, I was always amazed to find that the formally normal potato chip bags I had packed in our cooler where inflated and almost bursting by the time we each the Lake Tahoe area, which is over 5,000 feet in elevation. Have you ever wondered why this happens?

The potato chip bag was essentially cut off from the air outside. This means that it basically had its own environment, including its own air and pressure on the inside. Let's say the pressure inside the bag was 1000 mbs, standard pressure. At my house, the air outside of the bag was exerting a similar pressure, 1000 mbs. However, as the bag was taken up higher in elevation, and thus higher into the atmosphere, it encountered air that was exerting less and less pressure on it. Let's say that the pressure at our destination cabin was 800 mbs. That means the air outside the bag was exerting 20% less pressure (or weight) on the bag as it was at sea-level. The air and pressure inside the bag had not changed. So, you can now see why the bag was allowed to inflate. Less weight pushing down on it, the more it could stretch its wings. This, actually, is one of the reasons why astronauts cannot spend very much time in space. Here at the surface, we have pressure (and weight) pushing down on us. This keeps our bones and muscles close together and working. In space, astronauts are under much less pressure and thus their joints are allowed to expand and stretch. So, astronauts can return from space taller than they were before they left!

This leads me to one very important point. PRESSURE ALWAYS DECREASES WITH HEIGHT. This is a fundamental idea that cannot be forgotten. If you travel up in the atmosphere, you are getting above more and more molecules of that atmosphere. Thus, those molecules are no longer pushing down on your head, and the pressure on you has decreased. This is always true. It is more logical than mathematical.

Figure 1. Pressure with altitude.

Figure 1 shows what pressure does with height. For you math majors, you will immediately recognize this as an expontentially decaying curve. These types of curves have two distinct characteristics. First, as you can see, pressure of the atmosphere falls very rapidly the first few thousand feet of the atmosphere. This is true because the molecules of the atmosphere are packed very closely to the surface. So you do not have to go very high to get above a significant chunk of the atmosphere. Second, the atmosphere will never hit a pressure of 0 mbs (By the way, in the graph, they use a unit called hPa. This is the same as mbs). No matter how high you travel, you will never see a pressure of 0 mbs.

But, using this illustration of the atmosphere, it looks nice and simple. Well, let me through a wrench into the engine then.

Figure 2. Layers of the atmosphere by temperature. Credit: UCAR

You see, the atmosphere is actually more complicated than a simple expotentially decaying curve (as if that was simple). If you look at the atmosphere in terms of temperature (Figure 2), then several clear layers pop out at you. Let's take them from the bottom up.

Troposphere

The troposphere is the bottom layer of the atmosphere and extends from the surface to roughly 10 km. This top limit fluctuates greatly (for reasons we will get to in another lesson) and is called the tropopause. Here are some characteristics of the troposphere.

--Contains all of the weather you know. This is an amazing note. Every thunderstorm, cloud, tornado, hurricane, raindrop, hail, EVERYTHING you know about weather lives in the troposphere. So, we have only gone through the first 10 km, and we already hit the mother lode. Amazing, right?

--Temperature cools with height. The normal state of things during the day in the troposphere is a temperature that cools the higher up you go. This is because the source of heat for this portion of the atmosphere is actually the surface. The Sun's rays actually pass right through the atmosphere and are absorbed by the surface. So, the surface is the heater. The farther away you are from the heater, the cooler you will be.

Stratosphere

Some characteristics of the stratosphere:

--Temperature warms with height. This is the major characteristic of the stratosphere. Do you remember in the troposphere where I stated that the surface warms because it ABSORBS the heat from the sun (you better, you just read it)? Well, if the stratosphere is warming with height, then something must be ABSORBING something in this layer as well. Well, it turns out our old buddy ozone lives well in the stratosphere. In this layer, ozone goes about its business, absorbing ultraviolet radiation from the Sun, preventing it from reaching the surface. This same ultraviolet radiation is the reason why we smother ourselves with suntan lotion. Too much of it in our lifetimes can lead to skin cancer. So, ozone is up there, absorbing ultraviolet radiation, protecting us, and warming the stratosphere. This is the fabled ozone layer. Without it, we would be hit by much more hamful rays from the Sun.

--Little vertical movement. This is the reason why the troposphere houses all of the weather you are familiar with. Because of the warming temperatures in the stratosphere, it acts as a cap for any vertial movement in the troposphere. Ever wonder why the tops of thunderstorms are flat and spread out as if hitting a ceiling? Well, that is because they are meeting the stratosphere which literally acts like a celing to vertical movement.

Mesosphere

This is the coldest part of the atmosphere. If you ever laid on your back and looked up at a starry night only to see a shooting star dash across the night sky, then you have had some interaction with the mesosphere. Shooting stars have a hard time getting by the mesophere and are literally small chunks of rock that burn up high up above the surface.

Thermosphere

The thermosphere is interesting. If you look at the temperature of this layer, you will see that it is actually WARMER than the surface at about 110 kms in altitude. Yet, if you went up to the thermosphere and camped out without protection, you would freeze almost instantly. What's up with that?

You see, you are sitting in front of your computer or smartphone right now and the atmosphere is working on you without you even knowing it. As the molecules float in the immediate air around you, several of them will impact your skin. On impact, those molecules will impart their energy onto your skin, warming you up. The faster moving the molecule, the more energy it will impart on you, and the warmer you will be. But, if you remember our old "expotentially decaying curve" friend, the pressure in the thermosphere is very low. Because it is so low, there must be very little molecules up there. If there are few molecules floating around up there, then there are very few molecules available to impact your skin and keep you warm. Without these impacts, you will lose your warmth immediately and die. Get it? It is not a very good vacation spot.

There are also things like the ionsphere and exosphere, but I think I have filled your brain enough for the time being. On to Lesson 2 next week!

# Lesson 1, part 2: The Air We Breathe

By: shauntanner, 6:38 AM GMT on September 05, 2011

Thank you all for reading last's weeks opening blog in my ongoing educational series. For those of you just joining the class, welcome! My hope is that by the end of this series, you will all be experts in the atmosphere that you call home. Or, at the very least, you will have armed yourself with a few tools that you can use to make informed decisions on how you affect the environment.

If you missed my opening blog, please get caught up by reading it here. In it, I talk about my rationale of writing this blog series as well as give a brief introduction on what weather and climate actually are.

My goal is to write a new lesson every week. Since I have slightly more time on the weekend, these new blogs will more likely come out before the beginning of the workweek so you will have all week to read it. But, if Mother Nature intervenes (hurricane, tornadoes, stomping in puddles outside my house) then I will be late in posting the lessons. Okay, are you ready for this week's lesson? Last week we learned about weather and climate. This week we will learn about the air we all breathe.

What is in the air we breathe?

Several years ago, I was an avid watcher of "Who Wants to be a Millionaire" (oh be quiet, you know you were too). One of the questions on the show was, "What is the most abundant gas in the lower atmosphere?" The contestant stared at the four choices before him for quite some time before making the correct decision. After I got done jumping around my living room screaming the correct answer, I wondering to myself how much of the general public even knows what the answer is. I mean, isn't it important to actually know what our lungs pull in with every breath? Well, now it is your time to learn.

There are two categories of gases in the atmosphere:

Permanent:
These are gases that are constant in time and space. What that means is that their input is balanced by their output. So, if gas A is a permanent gas and I take one molecule of gas A out of the atmosphere, then somewhere else on Earth, one molecule of gas A was put into the atmosphere. These gases are constant...permanent.

Variable:
Don't make it harder than it sounds. These are gases that are...variable. If you want to make it difficult, then these gases are such that input and output are not balanced and thus can vary in time space. If you think really hard, you can probably come up with one very important variable gas that is literally part of your life everyday. It manifests itself as liquid in the atmosphere so you can see it. Water vapor! Sure, get it? Water vapor is a variable gas, obviously, because sometimes you have clear skies and the water vapor quantity is low, and sometimes you see clouds in the sky and everything feels more humid (humidity is the measure of water vapor in the air).

So, what is the most adundant gas in the atmosphere and is it a variable or permanent gas? Again, if I asked you to name some of the gases in the atmosphere, you could probably easily come up with several. One of those should be oxygen. Right? Right?

Figure 1. Table showing both the permanent and variable gases in the atmosphere.

In the table above, the left side shows the most abundant permanent gases and the right side are the variable gases. I want to draw your attention to the top two permanent gases. Nitrogen makes up 78% of our lower atmosphere. 78%! That number is amazing everytime I see it. Why it takes up so much of our atmosphere will be explained in a minute. The second most abundant gas in the lower atmosphere is oxygen at ~21%. For you math majors, I want you to add up the Nitrogen and Oxygen percentages. 99%! So, what I am telling you is that the two most abundant gases in the atmosphere account for 99% of the gases in the lower atmosphere by volume (as opposed to weight). So the rest of the gases in the table, both permanent and variable, take up only 1% of the atmosphere. Amazing, isn't it?

Before I get into specifics, I want to again draw your attention to the left hand column of the table. Argon, Neon, Helium, Xenon. If you know anything about chemistry (I don't blame you if you don't), then you should know that those elements have something in common. They are called Noble Gases. Without getting into the nitty-gritty, the basics you should know about Noble gases is that they don't like friends. They don't like to combine with other elements to create anything remotely exotic. So, when they do get put up into the atmosphere, they stay there because they don't like to make friends and combine with the other more friendly elements up there.

Now, for some specifics. I want to run down some sources and removal processes for some of the permanent gases.

Nitrogen?
Source:
Nitrogen is put into the atmosphere by volcanoes. In fact, volcanoes put a lot of stuff into the atmosphere.
Also, when animals and plants decay (including yourself when you pass on), Nitrogen is recycled back into the atmosphere.
Removal:
One of the big removal processes comes from soil bacteria. If you think about it, fertilizer is pumped full of Nitrogen because plants love it. Why do they love it? Because it is abundant in the atmosphere.
Another way Nitrogen leaves the atmosphere is by way of our friendly ocean plankton. They do a good job of eating it up.

Nitrogen is inert. What that means is that it doesn't like friends either. So, when it gets put into the atmosphere, it can stay for a long time.

Oxygen?
Source:
Think back to your second grade class and you should be able to answer this one. Photosynthesis! When plants exhale, they put oxygen in the air. We have a lot of oxygen because we have a lot of plants!
Removal:
Oxygen is a very friendly element. It combines with many other elements and when it does, the atmospheric form, 02, that makes up 21% of the atmosphere goes away. One of these chemical reactions occurs when plants and animals decay. Oxygen does a good job in breaking things down.
It is also removed during the breathing process, which is the other end of the photosynthesis reaction.

Now let's take a look at a couple of the variable gases.

Water Vapor?
Source:
The best way for water vapor to get into the air is by evaporation. Liquid water evaporates from the ocean (yes, and other bodies or water) and becomes water vapor up in the sky.
Also, volcanoes do a great job in pumping water vapor into the atmosphere.
It is removed from the atmosphere when it precipitates. It is that easy!
It is an interesting note that water is the only substance that can occur as gas, liquid, and solid in the lower atmosphere. That is quite a feat!

The last one I want to take a look at is carbon dioxide.

Carbon Dioxide?
Source:
You! When you breathe, you exhale carbon dioxide.
Also, burning fossil fuels is a big unnatural deal. We will cover this in a much later lesson.
Another big deal is deforestation. I usually say deforestation is a double-whammy because when you cut down a tree, the tree is no longer able to take carbon dioxide out of the air. Not only that, when trees are cut down, they begin to decay. When vegetation, such as trees, decays it RELEASES carbon dioxide into the atmosphere. So, it is a double-whammy...they can no longer absorb carbon dioxide AND they release it into the atmosphere when they are dead.

I will leave you with this last figure:

Figure 2. The Keeling Curve that shows the increasing concentration of atmospheric carbon dioxide over the last several decades.

What you see is one of the most famous curves in all of atmospheric science. It is called the Keeling Curve and was named after Charles David Keeling. Beginning in 1958, Mr. Keeling took measurements of atmospheric carbon dioxide twice a year at the Mauna Loa Observatory in Hawaii. Why Hawaii? Well, if you think about it, Hawaii is in the middle of a giant bathtub called the Pacific Ocean, well away from major influences from large cities and landmasses. It was a pristine an experiment place as you can get.

There is one observation you should see right away. That is, the curve has gone up relentlessly since 1958. Granted, carbon dioxide makes up only a tiny sliver of the atmospheric gases, but as we will see in Lesson 2, it doesn't take all that much carbon dioxide to shift the balance of the atmosphere.

There is another observation which I find even more fascinating. Do you see how the curve bounces up and down over the course of a year? Think for a minute before you read on about what could cause that. What could cause carbon dioxide to decrease rapidly throughout the year, only to see it increase later that year?

The answer, of course, is plants! During the Winter months in the Northern Hemisphere (the hemisphere with the most plant life), the plants die or become dormant and are no longer able to absorb carbon dioxide out of the atmosphere. Carbon dioxide concentration goes up. In the Spring and Summer, the plants come back to life and absorb the carbon dioxide. Thus, A VARIABLE GAS! This also illustrates how important plants are to the atmosphere.

Ah, I love it when a plan comes together.

See you next week everybody. Your homework is to think about what you are breathing.

Updated: 6:39 AM GMT on September 05, 2011

### Wunderground Meteorologist Shaun Tanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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