Last updated 10-1-07
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1st Negative Consequence: Pollution of Air, Water, and Soil:
Tutorial #D1:
Misconceptions About Air

Before you can understand air pollution, you need to have an understanding of air. One way to begin learning about air is to clear up the misconceptions of air.

Misconception #1: There's a lot of air

To us the atmosphere seems very thick, but compared to the Earth, it is only a thin skin of gas (red circle). Seeing how thin it should help us realize how easily it could be filled with pollutants.

Besides giving us the oxygen we need to breathe, it also sheilds us from harmful ultraviolet radiation and small meteors. The atmosphere also helps hold in the warmth of the sun and spreads the warmth more evenly over the Earth's surface.  It also carries water from the oceans to the land by way of clouds and rain.

Misconception #2: Air is light weight

Misconception #2: Air is light weight.

We don't feel the weight of air nor do balloons seem heavy.  It is true that for the same volume, air is lighter than liquid or solids.  But there are many miles of air above us pushing down with incredible weight.

Air is so heavy that it can lift this Jet. Even when the jet is sitting on the ground, the air underneath the jet is lifting with several thousand pounds of force. However, the air on top of the jet and on top of the wings is pushing it down with practically the same thousands of pounds of force.

This is how the plane can fly. The top drawing shows the wing not moving in the air. Because the top of the wing is curved, there is more surface area and more air hitting it. The 7 balls represent the number of air molecules hitting it. Underneath the wing there are only 6. The top 7 don't push down more than the bottom 6 because the 7 are hitting at an angle which decreases their downward push. For example, the top #3 molecule has the same force (pressure) as the bottom #3, but the top #3 hits at an angle reducing it's downward force.

But when the wing is moving, the air splits at 0 and rejoins at 7. So the top doesn't have 7 hitting it anymore, just 6. The six on the top of the curved wing hit at angles which reduces their downward force. The 6 under the wing hit the wing with a push that goes straight up. So the bottom 6 out-push the top 6 and the plane is lifted up.

Air gets heavier as it cools. For example, sometimes rain falling in a thundercloud cools the air fast and the heavy air comes crashing downward. The dropping speed is usually about 45 miles per hour but can reach 200 miles per hour endangering planes, people, and property. This is called a microburst.
Normally the air gets cooler as we go higher, but sometimes the air near the ground is colder than the air above it. Because cold air is heavier, it will stay close to the ground. This traps pollutants. This condition is common in Phoenix in the winter and results in the infamous brown cloud.
Misconception #3: Suction
Suction Cups
The way suction cups work is usually very much misunderstood. Contrary to popular believe, suction is not what holds suction cups to the surface.
Before a suction cup even attaches to a surface, there is air pressure pushing on the surface at about 15 pounds per square.
When the suction cup touches the surface, air is still between the suction cup and the surface (e.g., table top or window). Air underneath the suction cup presses in all directions, which includes pushing up at 15 psi. The suction cup can easily be pulled away at this point.

However, if the suction cup is pressed all the way down to expel all of the air, there is no longer any pressure pushing upward on the suction cup and the outside air pressure is now pressing down on the suction cup. This is what holds the suction cup in place.

So there is no sticking or attraction force that makes the suction cup stick to the surface. It is held there by air pressure. It just seems that the suction cup has some unseen stickiness holding it there.


Note: In a similar fashion, any time air gets squeezed out from between two objects, they will be held together by air pressure. For example, if you've ever walked through mud, you noticed how hard it is to lift your feet. That's not because mud is sticky or thick. It's because air gets squeezed out from between your shoes and the mud. Air pressure will try to hold your feet and shoes down.

Sometimes suction cups are used for lifting or pulling. Let's say the surface is that of a table. When a person pulls on the middle of a suction cup, they have to overcome the pressure of the 80 miles of air above the suction cup, which is 15 pounds per square inch. When they do, the suction cup lifts a little creating a vacuum gap. A vacuum is nothing, so it has no force. So it no longer pushes down on the surface. However, underneath that surface, air pressure is still pushing up at 15 psi, so it will lift the table (provided the table doesn't weigh too much). For example, if the vacuum gap covers 3 square inches, then 3 square inches under the table will push with 45 lbs (3x15 psi.) To lift a heavier table, you need a bigger suction cup that can make a vacuum gap with a bigger area.

Drinking through a Straw
Even though babies know how to drink from a straw, most people, young or old, don't know how it works. Most people think the suction caused by our mouth pulls the liquid up through the straw.

One clue to understanding this is to notice what happens to the cheeks of people drinking through a straw. This is especially noticeable if the drink is thick like a shake. You see that their cheeks are pushed in.

This is caused by the air pressure outside their cheeks being higher than the air pressure in their mouth.

Normally when your mouth is closed, there's not much air (blue spheres) inside your mouth. They bounce around causing 15 psi pressure in the mouth. However, when you drop your jaw and keep your lips closed, there's more room for the air to spread out (Roll mouse over image to see this). The air molecules are now spread out over a larger volume, so fewer are now striking each square inch of the inside of the mouth; so the air pressure inside the mouth is less (perhaps about 10 psi). Outside air at 15 psi is trying to get into the mouth. It pushes on the cheeks causing them to be sunken.
If you have a straw in your mouth, then air pressure pushing on your drink has more force than the force from the air in your mouth. The outside air pressure pushes onto the surface of the drink. This pressure pushes liquid up through the straw to your mouth. When you don't want to drink anymore, you will move your jaw upwards causing the air in the mouth to be more crowded, which increases the air pressure in the mouth to equal that of the outside air pressure. The liquid will stop flowing.
Misconception #4: Calm Air

Since air is invisible it's easy to think of it just being still because we've heard the phrases, "calm air" and "the air was still". However, even when air is not blowing, it is far from being still.

In a helium balloon, the helium atoms are travelling an average of 3,000 miles per hour! In air, the oxygen and nitrogen molecules* are travelling about 1,000 miles per hour! At these incredibles speeds, one atom will collide with other atoms and objects near them 7 billion times a second!

(*Note: Oxygen atoms travel in pairs as does nitrogen atoms. Two or more atoms combined is called molecules. So we say a gas of oxygen molecules and nitrogen molecules rather than saying oxygen atoms and nitrogen atoms)


This means that if a small air leak develops in the vacuum of space, air molecules rush out over a 1,000 miles per hour. There is no atmosphere to hold back the leak.
Misconceptions #4 & 5: Air is uniform & Hot Air Rises
Air is invisibile, so when we look at things through the air, we normally think the air is uniform. If that were true, this balloon would not float.

The air around us is being compressed by all of the air above us. The closer the air is to the ground the more weight is on it, and the more compressed (or dense) it is. That means the air at the top of an object is not as dense as the air under objects. In fact the air around us is constantly trying to lift everything because the air below objects press a bit harder than the air above the object.

In this picture, I replaced some of the balloons with bowling balls to make the point that air is pushing up on bowling balls or helium filled balloons the same amount. (as long as they are the same size). Again, that's because the air under a bowling ball or balloon is more dense (compressed) than the air near the top of the bowling ball or balloon.
The balloon rises because the air pushing on the bottom of the balloon has a greater force than the downward force of the air on top of the balloon plus the balloon's own weight. In the case of the bowling ball, the red weight arrow is so large, the upward lift from the bottom air goes unnoticed.
So the real reason hot air balloons float, is NOT "Hot Air Rises." It's because the air pressing on the lower half of the balloon is greater than the combined air pressing on the upper half and the weight of the balloon. All the hot air does is to reduce the weight of the air in the balloon. Hot air is more spread out than cool air, so it's lighter. When the burner is on the air inside is heated and as it expands much of it goes out the bottom of the balloon. This reduces the red arrow, which represents the total weight of the balloon.
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Since Feb 22, 2004