Lesson 2, part 1: Temperature

By: Shaun Tanner , 2:24 AM GMT on September 18, 2011

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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...

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3. reiven
12:28 PM GMT on October 02, 2011
Many of today's major mammalian orders emerged in the wake of the PETM cv writing help
Member Since: October 2, 2011 Posts: 0 Comments: 3
2. BirdsAreOff
5:11 PM GMT on September 24, 2011
Ok. Understand temperature is a general measure of particle speed, which would explain -- sort of -- how thermosphere can be cold yet contain molecules that would feel hot. But if temp of thermosphere is considered warmer than troposphere, and yet we would freeze there, then temp ceases to be a meaningful measure somehow, or what conclusions can be drawn from temperature need to be revised, or . . . . Anyway, again, this is interesting. Also, my husband is wondering what happened to the ionosphere! Thanks.
Member Since: August 29, 2011 Posts: 0 Comments: 2
1. Bogon
1:04 PM GMT on September 21, 2011
I don't come here to quibble, Shaun, so I hope you'll take my remarks as constructive criticism.

After your exposition of kinetic and potential energy, I expected your definition of temperature to be based on a statistical measure of the energy of the particles in a substance. Energy accounts for the differences in mass of individual particles. Thus you can get a similar measurement for helium and uranium, two elements whose atomic weights differ drastically. At a given temperature uranium atoms move much more slowly than helium atoms.

I say ‘particles’, not ‘molecules’, because not all substances are molecular. Helium is a monatomic gas. Uranium is a crystalline solid. Most of the visible matter in the universe is in the form of plasma, which is so hot that molecules dissociate, and even atoms break down into more fundamental particles.

Oh, yeah, the three cars' speeds in your example should be divided by 3, not 2.
Member Since: June 26, 2008 Posts: 80 Comments: 3822

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About shauntanner

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

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