What is heat?

Brace yourself.

This is a bit difficult to grasp. Heat is the random motion of atoms, molecules, and other subatomic particles in a system. A system maybe a glass of water or a cloud or the planet Earth and everything else in the universe. Temperature is how we measure the energy of this motion. We measure it in degrees Fahrenheit, Celsius, Kelvin, or

What is heat?electrically with millivolts or spectral wavelengths and even in more exotic ways. So, if we put more energy into a system, say, like heating a pot of water, the motion and speed of the atoms in the water increases, as does the energy of the molecules colliding at higher speeds. Faster speeds mean more collisions and more powerful collisions. Similarly, the same thing is happening with the water container and the stove rack it sits on, and all connected things start heating as well (Fig. 1).

Conversely, if we chill the same pot, the motions of those same atoms and molecules slow down, and the temperature goes down. If we chill the system, say, a beaker of water to almost absolute zero (-273 degrees C), nearly all that motion stops, and weird things start to happen, like super electrical conductivity and bizarre quantum stuff we can set aside. Currently, we do not know how to chill something to absolute zero. For now, let’s focus on heat as it relates to weather and climate.

What is heat? 1Now comes the question, how does heat move? How do we get heat from one thing to another? Like from the sun to the Earth or from the flame to the pot as pictured in Fig. 2?

There are three ways heat moves from hot to cold. They are convection, conduction, and radiation. In addition, you should know that heat never moves to a hotter body from a less hot body.

– Convection is what you experience when holding your hands over a bonfire, where the hotter air near the flame rises to the cooler air and warms your cold hands.

– Conduction is like warming your hands by wrapping them around a hot cup of coffee.

– Radiation happens when I put my hands beside the bonfire, and the infrared portion of the light waves travels from the flame to my hands by electromagnetic waves. Sometimes we get one or more of these methods working at the same time.

Now let’s take a look at the water in the pot and see what happens when we put water at 20 degrees C on a burner and turn on the heat. We look at the thermometer, and the red line starts going up. The question is, how high will it go? You might expect it will go to its boiling point at 100 degrees C. We are then surprised as we keep watching it and see that when it gets to about 90 degrees C, it stops. What happened? The heat from the warmer pot and water moves to the other cooler things in the room because nature does not like any temperature imbalance.

So, when we start the flame, it starts to heat the water, but it also starts heating everything around it. Not all of the heat goes from the flame to the water. Some of the heat goes to the pot, some of the heat from the flame, pot, and water goes to the air, you, the walls, etc. Moreover, if we turn on a fan and blow the warm air away from the container, it will accelerate this heat transfer to the surrounding stuff. Perhaps with the fan on, the water temperature in the container can only get to 70 degrees C. But we can get it back up to 90 degrees C or more if we turn up the flame. Do you recognize now all the variables in nature’s efforts to balance heat on the Earth itself?

We can also flip this heat/motion thing on its head. For example, in Fig. 3, we see that if you take a big hammer and

What is heat? 2 smash it on an anvil hard, then put your hand on the anvil or the hammer, you will feel it has turned hot. Or if you smash it with a little hammer, then it only heats up a bit. So, we can change heat (thermal energy) to kinetic energy (energy of motion) and back the other way. This is called a reversible reaction.

Now comes the fun part. Energy will always move in one direction and never the other way around (high to low, hot to cold, fast to slow). We use this phenomenon in thousands of different ways every day without even knowing it. Engineers and scientists call this the second law of thermodynamics.” If I put an ice cube on top of a hot pan, the pan’s heat moves to the ice cube by one or more of the three means described above until the temperature difference is eliminated. Similarly, in this case, the heat from the pan warms up the ice cube until it melts and keeps heating it until the water turns to steam and vaporizes. With all the water gone, the stove continues to heat all the other stuff in the kitchen until a new higher temperature equilibrium is reached.

In weather and climate, this is a fundamental concept, as it explains many things. It describes how the sun warms the air, sea, and land during the day and why and how it cools at night. It also helps to explain how the oceans store heat and how that heat is redistributed around the planet in a vain effort to eliminate all heat and energy differences. Another thing it explains is how that heat is distributed around the globe in a futile attempt to reduce all heat and energy differences.


How about the Earth itself? There is a considerable scientific debate on how much or how significant this heat is. In a July 19, 2011 article in Physics World, a group of scientists estimated this internal heat to be approximately 50 percent of the total heat that is radiated from the Earth into outer space. Others consider that figure to be only a few percent. The debate will likely continue for a long time because it appears impossible to measure, or even to estimate. Also, what remains a significant unknown is how much of this core heat is generated by the radioactive decay of Uranium and Thorium versus how much is from the primordial energy left from Earth’s formation.

Eons ago, kinetic energy from the asteroids and meteors smashing to the Earth (remember the hammer hitting the anvil?) imparted a lot of energy into the Earth’s core. We know that this internal heat (nuclear plus residual) accounts for the molten core that generates the Earth’s magnetic field and protects us from some of the sun’s solar cosmic rays and solar winds. It’s also the same energy that moves the tectonic plates around the planet and occasionally results in volcanic eruptions, earthquakes, and tsunamis. It is also the source of the geothermal power used in countries like Greenland and Iceland to make electricity and entertains us as we watch “Old Faithful” periodically explode at Yellowstone National Park. For climate change purposes, we can’t control this internal heat, which we can’t even measure. Therefore, we’ll treat it as a constant and “ignore it” until better data becomes available. Other sources can be natural

like a forest fire or human-made sources such as factories and automobiles.

Human-made heat comes in two varieties. The first is the direct sources, like the chemical discharge of combustion at power plants, or cars, trucks, and waste heat from electrical power plants. Then there is indirect “heating” by the retention and storing of heat by the greenhouse effect. Note that CO2 does not provide any heat by itself, but it does help the Earth retain some heat by preventing it from escaping to outer space too quickly.

At our visual level, you can see heat as a simple subject. At the Atomic-level it is very complicated. For nature, it is the reason for everything that occurs in our weather and climate.

For more information on the subject, we recommend our book A Hitchhikers Journey Through Climate Change.

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