What is Heat?

Daniel Richardson 2025 -- 2026

Introduction: Unless you've been living in a temperature-controlled environment, I am sure that you have noticed a progressive increase in warmer weather over the past decade or so along with just about everybody else on the planet. In fact, the warmest years over the past 174 years that temperature records have been kept, have all occurred over the past decade. The rising heat that the world has been experiencing is referred to as "global warming", a phenomenon which is of major concern due to the fact that, as reported by the National Weather Service, heat is the leading cause of death among all weather-related events.

To give you an idea of how deadly heat can be, in August of 2003 a heat wave settled over Europe, in particular France, driving temperatures up into the high 90s with spikes over 100. Within two weeks in France alone around 15,000 people died as a direct result of the heat.

This is not the first time that the earth has experienced a period of global warming. On this note, geologic evidence has shown that the earth has gone through a dozen such periods over the past million or so years with each warming period followed by a cooling period. In other words, the earth is no stranger to periods of rising heat. This being the case, it might be beneficial to have a closer look at heat in relation to global warming.

Accordingly, this essay is about heat itself. What is it. How is it transferred from warm to cool objects. And, how does it work to warm the earth.

What is heat: The still valid notion as to how heat is generated was given by Sir Isaac Newton who back in the eighteenth century stated that heat comes about by way of the "trembling agitation of the smallest parts of all bodies." This is what we now understand as vibration of the atoms that bond together to make up molecules (e.g. the molecule carbon dioxide [CO~2~] is one carbon atom and two oxygen atoms.). Rather than saying the vibration of the atoms that comprise molecules, we simply say the vibration of molecules.

So, basically, heat is a form of energy produced by the vibration of molecules that make up any object, such as a plant or a rock. That is a very broad picture of heat the details of which are quite controversial. The following represents my consensus of the literature on this subject which may or may not be entirely correct, but it at least provides an ample notion of what heat is, which serves the purpose of this essay.

Owing to modern physics and chemistry, we know that at any temperature above absolute zero, which is a very cold -459.67^O^ F, molecules are always in a state of motion and emitting energy, which, in keeping with Newton's notion, is termed thermal radiation. The more intense the molecular vibration the more intense the thermal radiation and the higher the temperature associated with the radiation. However, it is not until temperatures reach about 80^o^F that molecular vibrations are strong enough to emit thermal radiation in the form of infrared rays, which are what we feel as heat.

As a side bar, infrared rays are a component of what is known as the electromagnetic spectrum. This is a series of energetic waves that are all around us ranging from low energy radio waves to high energy gamma rays. Infrared rays are about midrange in this series and occur next to the energy range of visible light and ultraviolet (UV) light. Individual components of the electromagnetic spectrum enable us to do things like watch television, talk on a cell phone, heat a cup of coffee in a microwave and see the world in technicolor.

The reason we feel infrared radiation as heat is because these rays penetrate our bodies where they elicit the vibration of molecules within us eliciting heat also in the form of infrared radiation. In this manner infrared radiation can be thought of as both the cause and the effect of heat.

There are several ways in which molecular vibrations are increased to the point of producing infrared rays. For example, when an object is struck by radiation coming from the sun or from a nearby source of heat, such as the furnace in your home, heat is produced as the radiation increases the vibration of molecules.

A completely different source of molecular vibrations, hence heat, for us is the metabolism that occurs in our bodies as well as in all warm-blooded creatures (e.g. all mammals). In this case heat comes about as a byproduct of the action of the high energy molecule adenosine triphosphate (ATP). The breakdown of this molecule to adenosine diphosphate (ADP) releases the energy that propels a large number of biological activities, such as the contraction of muscle fibers or the digestion of food in the GI tract. However, in terms of bodily functions, this is an inefficient system in which only about 40% of the energy released by the conversion of ATP to ADP is used to drive biochemical reactions, with the remaining 60% spent in causing molecular vibrations in the ATP/ADP structure which results in generating the heat which keeps us warm on the inside.

In brief, any heated object in the temperature range of 80 to 900^o^ F, such as the furnace in our home or the coals in campfire, produces infrared rays which warm up the environment, including us if we are part of the environment.

One additional note about infrared ration is that it is invisible to us but not to snakes, in particular rattle snakes. This is something that hikers, particularly in the southwest, should be aware of because rattle snakes use the infrared radiation coming from warm bodies to locate their pray or the leg of a hiker who has gotten too close.

Infrared rays are not the only forms of electromagnetic radiation associated with heat producing thermal radiation. At temperatures from 900^o^ on up, radiations in the visible range of colors are emitted, and because they contain more energy than infrared rays, they elicit a more intense vibration of molecules within objects that they penetrate and hence a more intense heat which when intense enough can "burn" the associated object. A candle flame, for example, emits red, orange, yellow and blue light with red being at the lowest temperature (1200^o^ F) and blue the highest (2600^o^ F) all of which can cause burns, as anyone who has stuck their finger in a candle flame knows.

Exactly how is heat transferred from warm to cool objects: In the late eighteenth century the French chemist Antoine Lavoisier proposed what he called the "caloric theory of heat". This notion held that heat was a substance similar to water or a gas in that it flows from warm to cooler objects much like water flowing downhill.

The idea of heat flowing from warm objects into cooler ones remains a popular notion today. But is it correct? Well, that sort of depends on your point of view. To get at this, we will illustrate using a common everyday occurrence experienced by my household and our neighbors. We live in a patio home complex in Tempe, AZ, part of the Phoenix municipality. Each home has its own metal mailbox. In the summertime these things get quite hot as the intense radiation from the sun elicits strong vibrations of molecules within the metal of the mailbox. These vibrations generate considerable infrared radiation (heat) within the metal. When someone comes along and grabs the handle of the mailbox lid to open it, they find the handle hot to touch. What is happening is that vibrations of molecules within the metal of the mailbox generate heat carrying infrared radiation, which then penetrates into the fingers of the person opening the box thereby eliciting similar vibrations within molecules of the person's fingers. These vibrations in turn generate even more infrared radiation, hence heat, within the fingers thereby causing the unsuspecting mailbox opener to have a sudden owie.

With all that going on, why is it that the mailbox, or any hot object, feels hot as soon as you touch it? Fundamentally, this is because collectively all the steps involved from the penetration of an infrared ray into the finger to the registration of pain in the brain take less than 15-thousandths (.015) of a second. Since we cannot sense such short a time interval, the heat we feel when touching a hot object seems instantaneous to us.

In terms of basic thermodynamics, when a hot object, such as a metal mailbox in the summer, is touched heat energy is transferred to the cooler object, this being the fingers of the person doing the touching. Given that this is a transfer of energy, it must conform to the first law of thermodynamics which states that energy can be transferred from one object to another but it cannot be created nor destroyed. Accordingly, in our mailbox example, the mailbox loses heat energy while the finger gains an equal amount. In this manner, the sum total of heat within the mailbox and the fingers combined remains the same before and after opening the lid. Simply put, the metal in the mailbox becomes a bit cooler while the fingers become a bit warmer.

But, did heat actually flow from mailbox to fingers? Although, there was no actual flowing of heat as a substance, like water, if you consider that the infrared radiation generated within the mailbox penetrated into the fingers with virtually no resistance, then heat does flow from warmer to cooler objects in a manner similar to the way water flows downhill. In my opinion, that's a legitimate way to look at it.

Obviously, any area of skin, not just the fingers, will heat up when exposed to radiation from a source, such as the sun. Accordingly, when out on a hot summer day radiation from the sun will heat up your skin in a manner similar to the mailbox-finger illustration. While clothing does a good job in blocking the UV rays that cause the skin to tan or burn, up to 88% of the sun's infrared rays will pass through clothing and penetrate your skin causing molecular vibrations which further heats you up. Remember, you already have an internal heat source in the form of body metabolism which is added to by the heat generated from the penetration of thermal radiation in the form of infrared rays.

At this juncture, we have pointed out that heat as we know it is produced by thermal radiation mainly in the form of infrared radiation. To simplify the remainder of this essay, let's refer to this simply as heat.

How does heat work to warm the earth: Now that we know what heat is and how it is transferred from warm to cool objects, let's have a look at how heat from the sun keeps the earth, and all within it, warm. This occurs by way of a phenomenon known as the greenhouse effect which has been going on ever since plant life first appeared on the earth some 460 million years ago. It is important to point out that in the absence of a greenhouse effect, the global temperature now and in the geologic past would be consistently below freezing. Accordingly, it is the greenhouse effect that has enabled life as we know it to have evolved on the earth. The important point about the greenhouse effect for our consideration is that it is the main driver of the global warming that we are experiencing today.

To illustrate how the greenhouse effect works, consider an actual greenhouse. The rays from the sun pass through the greenhouse glass windows and strike the plants being raised within causing molecules in the plants to vibrate. These vibrations then generate heat, which is dispersed into the surrounding air. The heat so generated is absorbed by the greenhouse glass which then re-emits some of it back into the greenhouse causing more vibrations, hence heat, within the plants which then emit more heat into the environment. This action creates a back-and-forth cycle continually generating heat some of which stays within the greenhouse keeping the plants warm, while a portion of the generated heat escapes into the outside atmosphere via vents which can be adjusted so as to maintain a desired steady state temperature within the greenhouse.

The greenhouse effect in our world is similar to that of a real greenhouse, but instead of greenhouse glass we have greenhouse gas -- volatile air-born molecules such as carbon dioxide.

The way this works is as follows:

Energy from the sun's rays cause molecules within all matter on earth, such as plant life, to vibrate.

This vibration emits heat in the form of infrared radiation, which is projected in all directions including back up into the atmosphere.

Some of the upward projected radiation is absorbed by certain molecules in the atmosphere, such as carbon dioxide and water vapor causing these molecules to vibrate and generate heat.

A portion of the heat absorbed and generated by the atmospheric molecules is emitted back toward earth where it is reabsorbed by earthly matter again eliciting more molecular vibrations, hence heat, which is then projected back into the atmosphere some of which is again absorbed by molecules, such as carbon dioxide, which then again reemits heat back toward earth further heating up our atmosphere.

This back-and-forth is known as the greenhouse effect, and the participating atmospheric molecules are known as greenhouse gases.

The greenhouse effect continues throughout the day as the sun rises producing our steady increasing temperature, then reverses as the sun goes down. Furthermore, the greenhouse effect has a seasonal rhythm, being stronger in the summer months when the sun is more directly overhead.

More about greenhouse gases: According to the above scheme, the more greenhouse gasses that are present, the more infrared radiation exists to heat up the atmosphere. Although there are several greenhouse gases, the main two are Carbon Dioxide (CO~2~) and water vapor. Where water vapor is abundant in humid air and in clouds it can account for as high as 85% of the total greenhouse effect. However, CO~2~ is the main greenhouse gas in a dry environment. Furthermore, it is abundant world-wide and relatively easy to measure and track. Accordingly, CO~2~ is the one used in discussions of things like the greenhouse effect and global warming.

In addition to the abundance of water vapor and CO~2~ as greenhouse gases, methane (CH~4~) and nitrous oxide (N~2~O) are also abundant and have considerable greenhouse effects. Nitrous oxide mainly comes from microbial action in soil, but also from livestock manure and the use of nitrogen containing fertilizers.

Methane, the main constituent of natural gas, is emitted into the atmosphere from decaying organic matter, including that found in landfills (your garbage at work), and from a digestive process in cattle called "enteric fermentation", the source of cow burps. A single cow can release up to 500 liters of methane a day in burps. Additionally, all livestock, including cows, release methane through flatulence and manure.

So, cows release methane at both ends of their digestive system, and there are a lot of cows world-wide, about 1.5 billion according to the U.S. Department of Agriculture. That's a lot of methane getting into the atmosphere which is of concern because molecule-for-molecule methane is a more potent greenhouse gas than CO~2~. However, for most of the world there is more CO~2~ around than methane except for in the Artic where thawing of the permafrost is releasing massive amounts of methane along with CO~2~.

At this point, we have learned what heat is, how it is generated and how it is transported from warm to cool objects. Additionally, we looked into the greenhouse effect, and how it serves to warm the earth, an important consideration given that the greenhouse effect is thought to be the main driver of global warming. Whether or not all these points were adequately covered, I will leave to your judgment.

Before closing, we need to consider another form of electromagnetic generated heat that comes from the sun, this being ultraviolet (UV) radiation. Like infrared radiation, UV rays can penetrate objects, including us, causing the vibration of molecules which then generate thermal radiation (heat). However, in mass, they are less affective in doing this compared to infrared rays, even though UV rays are more energetic than infrared rays. This is because, thankfully, a high percentage of UV rays coming from the sun are absorbed by our atmosphere, then emitted toward earth as visible light or infrared radiation. Accordingly, on the whole, much more infrared than UV radiation strikes us.

Our main concern with UV radiation has to do with its biological effects on human skin. Exactly which effect depends on which form of UV radiation is doing the damage. On this note, UV rays come in three flavors: UVA, UVB and UVC.

UVA rays are the most abundant due to the fact that a full 95% of them emitted from the sun reach the earth, hence us. They can penetrate deep into the skin where elastic fibers, nerve cells, blood vessels and fat tissue are located. Damage of these structures by UVA rays causes the wrinkling and loss of skin elasticity associated with aging.

UVB rays are mostly absorbed by the ozone layer of air, with only about 15% of them reaching the earth surface. Since they have a higher energy level than UVA rays, they can be more damaging. However, they penetrate mostly the outermost layer of skin, the epidermis, where they can cause considerable damage in the form of sunburns and more importantly skin cancer. With this in mind, it is understandable why the thinning of the ozone layer due to air pollution over the last few decades is of such concern.

UVC rays are the most energetic and damaging of the UV rays. But, as of now, they are completely absorbed by the ozone layer, and never reach the earth's surface. However, artificial sources of UVC rays do exist and can be quite damaging. For example, the UVC rays that come from a welding torch can cause corneal inflammation in welders who don't have their eyes properly protected.

As previously mentioned, like infrared rays, UV rays elicit molecular vibrations, hence heat. However, along with molecular vibrations, UV rays damage skin cell DNA which leads to an inflammatory response. In the case of epidermal cells this response consists of an increase in blood flow to the area along with the pain and burning sensation associated with sun burn. A protective mechanism against sun burn is that UVB rays stimulate the secretion of the pigment melanin from melanin producing cells, called melanophores, within the epidermis. Melanin provides the suntan that blocks the penetration of UVB rays into the skin. However, over stimulation of melanophores by excessive sun exposure can result in a melanoma, the most serious form of skin cancer.

Just the right amount of a melanin producing suntan can offer good protection against UV rays. But finding the right amount can be very tricky. A much safer and more effect way to block UV rays is to use sun screen which is available at most any drug store. Most brands of sun screen show the sun protective factor (SPF) on the bottle or tube, which is specific for UVB rays. The protective factor formula is: 1/SPF = the fraction of UVB rays that will penetrate into the skin. For an SPF of 50 (SPF = 1/50 = 0.02), meaning that only 2% of the UVB rays striking us will penetrate into our skin. Not bad!

Admittedly, the preceding treatment of UV rays had little to do with the main theme of this essay, what is heat. But, UV rays are a source of thermal radiation as well as being potentially damaging to the skin, in particular being a source of skin cancer. As such, I felt that treatment of them would be worthwhile. I hope you agree.

Sources of information:

Internet searches provided most of the information used in this document.

Additionally, the first section of Neal DeGrasse Tyson and Lindsey Nyx Walker's book titled:

"To Infinity and Beyond" (National Geographic, 2023) provides information on the electromagnetic spectrum and the greenhouse effect.

For extra reading, Jeff Goodell's book titled: "The Heat Will Kill You First" (Little Brown & Co, 2023) provides a thorough treatment of the hazards of living in a hot environment.