Heat Transfer

images-6Although you may not realize it consciously, much of your daily effort is in how to take heat from “here” and move it “there.” Your body certainly does this on it’s own, for keeping your internal temperature at precisely 98.6F is quite an orchestration of our warm blooded physiology; any small deviation from this temperature makes a bazillion vital biochemical reactions impossible. Outside of our human endeavor, the indifferent hand of nature acts to move the heat created in the nuclear oven of the sun from “there” to the earth, “here.” One false move any step of the way and we are  goners. The truly extraordinary thing is that heat energy, wherever it is created, inherently wants to dissipate, not concentrate, as it moves from one place to another. This is the law of entropy, one of the fundamental laws of physics; even as you may gather energy in one place, like in the logs in front of your fireplace on a cold winter evening (here), on a larger scale, more heat is randomly dissipated into the wider world (there). It’s an uphill battle. But the transfer of heat, and hence, how we might harness it, is Unknown-3rather simple. There are just three ways: radiation, conduction and convection (or advection). In understanding these three methods of heat transfer you will not just make what was a subconscious event, conscious. You will gain more control over your environment. Let’s look at each of these three.

Radiation

With only the most minor exceptions, all the energy that is exchanged between the earth and the rest of the Universe is through electromagnetic radiation, a concept, along with blackbodies, which I described in my essay on Fire. Only radiation can travel across the vacuum of space moving energy from one spot to another and at 186,000 miles/second (~300,000 km/sec). Without the energy from the sun, the surface of the earth would be at nearly Absolute Zero (-459.67°F). As it is, with the sun, the earth has an average surface temperature of 61°F (16°C), and why is that? Why doesn’t the earth just keep heating up as it receives more radiative energy from the sun? When something is coming into a pot, but the pot is not filling up, then you know that that something is also leaving in some way. And indeed, that is happening here. The earth is a blackbody – an object that can receive and emit radiation at all wavelengths but that emits most radiation at a wavelength determined by the object’s temperature. The sun, at 10,000°F, sends to earth radiation which is centered on the visible range. The earth, much cooler, reemits this energy back out into space in the lower energy/longer wavelength infrared range. The temperature on earth where the incoming radiation matches the outgoing radition happens to be at 61°F. Should the sun heat up, so would the earth, until it settled at a higher equillibrium temperature and was emitting slightly higher energy/shorter wavelength radiation. You can guess I will be revisiting this idea of an equillibrium temperature when I write a post on Climate Change!

Let’s look more closely at how radiation from the sun is absorbed as it encounters the earth, and exactly where and how this radiative energy is turned into heat energy, raising the temperature. When radiation is absorbed at the atomic level, it can make electrons jump into higher energy orbitals around the nucleus. Radiation that can do this tends to be in higher energy visible to ultraviolet to X-ray end of the electromagnetic spectrum. Radiation can also be absorbed, however, by increasing a molecule’s kinetic energy. “Kinetic” refers to movement, and so when the kinetic energy of a molecule rises it is moving more. It can do this in one of two ways: it can rotate on its axis faster or it can vibrate more (what I have previously referred to as “jiggling”). Causing a molecule to vibrate faster usually occurs at the lower energy levels associated with infrared radiation. Rotational changes are associated with microwave radiation. In any event, this increase in kinetic energy is associated with a higher temperature, and it is the way that radiation from the sun heats the earth. Exactly where in the earth/atmosphere system the various wavelengths of radiation are absorbed is a topic for a full discussion which I’ll address in a post on the “Greenhouse Effect.” Suffice it to say for now, that radiation from the sun passes nearly unseen through the earth’s atmosphere (if it isn’t absorbed or reflected by a cloud, which is liquid) and it is either absorbed at the ground or reflected back into space.

After the earth’s surface is warmed, the infrared radiation it gives off can be absorbed by such asymmetric molecules in the atmosphere as CO2 (carbon dioxide), H2O (water), CH4 (methane) and O3 (ozone), making them vibrate or rotate more, thus increasing their temperature. On a much smaller scale, heat lamps use infrared radiation to heat up the air in a room.

Convection and Advection

The second way to move heat energy around is much slower and less mysterious. Move this hot thing from one place and physically put it in another place. Take warm air from your furnace and blow it into your bedroom. Although all movement of this type is sometimes subsumed under the name convection, convection really refers to vertical air movement, while the horizontal movement of air (wind) is called advection. Convection can occur when advecting air meets a mountain and is forced upward. More often, however, the vertical mixing of air occurs when the temperature of an air “parcel” (imagine an invisible balloon) increases and it expands making it lighter; cooler, heavier air pushes in around it and the bubble of warmer air is again forced to rise.

The heat content of a bubble of air is composed of two types. The sensible heat is the heat that you can feel on your skin and which we regard as temperature. As we have seen, this is associated with the kinetic or vibrational energy found with jiggling molecules. A more subtle form of heat found in the atmosphere is latent heat. In our atmosphere, this is derived from the phase of the water content and would be zero in perfectly dry air. As liquid water leaps from the liquid to vapor state it needs an extra “kick” of energy called the latent heat of vaporization. It gets this from the sensible heat in the atmosphere, so that when water is evaporating into the air, the air tends

Latent heat. Energy "hiding" in the respective phases of water.

Latent heat. Energy “hiding” in the respective phases of water.

to chill. The total energy hasn’t changed; some if it has just been “hidden” as latent heat. As water vapor is condensed into a liquid on the other hand, it gives off this same kick, the latent heat of vaporization in reverse, and it deposits it in the sensible heat of the air. As raindrops form inside a cloud, the surrounding air warms. As the sweat on your skin evaporates it absorbs this latent heat from your body. If the air is too humid for the sweat to evaporate efficiently then it can’t help you cool down. This is why dry desert heat feels less oppressive.

The reason for bringing up latent heat is not trivial. As the wind blows from the tropics, I think it is rather intuitive how sensible heat is moved. However, it is also bringing a great deal of latent heat energy hidden in the water vapor. This becomes manifest as sensible heat when the water vapor releases it as it condenses out as rain. Latent heat, moving around with the atmosphere’s water molecules, is a vitally important factor in understanding the global heat distribution.

Conduction

Conduction operates on the smallest, molecular, scale. Recall when I have explained how molecules vibrate more rigorously as they receive heat energy and their temperature increases. This kinetic energy can be transferred from molecule to molecule when they collide. If you take a metal spoon and place one end on the coil of a hot electric stove, the heat absorbed by the spoon at the surface of the stove will move molecule by molecule to the other end where your finger is holding it. You will feel this heat as the molecules in your own skin start to vibrate faster. Ouch. Now suppose the far end of the spoon is resting on a piece of dry ice (frozen carbon dioxide, which is at least -109.3°F) instead. Soon your finger will start to feel cold as the temperature gradually falls across the length of the spoon. It would appear that “cold” is moving toward your hand this time, but that is not the case. In fact, heat energy only flows from hot things to cold things, never in reverse. This is best understood, again, by the transfer of kinetic energy. This time the heat is flowing in the opposite direction. Vibrating molecules in the spoon hit the more sedately vibrating molecules in the dry ice, transfer some of their kinetic energy to the ice, and slow down. Again this happens all the way to the end of the spoon you are holding. Here, the molecules in your finger transfer vibrational energy to the spoon and you can feel the temperature drop. In both cases the energy flowed from the higher kinetic energy to lesser kinetic energy.

Heat transfer by conduction is something that we feel often in our daily life. When we are cold and jump into a hot shower. When we are hot and jump into a cold shower. When we hop into bed at night and tense momentarily as our warm body hits the cold sheets. In the atmosphere, however, most heat transfer by conduction dominates in only two places. One of these occurs at about 100 km above the earth’s surface where the atmosphere is extremely thin, and the description is too esoteric for this essay. The other occurs precisely at the earth’s surface. Air flowing less than a centimeter from the ground tends to come to a halt due to friction. When this happens it allows vibrational heat energy to be transferred just like I described above with the spoon. Some materials are better conductors of heat energy than others. If you touch a hot pan you will obviously get burned quickly. But have you ever touched a piece of aluminum foil just out of the oven? No problem! The aluminum molecules in the foil can dance very fast, but they have a difficult time moving the molecules in your finger faster, at least compared to other materials we routinely cook with.

I’ll give a couple of examples to make a clear image. On a spring day, usually when air is being advected from the South, the warmth from the air is absorbed into the snow surface. As the ice reaches 32°F, it melts. The air cools as some of the heat energy it is carrying goes both toward increasing the water temperature and providing the latent heat of freezing to melt the ice. During the summer, the radiation from the sun is absorbed at the ground where it warms the hot tarmac of a parking lot, for example. The surface soon becomes very hot, this time warming the air that comes into contact with it.

Putting It All Together

Test question!

A bird is sitting on a branch in a tree about fifteen feet above the ground on a cool dark night just before dawn. As morning arrives and it gradually gets lighter, the bird, hidden under the leaves, begins to notice that the air around it has started to warm.  Describe the processes which are responsible for warming the air around the bird.

Give it a try!

Answer:

1. Energy leaves the hot surface of the sun in the form of electromagnetic radiation and travels for approximately eight minutes across the vacuum of space until it reaches the earth. Upon entering the earth’s atmosphere, most of this energy passes directly to the surface where it is absorbed (the blue part of the energy is scattered, which explains why the sky is blue).

2. As the very surface (only molecules deep!) absorbs this radiation, the molecules start to jiggle and the temperature goes up. This difference is sensible to the touch.

3. The vibrating molecules on the surface bump against the air molecules, sharing this heat energy, and they start to jiggle as well.

4. Now the layer of air touching the ground grows warm and expands making it less dense. Less dense bubbles of air leave the ground and convect up into the tree. The bird is warmer.

Posted on June 19, 2013, in Uncategorized and tagged , , , . Bookmark the permalink. 1 Comment.

  1. As always, it is a pleasure to be enlightened by you, Craig!

    How odd that we have different words for horizontal and vertical transfer of heat. Bizarre.

    So, what I don’t understand is how radiation can excite an atom to jiggle, I understand (somewhat) how radiation can excite an electron to jump an energy layer, but not how it makes the whole atom move more… Explain?

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