Atmospheric Heat Engines—Global Atmospheric Circulation
Written by Dr Jerry L Krause
C. Sutcliffe (Weather and Climate) wrote: “It was from the study of the prevailing winds as they were observed, especially the surface winds from the days of world exploration in sailing ships, that the concept of the general circulation of the atmosphere, the most important concept in the whole of meteorology, arose.”
And, “If the general circulation is, by definition, a matter of average winds of the world it immediately embraces all the other major physical quantities and physical processes. One cannot go far in the study of winds without introducing the driving force, the pressure; and one cannot go far in the study of pressure without introducing air density and air temperature for the pressure of the air above. Again the study of temperature introduces energy and its transformation, the absorption and emission of radiation, evaporation and condensation and the conversion of latent heat: vertical motions introduce clouds and rainfall, and so on until the whole of weather science is drawn into the argument. Thus, a complete understanding of the general circulation of the atmosphere implies a complete understanding of every process in world climatology and it is for this reason that it may justifiably be called the central problem of the science.”
If I were to ask—What is the prime mover of this atmospheric circulation?, what might one answer? In C. Donald Ahrens very popular meteorology textbook—Meteorology Today 9th Ed—I find the statement: “We know the primary cause of the atmosphere’s general circulation is the unequal heating that occur between tropical and polar regions.”
Steven A. Ackerman and John A. Knox in their popular meteorology textbook, Meteorology 3rd Ed, introduce the idea of conceptual models. And they state: “A simple conceptual model of the global wind pattern must explain the steady winds that have long been observed by mariners. A model of global atmospheric circulation must also be consistent with other observed pattern, such as the position of deserts and regions of high precipitation in January and July (Figure 7-4, shown below).
From the figures above can be seen how small these regions of great precipitation are relative to the total area of the earth’s surface; we see where they are (generally) and where they are not.
The titles of the first five chapters of Physical Chemistry 2nd Ed. by Farrington Daniels and Robert A. Daniels were: 1. Introduction; 2. Gases; 3. First Law of Thermodynamics; 4. Thermochemistry; 5. Second and Third Laws of Thermodynamics. In the first chapter I learned that physical chemistry was composed of thermodynamics, kinetics, quantum theory, statistical mechanics, and molecular structure. In the second chapter, while there was considerable mathematical equations and reasoning, about the only thing I could claim to have learned was PV=nRT (The Ideal Gas Law).
The third chapter began: “This chapter provides an introduction to the applications of the concepts of heat, work, energy, and heat capacity in physical chemistry. It is followed by a chapter on the measurement of the heats of chemical reactions. Much of physical chemistry is based upon thermodynamics, which deals with the heat and work accompanying chemical and physical processes. In this chapter I learned there were five types of work: Mechanical; Volume expansion; Surface increase; Electrical; Gravitational. I learned that “Only work and heat can be measured directly, but it is helpful to define energy, which includes the internal energy of a substance as well as heat and work.” And I learned the First Law of Thermodynamics: “A cyclic process is a process in which a system is carried through a series of steps which eventually bring the system back to its initial conditions. The change in internal energy for a cyclic process is zero since the internal energy is a function only of the state of the system.” And there were more mathematical equations which I cannot claim to fully comprehend as I know others do.
In these general comments I pass over the 4th chapter because it is Thermochemistry and not so directly related to the physical properties of the atmosphere. And the 5th chapter is the primary reason I have briefly reviewed the first three chapters and the little I learned in their study. The 5th Chapter began: “The first law of thermodynamics states that when one form of energy is converted into another the total energy is conserved, but it does not indicate any other restriction on this process. Although various forms of work can be converted completely into heat and ideally may be transformed completely into one another, it is found that only a fraction of a quantity of heat may be converted into work in a cyclic process.”
I have reviewed how little I learned in the first three chapters to state: This last statement I consider I fully understand and it is something I have never read in the context of the endless 2nd Law arguments that are made in the context of the greenhouse effect. I have never read that a thunderstorm, during which atmosphere is lifted from the surface to the top of the troposphere, is a clearly defined heat engine except the cyclic is not completed during its brief existence.
It is written that Solomon long ago stated: “The sun rises and the sun sets, and hurries back to where it rises. The wind blows to the south and turns to the north; round and round it goes, ever returning on its course. All streams flow into the sea, yet the sea is never full. To the place the streams come from, there they return again.” (Holy Bible, Ecclesiastes 1: 5-7 (NIV)
Hence, there is little doubt that when we study weather and climate we are studying cyclic processes. So, the purpose of this article is consider what we know about heat engines and their functioning. I am sorry it took so long to get to this point, but it seems an observable fact that those who debate weather and climate have never gotten to this point. However, these debaters might consider that what I will review is too unique and therefore term if crazy. While yet others might claim that thunderstorms have been described in detail so nothing of worth can be added to what is known.
However, I believe we can generally agree that thunderstorms are small, isolated, atmospheric events of short duration. I believe we can agree that after a thunderstorm has ‘passed’ there remains an observable ‘thin’ cloud high in the atmosphere indicating that all the matter lifted from the surface has not been cycled back to the surface in the vicinity of the thunderstorm. But there is a rule: what goes up must come down. So there is the question: Where does that cloud, which remains after the event of the thunderstorm long passed, come down? But this will be a question for later.
The first question that it seems to need be answered is: Why does this small thunderstorm, which so quickly does work in lifting atmosphere from the surface to the top of the troposphere, have such a brief existence? The answer, I believe, is: “it is found that only a fraction of a quantity of heat may be converted into work in a cyclic process.”
The sun heats the earth’s surface and the surface heats the atmosphere in contact with it and according the ideal gas law this heated atmosphere at the base of the atmosphere expands so that it density becomes less than nearby surface atmosphere which has been heated as much.
So by the principle of buoyancy the more heated surface atmosphere is lifted by the denser, cooler, atmosphere away from the surface. Of course, this process has been described over and over. And we understand that the temperature of this lifted parcel of atmosphere decreases as its internal kinetic energy is converted to gravitational potential energy.
And we understand (observe) that at some point, as this parcel of atmosphere is lifted, water vapor will begin to condense to form cloud. And we understand that this condensation of water vapor releases energy (termed the latent heat of condensation) which slows the cooling of the temperature as the parcel continues to be lifted through an atmosphere whose temperature is less than that of the parcel.
And we understand this lifting process would continue to the limit of the atmosphere if the temperature of the atmosphere through which the parcel is being lifted did not begin to warm so that at some point the temperature of the parcel was no longer greater than that of the surrounding atmosphere which had been lifting the parcel.
What has been lost in this common description of a thunderstorm event is: “that only a fraction of a quantity of heat may be converted into work in a cyclic process.” And I do not know how to find it except by drawing upon our common experiences (observations) of the heat engines that we have invented. For what happens to these heat engines, if the energy of the fuel being consumed that is doing a small amount of work, is not removed from the vicinity of the engine?
We know, I believe, that the engine will quickly self-destruct. So my answer to the question— Why do thunderstorms have such a short duration?—is that, if they have no process by which the excess heat can be removed from their vicinity, they self-destruct. This is the case if they are isolated from another very important, unique, sometimes feature of the atmosphere.
This feature is termed a jet stream. So, as the parcel rises into it, the excess heat, as well as some of the cloud, is quickly carried away from the top of the thunderstorm so that atmosphere can be continuously lifted from the surface to the base of this jet stream and the lifetime of the thunderstorm can be much longer. However, jet streams, part of the atmospheric circulation system, are rarely, if ever, observed in tropical regions but thunderstorms certainly are.
Another thing we know about the heat engines, which we have invented, is that they need a fuel to do work. There seems to be little doubt, once one considers it, that water vapor is the fuel of thunderstorms. And I quickly jump to the conclusion that precipitation is the exhaust of these natural heat engines known as thunderstorms. For we observe that even though thunderstorms can have a short lifetime, they often produce a great amount of precipitation in their short lifetime. Hence, the great importance of Ackerman and Knox’s Figure-4. For in this figure we can see (observe) the quite small localized regions where thunderstorms exert their tremendous influence in driving the global atmospheric circulation. The fewer thunderstorms, which occur at higher latitudes, than those of the tropics, do drive certain regional atmospheric circulations. And it has been observed how these higher latitude thunderstorms can strongly influence the localized atmospheric circulations of their ‘vicinities’.
I love the Sutcliffe quote—“All this may seem a far cry from the general circulation of the world’s atmosphere but the detail serves to point the moral, that one cannot explain the broad features of world climate if one does not know the actual mechanisms involved.”