Chapter 3 Thermodynamics – ATMO 200

Chapter 3: Thermodynamics
Alison Nugent and David DeCou
Learning Objectives
By the end of this chapter, you should be able to:
1. Define and describe four methods of energy transfer
2. Describe the change in energy associated with changes in water state
3. Define and apply the first law of thermodynamics
4. Differentiate Eulerian and Lagrangian frameworks
5. Describe the importance of the dry adiabatic lapse rate, and recall what sets
its constant value in the atmosphere
Chapter 3: Thermodynamics – ATMO 200 http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodyn...
1 sur 33 03/08/2020 à 00:29
http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodynamics/

8. Compute the Bowen ratio, and define latent and sensible heat flux
Introduction
What is Energy? You may hear frequently about “green energy”, “clean energy”, “re-
newable energy”, and “solar energy” in the media, as energy is currently a hot topic.
Power plants, wind turbines, and solar panels may come to mind. The first paragraph
in Roland Stull’s Practical Meteorology Chapter 3 discusses several types of energy
right off the bat, but first of all, what is energy?
Convective clouds in the atmosphere are driven by an enormous source of energy called latent heat (CC BY-SA
2.0).
Your intuition will probably tell you that we need energy in order to do things. Energy
makes stuff “go”. Your appliances, your car, and your body all need energy. Your body
utilizes energy to maintain its various functions even as you read this sentence. You
Chapter 3: Thermodynamics – ATMO 200 http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodyn...
2 sur 33 03/08/2020 à 00:29

all evidence of energy at work, on scales both micro and macro.
Energy can be defined as the ability to do work. When you apply a force on an ob-
ject, it is said that work is done on the object if that object is displaced, meaning it
moves from its original location. For example, when you pick up a book, you exert a
force against gravity causing the book to change position, and you do work on the
book. The higher you lift the book or the further you throw it (should you decide to),
the more work you do. However, if you apply a lot of force on a heavy piece of furni-
ture but it doesn’t move from its original place, no work has been done regardless of
the amount of effort.
Note: ∆ is the Greek letter delta, and denotes a change. Here, ∆d is the change in position,
or the displacement. The standard unit of force is a Newton (N = kg·m·s ), which is de-
fined as the force required to make a mass (m) of 1 kg accelerate at 1 m·s . Force (F) is
equal to the mass (m) of an object times its acceleration (a): F = m*a.
Internal energy is the total amount of energy stored in any object and determines how
much work the object is capable of performing. This includes both kinetic energy
(energy that an object has when it is in motion) and potential energy (energy that is
stored). For example, a bowling ball sitting on a table contains energy despite the fact
it is not in motion. It does not have kinetic energy because it is still, but it contains po-
tential energy simply because of where it is situated. Were it nudged off the table, the
bowling ball will do work because it will be pulled downward by gravity. This is an ex-
ample of gravitational potential energy. The potential energy (PE) due to gravita-
tional pull is given by the following equation:
-2
-2
Chapter 3: Thermodynamics – ATMO 200 http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodyn...
3 sur 33 03/08/2020 à 00:29

Gravitational potential energy (CC BY 2.0).
Anything that moves contains kinetic energy (KE), which is given by the following
equation
where m is the mass of an object in kilograms (kg) and v is the velocity of an object in
meters per second (m·s ). From this relationship you can see that objects with more
mass or objects that are moving faster have more energy.
Energy takes on many forms and often changes forms from one to the other, but the
total amount of energy in the universe remains constant. Energy cannot be created or
destroyed. This means that the energy lost in a process must be the same as the energy
gained in another. This is what the law of conservation of energy means, and this is
what is known as the first law of thermodynamics. The first law of thermodynam-
ics frequently comes into play in atmospheric motions and will be discussed further
later in this chapter.
In short, energy is the capacity of a system to perform work. Energy is always con-
served and cannot be created or destroyed. We begin this chapter with a short review
of energy because energy is ultimately responsible for Earth’s weather from tempera-
ture changes in the atmosphere to the resulting air motions. Without energy, no
weather would occur.
-1
Chapter 3: Thermodynamics – ATMO 200 http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodyn...
4 sur 33 03/08/2020 à 00:29

One important example of kinetic energy is thermal energy
, which comes from the
tiny movement of many molecules in a system. In Chapter 1, we discussed how
tem-
perature is a measure of the average speed of atoms and molecules in a system. Here
we can further describe temperature as being proportional to the average kinetic en-
ergy of the random motions of the molecules in a system. The faster the molecules
move, the higher the temperature.
The transfer of thermal energy due to the temperature difference between two objects
is what is known as heat. Heat is a form of energy in transit, and once transferred, it is
stored as internal energy. There are four main methods of heat transfer: conduction,
convection, radiation, and the absorption or release of latent heat.
Conduction, convection, and radiation are all methods of heat transfer (CC BY-SA 4.0).
When you directly touch a hot object, such as the stovetop, the energy from the hot
stove top is immediately transferred to your cool hand due to a difference in the speed
of the molecules, causing you to feel a burn. This is an example of conduction: en-
ergy directly transferred through a substance without the movement of material.
Certain materials are better conductors of heat than others. Metal, for example, con-
ducts heat very efficiently, while air, which acts as an insulator, is a very poor conduc-
tor of heat.
Chapter 3: Thermodynamics – ATMO 200 http://pressbooks-dev.oer.hawaii.edu/atmo/chapter/chapter-3-thermodyn...
5 sur 33 03/08/2020 à 00:29