Types of energy, energy transport, energy balance

It's a lucky thing we don't use a textbook in this class.  If we did we would just be moving into Chapter 2.  During the next week and a half we will be concerned with energy, temperature, heat, energy transport, and energy.  We'll be especially interested in the flow of energy back and forth between the earth's surface and it's atmosphere; the greenhouse effect is a big part of that.

It is easy to lose sight of the main concepts because all the details.  The following is an introduction to this new section of material and most of the figures are found on pages 43 & 44 in the photocopied ClassNotes.

1. Types of energy

Kinetic energy is energy of motion.  Some examples (both large and microscopic scale) are mentioned and sketched above.  This is a relatively easy to visualize and understand form of energy.




Radiant energy is a very important form of energy that was somehow left off the original list in the ClassNotes .  Electromagnetic radiation is another name for radiant energy.  Sunlight is an example of radiant energy.  It's something that we can see and feel (you feel warm when you stand in sunlight).  Something that is not quite so obvious is that everyone in the classroom is emitting radiant energy.  This is infrared light, an invisible form of radiant energy.  And actually it's not just the people; the walls, ceiling, floor and even the air in the classroom are also emitting infrared light.  We can't see it and, because it's there all the time, I'm not sure whether we can feel it or not.

Latent heat energy is an, important, under-appreciated, and rather confusing type of energy.  The word latent refers to energy that is hidden.  That's part of the problem.  Another part of what makes latent heat energy hard to visualize and appreciate is that the energy is hidden or stored in water vapor or water - that seems an unlikely place to find energy.

It might be helpful to think of latent heat energy as being a form of potential energy.

Gravitation potential energy is something I suspect you're familiar with.






It would take a lot of energy to push a rock up a hill.
Once at the top of the hill the rock has a lot of stored, potential energy (the energy that it took to get it there). 
This energy would reappear as kinetic energy if you were to push the rock and start it rolling down hill.
Energy is being added in the left figure, stored energy is shown in the middle figure, and the energy reemerges or is released in the final picture.

Latent heat energy






Some of the sunlight energy
hitting water warms the water. 
The rest is used to evaporate water.

The water vapor contains a lot of stored, "latent heat", energy (sunlight energy that was added during evaporation).
The stored energy is released when the water vapor condenses and turns back into water.

The same kind of scenario is shown here except that it involves water, water vapor, and sunlight.  Energy is added in the left figure and is used to evaporate some water.  The added energy is stored or hidden in the water vapor, and the energy is released when the water vapor condenses and turns back into water.  It takes a lot of energy to evaporate water. 

Here are three examples showing energy originally hidden in water vapor reemerging in a tornado, water rushing down a mountain or wash, and a hurricane.  A big part of the energy in the tornado, flash flood, and hurricane was initially hidden in the water vapor.

2. Energy units
Next just brief mention of units of energy

Joules are the units of energy that you would probably encounter in a physics class.  Your electric bill shows the amount of energy that you have used in a month's time, the units are kilowatt-hours.  We'll usually be using calories as units of energy.  1 calorie is the energy need to warm 1 gram of water 1 C (there are about 5 grams of water in a teaspoon). 

Here's a little miscellaneous information that you don't need to worry about remembering.  You've probably seen the caloric content of food on food packages or on menus in restaurants.  1 "food calorie" is actually 1000 of the calories mentioned above (food is probably a form of chemical energy, the energy is released when the food is consumed). 

A 150 pound person would burn almost 500 food calories while sleeping during the night (8 hours x 60 minutes per hour x 1 food calorie per minute).  This is about the energy contained in one donut.

3. Energy transport processes

By far the most important process is at the bottom of the list above.  Energy transport in the form of electromagnetic radiation (sunlight for example) is the only process that can transport energy through empty space.  Electromagnetic radiation travels both to the earth (from the sun) and away from the earth back into space.  Electromagnetic radiation is also responsible for about 80% of the energy transported between the ground and atmosphere.

You might be surprised to learn that latent heat is the second most important transport process.  This term latent heat can refer to both a type of energy and an energy transport process (the energy is hidden in the water vapor, the water vapor can move around and carry that energy with it).

Rising parcels of warm air and sinking parcels of cold air are examples of free convection.  Because of convection you feel colder or a cold windy day than on a cold calm day (the wind chill effect).  Ocean currents are also an example of convection. 

Convection is also one of the ways of rising air motions in the atmosphere (convergence into centers of low pressure and fronts are two other ways we've encountered so far) 

Conduction is the least important energy transport at least in the atmosphere.  Air is such a poor conductor of energy that it is generally considered to be an insulator.

4. Energy balance

The next picture (the figure in the ClassNotes has been split into three parts for improved clarity) shows energy being transported from the sun to the earth in the form of electromagnetic radiation.  On average about half of this sunlight passes through the atmosphere and is absorbed at the ground.  This causes the ground to warm (sunlight energy striking the ocean warms the oceans but is also used to evaporate ocean water).  Measuring the energy in sunlight arriving at the surface of the earth is the object of Experiment #3.

We are aware of this energy because we can see it (sunlight also contains invisible forms of light) and feel it.  With all of this energy arriving at and being absorbed by the earth, what keeps the earth from getting hotter and hotter?  If you park your car in the sun it will heat up.  But there is a limit to how hot it will get.  Why is that? 

It might be helpful when talking about energy balance to think of a bank account.  You open a bank account and start depositing money.  The bank account balance starts to grow.  But it doesn't just grow without limit.  Why not?  The answer is that once you find money in the bank you start to spend it.  The same is true of energy and the earth.  Once the earth starts to warm it also starts to emit energy back into space (the orange arrows in the figure below).  Radiant energy is emitted by the ground, the oceans, and the atmosphere. 

Energy is emitted by the earth in the form of infrared light, an invisible form of energy (to human eyes anyways).  A balance between incoming and outgoing energy is achieved and the earth's annual average temperature remains constant.

We will also look closely at energy transport between the earth's surface and the atmosphere (see the figure below). This is where latent heat energy transport, convection and conduction operate (they can't transport energy beyond the atmosphere and into outer space).

This is also where we will find the atmospheric greenhouse.  That will be a important goal - to better understand how the atmospheric greenhouse effect works.

5. The atmospheric greenhouse effect



The greenhouse effect is getting a lot of "bad press".  If the earth's atmosphere didn't contain greenhouse gases and if there weren't a greenhouse effect, the global annual average surface temperature would be about 0 F (scratch out -4 F in the figure above and put 0 F, it's easier to remember).  Greenhouse gases raise this average to about 60 F and make the earth a much more habitable place.  That is the beneficial side of the greenhouse effect.  That's mostly what we'll be concentrating on - how can the greenhouse effect cause this warming, how can it produce this much warming.

The detrimental side is that atmospheric greenhouse gas concentrations are increasing (no real debate about that).  This might enhance or strengthen the greenhouse effect and cause the earth to warm (some debate here particularly about how much warming there might be).  While that doesn't necessarily sound bad it could have many unpleasant side effects (lots of debate and uncertainty about this).  That's a subject we'll explore at various times during the semester.