Friday Feb. 15, 2013
click here to download today's notes in a more printer friendly format

Calexico Quattro (live at the Barbican Theater in London).  Calexico is one of my favorite local groups.  They'll probably be back for an encore performance later this semester.

Here's a good video of the meteorite that hit somewhere in Russia earlier today.  It caused some damage and injured several hundred people apparently.  And a second video with sound of the explosion.  After reading some more about the meteorite, it sounds like the explosion was mostly just the shock wave produced as the meteorite fell through the atmosphere rather than the sound of impact.  Much of the meteorite probably burned up in the atmosphere, though some fragments did reach the ground.

Quiz #1 has been graded and was returned in class today.  There were some very high grades and also many very low grades.  I suspect that many students aren't putting in the effort that is needed to do well on the quizzes (such as reading the online notes when they appear after class).  Please come see me by during office hours if you did poorly on the quiz and are working hard at trying to understand and learn the course material.  In many cases it isn't a question of not devoting enough time but studying the material in the right kind of way.

An "in-class Optional Assignment" was handed out in class today.  The idea is to work on the assignment in class and turn it in at the end of the period.  If you weren't in class and would like to do the assignment you can download a copy here and turn in the completed assignment at the beginning of class next Monday.  You'll receive at least partial credit.

OK we're done with weather maps for the time being.  Though if interesting weather appears imminent I'll try to mention it in class.

If we were using a textbook in this class we'd be moving into Chapter 2!  During the next couple of weeks we will be concerned with energy, temperature, heat, energy transport, and energy balance between the earth, atmosphere, and space.

It is easy to lose sight of the main concepts because there are so many 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.

Types of energy
We will learn the names of several different types or forms 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.




Latent heat energy is an under-appreciated and rather confusing type of energy. The word latent refers to energy that is hidden in water and water vapor.  The hidden energy emerges when water vapor condenses or water freezes (the energy had been added earlier when ice was melted or water was evaporated).  The fact that the energy is hidden is part of what makes it confusing.







Radiant energy is a very important form of energy that was for some reason left off the original list in the ClassNotes.  Sunlight is an example of radiant energy that we can see and feel (you feel warm when you stand in sunlight).  There are many types of radiant energy that are invisible (such as the infrared light that people emit).  Electromagnetic radiation is another name for radiant energy.

Energy transport
Four energy transport processes are listed below

 

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.

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.  Ocean currents transport energy from the warm tropics to colder polar regions.

Convection is a 3rd way of causing 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.

Energy balance and the atmospheric greenhouse effect
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.




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.  If you periodically deposit money into your account why doesn't the balance just grow without limit.  The answer is that you also take money out of the account and spend it.  The same is true of energy and the earth.  The earth absorbs incoming sunlight energy but also emits energy back into space (the orange and pink arrows in the figure below).  Energy is being emitted by both the surface of the earth and the atmosphere.


Energy emitted in the form of infrared light is an invisible form of energy (it is weak enough that we don't usually feel it either).  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).



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



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 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.

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 also).  That's a subject we'll explore briefly later in the semester.

When you add energy to an object, the object will usually warm up (or if you take energy from an object the object will cool).  It is relatively easy to come up with an equation that allows you to figure out what the temperature change will be (this is another equation I'll write on the board before  the next quiz if you ask me to - try to understand it, you don't have to memorize it).




The temperature change, ΔT,  will first depend on how much energy was added, ΔE.  This is a direct proportionality, so ΔE is in the numerator of the equation (ΔE and ΔT are both positive when energy is added, negative when energy is removed)

When you add equal amounts of energy to large and small  pans of water, the small pan will heat up more quickly.  The temperature change, ΔT, will depend on the amount of water, the mass.  A small mass will mean a large ΔT, so mass should go in the denominator of the equation. 

Specific heat is what we use to account for the fact that different materials react differently when energy is added to them.  A material with a large specific heat will warm more slowly than a material with a small specific heat.  Specific heat has the same kind of effect on ΔT as mass.  Specific heat is sometimes called "thermal mass" or "thermal capacity."  You can think of specific heat as being thermal inertia - a substance with high specific heat, lots of thermal inertia, will be reluctant to change temperature.

Here's an important example that will show the effect of specific heat (middle of p. 45).


Equal amounts of energy (500 calories, note that calories are units of energy) are added to equal masses (100 grams) of water and soil.  We use water and soil in the example because most of the earth's surface is either ocean or land. Before we do the calculation, try to guess which material will warm up the most.  Everything is the same except for the specific heats.  Will water with its 5 times larger specific heat warm up more or less than the soil?

The details of the calculation are shown below.

With its higher specific heat, the water doesn't heat up nearly as much as the soil.  If we had been removing energy the wouldn't cool off as much as the soil would.

These different rates of warming of water and soil have important effects on regional climate.


Oceans moderate the climate.  Cities near a large body of water won't warm as much in the summer and won't cool as much during the winter compared to a city that is surrounded by land.  Water's ΔT is smaller than land's because water has higher specific heat.
The yearly high and low monthly average temperatures are shown at two locations above.  The city on the coast has a 30o F annual range of temperature (range is the difference between the summer and winter temperatures).  The city further inland (assumed to be at the same latitude and altitude) has an annual range of 60o F.  Note that both cities have the same 60o F annual average temperature.  We'll see a much more dramatic example of the moderating effect of water on climate in a couple of weeks.

Here's another situation where you can take advantage of water's high specific heat to moderate climate on a smaller scale.



It's going to be warm this weekend and I am going to plant the rest of my tomatoes.  I do this so that they can start to make tomatoes before it starts to get too hot in May.  In February it can still get cold enough to kill tomatoes so they need some protection (the broccoli and lettuce in the background can handle a light frost).





Here's one way of doing that.  You can surround each plant with a "wall o water"  -  a teepee like arrangement that surrounds each plant.  The cylinders are filled with water and they take advantage of the high specific heat of water and won't cool as much as the air or soil would during a cold night.  The walls of water produce a warm moist micro climate that the tomato seedlings love.  The plastic is transparent so plenty of sunlight can get through.

Adding energy to an object will usually cause it to warm.  But there is another possibility (bottom p. 45),  the object could change phase or state (change from solid to liquid or gas).  Adding energy to ice might cause the ice to melt.  Adding energy to water could cause it to evaporate.

The equation at the bottom of the figure above allows you to calculate how much energy is required to melt ice or evaporate water or sublimate dry ice.  You multiply the mass by the latent heat, a variable that depends on the particular material that is changing phase.  The latent heat of vaporization (evaporation) is the energy required to evaporate 1 gram of a material.

If you add energy to or remove energy from an object, the object will usually change temperature.  You can calculate the temperature change if you know the object's mass and its specific heat.  That's the equation we used in the example calculation above.  It's shown again below.



We (actually you) will conduct an experiment in class next Monday and will try to measure the energy needed to evaporate liquid nitrogen.  We'll need a way of measuring energy.

If you know the mass and specific heat of an object and measure a change in temperature you can use the rearranged equation in the figure above to calculate how much energy was added to or removed from the object.