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 meant to introduce some of what we will be covering. 

We will learn the names of several different types or forms of energy. Kinetic energy is energy of motion. Some examples are mentioned and sketched above.  It is a relatively easy to visualize and understand form of energy.

Latent heat energy is an underappreciated and a somewhat confusing form 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.  Energy may be taken from and cool the surroundings when water evaporates or melts.

Radiant energy is a very important form of energy.  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.

The figure above emphasizes the fact that water vapor is a particularly important form of invisible energy.  When water vapor condenses to produce the water droplets (or ice crystals) in a cloud, an enormous amount of latent heat energy is released into the atmosphere. 

It is hard to visualize or appreciate the amount of energy released into the atmosphere during condensation.  You can imagine the work that you would do carrying a gallon of water (8 pounds) from Tucson to the top of Mt. Lemmon.  To accomplish the same thing Mother Nature must first evaporate the water and (if my calculations are correct) that requires about 100 times the energy that you would use to carry the 8 pounds of water to the summit of Mt. Lemmon.  And Mother Nature transports a lot more than just a single gallon.

Four energy transport processes are listed below.
 

By far the most important process is electromagnetic radiation (light is a familiar form of electromagnetic radiation).  This 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 and 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.  Energy is transported when water vapor or water (which contain hidden latent heat energy) move from one location to another.

Rising parcels of warm air and sinking parcels of cold air are examples of free convection.  Because of convection you feel colder on a cold windy day than on a cold calm day.  Ocean currents are also an example of convection.  Ocean currents transport energy from the warm tropics to colder polar regions.

Note that convection is one of four ways of causing rising air motions in the atmosphere (convergence, fronts, and topographic lifting are the others). 

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

The next picture 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?  The answer is that the earth also sends energy back into space (the orange and pink arrows in the figure below)


This 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. This is where latent heat energy transport and convection and conduction operate (they can't transport energy beyond the atmosphere into outer space).


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


Without the greenhouse effect, the global annual average surface temperature on the earth would be about 0o F.  With greenhouse gases the annual average is much warmer, about 60o F.  There is concern that observed increases in greenhouse gas concentrations in the air might enhance the greenhouse effect and cause global warming.  That could have deleterious effects.

When you add energy to an object, the object will usually warm up (conversely when 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.



The temperature change will first depend on how much energy was added.  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 taken from something)

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 mass.  A small mass will mean a large ΔT, so mass should go in the denominator of the equation.  

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."

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


Equal amounts of energy, 1000 calories (note that calories are units of energy), are added to equal masses, 500 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. Water has a higher specific heat than soil, it only warms up 2o C.  The soil has a lower specific heat and warms up 10o C, 5 times more than the water (there is a factor of 5 difference in the specific heats of water and soil).

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.

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.

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




Many people in the Tucson area plants tomatoes in February so that the plants can mature and produce fruit before it starts to get too hot in May.  It can still get cold enough at night in February and March to kill tomatoes, so you must provide some kind of frost protection.




One way of doing this is to put a "wall of water" around each plant.  The tepee like arrangement is made up several cylinders about 2 inches in diameter that are filled with water.  The high specific heat of the water means the water and the plants inside the tepee won't cool nearly as much as the soil and the outside air. 

Adding energy to an object will usually cause it to warm.  But there is another possibility,  the object could change phase (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. 



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 earlier.

We will be using the equation next in a slightly different way in a class experiment/demonstration.  The equation is also used Latent Heat of Fusion of Ice Experiment included in this week of the course. We will measure temperature change and use that to determine the amount of energy gained or lost by an object.

The experiment is normally conducted by a handful of student volunteers in the classroom version of the course.  The object is to measure the latent heat of vaporization of liquid nitrogen.  That just means measuring the amount of energy needed to evaporate a gram of liquid nitrogen.  In the Latent Heat of Fusion of Ice Experiment you will measure the energy needed to melt one gram of ice.  Before beginning the experiment, a sealed envelope containing the known latent heat of vaporization of nitrogen is given to a student in the class. 



Some data from an actual experiment are shown above.  The various steps of the experiment are explained below.

(a)
Some room temperature water poured into a styrofoam cup weighed 104.4 g.  The cup itself weighed 3.4 g, so we had 101.0 g of water.

(b)
The water's temperature was 21.0 C  (room temperature).

(c)
  40.0 g of liquid nitrogen was poured into the cup of water.

It takes energy to turn liquid nitrogen into nitrogen gas.  The needed energy came from the water.  This flow of energy is shown in the middle figure above.  We assumed that because the experiment is performed in a styrofoam cup that there is no energy flowing between the water in the cup and the surounding air.

(d)
After the liquid nitrogen had evaporated we remeasured the water's temperature.  It had dropped to 1.0 C.  That is a temperature drop of 21.0 - 1.0 = 20.0 C.

Because we knew how much water we started with, its temperature drop, and water's specific heat we can calculate how much energy was taken from the water.
101.0 g x 20 C x 1 cal/g C  =  2020 calories

We then divide that number by the amount of liquid nitrogen that was evaporated.
2020 calories / 40 grams = 50.5 calories per gram

A trustworthy student in the class informed us that the known value is 48 cal/g.  The student's measurement was pretty darn close to that value.