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 (found on pps 43&44 in the photocopied
Class Notes) is meant to introduce some of what we will be covering in
class. Click here
to download a more
printer friendly version of this section.
Latent heat energy is perhaps the most underappreciated and most
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.
Radiant energy is a very important form of energy that was for some
reason left off the original list. 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.
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 common
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 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.
Ocean currents are also an example of convection. Ocean currents
transport energy from the warm tropics to colder polar regions.
Note that convection is
a 3rd way of causing rising air motions in the atmosphere (convergence
into centers of low pressure, and fronts were the other two
ways).
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.
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.
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 outside
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.
Remember that without the greenhouse effect, the global annual
average surface temperature on the earth would be about 0o F
rather
than 60o F.
