Article of the Week #7
Directions:
1. Mark your confusion.
2. Show evidence of a close reading. Mark up the text with questions and/or comments on ALL 5 annotations
this week!
3. Write a one-page reflection on your own sheet of paper. (10-15 typed lines; 12-point font)
Where Does The Sun’s Energy Go?
As you read previously, the energy for pretty much everything that happens here on Earth is provided by the
sun. As you have also previously learned, we must be losing that energy back into space at the same rate that
it’s coming in, otherwise the Earth would either burn or freeze with too much or too little energy. The loss back
into space is accomplished through low-frequency (infrared) radiation that is emitted by all matter that is above
absolute zero (everything on the planet). A common analogy for this is water filling a bathtub with the drain
open. The more water that is in the tub, the faster the water goes out through the drain (trust me). The bath
will fill until there is enough water that it is going out the drain as fast as it is coming in through the water spout.
This means there is a relatively constant amount of water (energy) in the tub (Earth), but there is always some
coming in and going out. The question then becomes, what does that energy do while it is here on Earth before
it goes out that drain?
The Different Paths
The answer to that question is: it depends. While there are many, many different variations of the possibilities,
they all fall into one of just a few different categories. Generally speaking, the light either bounces off of
something and goes back out into space, is absorbed by water, is absorbed by a solid object, or is used by a
photosynthetic organism. From these starting points, the energy can go down very different (or very similar)
paths before winding up becoming heat again that is turned into infrared radiation and sent back out to space.
For each one, that is one of the possibilities at the very beginning- to warm whatever the light hits, and that
thing sends the light energy back out as infrared light.
Reflection
Lights bounces off of things. We are all familiar with this, since otherwise no one would be able to see. The
light reflecting from the Earth is visible from the moon and elsewhere in space. The fraction light that hits the
Earth and bounces back into space is called the Earth’s albedo. Albedo is measured as a number between 0
and 1, where 0 means that all of the light is absorbed and a 1 means that all of the light is reflected. The more
white any object is, the greater its albedo, and the darker the object is, the lower its albedo. The Earth’s albedo
is around 0.3 on average, but this changes as various parts of the earth reflect differently. Bodies of water and
forests absorb most light, while snow-covered land and deserts reflect most light. Clouds also increase the
planet’s albedo.
Water
From your previous reading, you are already familiar with some of what happens to the energy that strikes
water. Specifically, it pushes that water along the water cycle. The light energy becomes thermal energy as
the water evaporates and gravitational potential energy as that warm air rises and eventually condenses into
clouds. That potential energy is lost as the water falls as precipitation, and the energy becomes kinetic energy.
Both while the water is falling through the air and when it is running over the ground in streams and rivers,
it encounters resistance to its motion, called friction, that slows it down. The friction turns the kinetic energy
back into thermal energy, which can then turn into light and radiate out into space as mentioned above in the
introduction.
But there is still something else that can happen to water that has been warmed by the sun. Thermal energy
tends to spread, so that thermal energy can also spread directly to the air above the water. It takes a lot of
energy movement to change the temperature of water, which makes warm water very effective at warming its
surroundings, since it contains so much energy.
Note the warmer areas along the coast in this temperature map.
What happens when water warms the air above it is just like what happens when ground is heats and warms
the air above it, which brings us to...
Solid Objects (generally speaking, the ground)
Thermal energy spontaneously transfers from areas of higher temperature to lower temperature. So when
light from the sun hits the ground, it gives the ground greater thermal energy than the air above it, so the
heat will transfer to the air. There are a couple ways to explain what happens next. One way would be to say
that the increased energy makes the air particles move faster, so they spread out further than the air above.
This decreases the heated air’s density, and less dense stuff floats. The other way would simply be this:
warm air rises. It so happens that rising air cools down, and anything that rises gains potential energy. This
demonstrates that the thermal energy is becoming gravitational potential energy. Eventually as the air cools,
its density drops enough that it cannot stay up anymore, so it falls. Just like anything that falls, gravitational
potential energy becomes kinetic energy. As that moving air hits the ground and spreads (towards where other
air is rising) we know that air that’s moving along the ground as wind. The wind is slowed down as it moves
over land by friction with the ground, creating heat, eventually leading to infrared radiation out into space.
During the day, the sun warms the ground faster, so the air above solid land rises, leading to a breeze coming
from the sea. At night it switches, and the air rises over the water leading to a land breeze.
Please answer question 1 AND choose one of the bulleted questions for your response.
1. How is this text organized? How do you know?
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What effect do you think lowering the Earth’s albedo would have on the Earth’s energy?
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During the summer, do you think areas around water are cooler or warmer during the day than those
far from water? Why?