Module V - Reason for the Season

ENGAGE

Reason for the Season
As we begin to apply the physics of thermal energy to the ocean, let's start by reviewing why the equatorial region gains so much more thermal energy (heat) than higher latitudes.

Most of us know that the tilt of the earth is the reason for the seasons--that days are longer in the summer and shorter in the winter as Earth is inclined toward and away from the sun, respectively, in its annual orbit. The same is true for both hemispheres, it just occurs at opposite ends of the calendar.



EXPLORE

Here are a few resources that can help you and your students to better visualize how Earth's inclination leads to its differential heating.

The first is a great TD interactive resource called Global View of the Seasons. Be sure to check out its different tabs and features.

Global View of the Seasons







The next two resources are hosted on YouTube. First is a product of Ignite! Learning called What causes Earth's Seasons. And NOAA Visualization has a video titled, Seasons on Earth.

What causes Earth's Seasons

Seasons on Earth




Helpful Hint: Many schools intentionally block access to YouTube because of some of some of its objectionable content. Regardless of how you may personally feel about blanket school censorship of content, most school IT professionals can provide you with an internet "backdoor" so you can access the media you need to teach.


EXPLAIN

• What causes the seasons on Earth?
  
• What do seasonal patterns tell you about differential heating of the planet?
  

EXTEND

• What do seasonal patterns of change indicate for cultures living in different climate zones?

EVALUATE

• How informative and useful are these resources for your purposes?
  

ENGAGE

Changes in Latitude--Where It's Hot and Where It's Not!
It's reasonable to figure that more hours of daylight means more heating in that hemisphere. True enough, but hours of day light and the amount of light absorbed and turned into heat does not necessarily follow. So, what role does latitude play in the differential heating of the planet?

We know that above the arctic circle, daylight occurs 24/7 for part of the summer. Yet this does not translate into a climate that is warmer than at the equator with its fairly consistent diurnal rhythm of sunrise and sunset. What gives?

What's Your Angle?
Light arrives in the equatorial region more or less perpendicularly--that is straight-on. Therefore, more light energy arrives per unit of area.

Because the curve of the Earth, light arriving at the surface at higher latitudes is spread over a proportionally larger area, resulting in less solar energy per unit area. Huh?


Try This Trick!
Using a projector or some other light source, hold a circular piece of paper or lid so that the light hits it straight-on, then look at the shadow it projects on the wall. Now try tilting the paper so that light strikes it at an angle. The surface area of the circle has not changed, but the area of its shadow is much smaller.
Less light per unit area = smaller shadow = less thermal energy per unit area.



Reflecting on Climate
Reflecting on another angle, because of the curve of the Earth, solar energy at higher latitudes arrives at a greater angle of incidence, and is more more easily reflected off any surface. Much like a bullet's ricochet.

To put it in black and white terms, most of us know the difference between wearing a white or black shirt on a hot, sunny day. As though polar regions were designed to stay colder, what little solar energy actually makes its way through the atmosphere to the surface encounters a bright, white surface that reflects it back into the atmosphere before it can be absorbed and turned into heat.

This reflective quality of the Earth's surface and its atmosphere is referred to as albedo. (Incidentally, we'll be re-visiting this important phenomenon in a later modules.)


Red Sky at Night, Red Sky at Morning
We are finding that a solar photon has a perilous journey to high latitudes. Even before any light reaches the surface at higher latitudes, it must travel through more air than at the equator. This longer journey through the envelope of gases diffuses and scatters more of light's energy into the atmosphere before it can be absorbed at the surface and turned into heat. This, incidentally, is also why sunrises and sunsets are more colorful and dimmer than the sky at noon.

Bottom line - whether its Earth's atmosphere, albedo or light's angle of incidence, because polar regions receive less solar energy over time, they are colder and have year-round snow and ice.

And that's is a good thing for regulating Earth's climate, as we will learn over the next few modules.



EXPLORE

Check out this short, simple TD video capturing the rising and setting sun.
Observe Sunrise and Sunset






Let's begin to consider some pathways for the movement of thermal energy by viewing this TD interactive resource, Sea Surface Temperature.

Sea Surface Temperature






EXPLAIN

• What causes the annual patterns you observe.

EXTEND

• How can you determine the direction of prevailing winds at the Equator?
• How might you use either of these resources in your class?
  

EVALUATE

• How useful are these digital resources for teaching and learning about global systems?