Name: ____________________________
Date: _____________________________
Seasonal Changes Worksheet
Background Information:
Reasons for the Seasons:
Have you ever wondered why the earth has seasons?
Here is your chance to find out! We experience seasons
because of the tilt of the earth's axis (which is about 23.5°).
The diagram to the right shows this effect. The solid black
line is the equator, and the short black lines are the poles.
(The blue lines show where the equators and poles would be
if the earth were not tilted.) If this were summer in the
northern hemisphere (on June 21st), the sun would be
directly to the left. However, as the earth moves around the
sun, the tilt is toward the same direction in space, but that
direction changes relative to the sun. On June 21, which is
the Summer Solstice in the Northern Hemisphere, the North
Pole is "leaning" toward the sun the most. On December 21,
the Winter Solstice in the Northern Hemisphere (and
therefore the Summer Solstice in the Southern Hemisphere),
the North Pole is "leaning" away from the sun the most.
As light travels through the atmosphere, some of
the energy is absorbed. This is why the sun feels
less intense in the winter than in the summer.
Notice how light ray # 1 doesn't travel very far
through the atmosphere. This corresponds to the
sun being "directly overhead". Rays # 2 travel
slightly farther through the atmosphere, and rays #
3 farther still. Finally, rays # 4 have to travel
through a great deal of atmosphere. Thus, they
have much less energy left when they reach the
ground. This is part of the reason why the polar
regions are colder than other parts of the earth.
Here is another factor that causes the climate to be colder
for places farther from the equator (or when that
hemisphere is tilted away from the sun). A given amount of
light (and heat) is spread over a larger area, so the energy
is less concentrated. This is also why the highest
temperatures on a given day are on or slightly after noon.
Notice how area # 2 is slightly larger than area # 1, and
area # 3 is much larger. The energy per square meter
reaching the earth at a latitude of 60° is only half that of the
energy reaching the earth at the equator (ignoring the
atmospheric effect mentioned above).
Seasonal Changes, Fall Colors, and Photosynthesis:
It's almost wintertime! That means shorter days and cooler nights. And, in many parts of the
United States, it means that trees have shed their leaves after a final colorful salute to summer.
Did you know that a tree's leaves are orange and yellow -- even in summer? Most leaves
include the pigments of all three colors. It's just that the pigment chlorophyll (the pigment that makes
the leaves green) is a much stronger pigment than the others. It covers the yellow (xanthophyll) and
orange (carotene) pigments that are natural to a tree's leaves. But, come fall, a tree's leaves produce
less chlorophyll. Now the other colors can show through. And that they do -- in brilliant explosions of
color!
Another pigment (anthocyanin), which produces reds and purples, isn't present all year long in
most green leaves. It only shows up as the nights get cooler.
Fall is a great time of year for learning about the process that gives life to trees. As you know,
this process is called photosynthesis, which translated means "putting together with light." As winter
nears, less sunlight and less water -- elements essential to the process of photosynthesis -- will be
available to trees. That means less food for deciduous trees! Soon the tree's photosynthesis (foodmaking) "factory" will shut down and the tree will rest until spring when water and light again awaken
the process.
Answer the following questions using complete sentences in the space provided.
1. According to the information below, why do we experience seasons here on the earth? (3 points)
2. Draw a diagram showing the position of North America with respect to the Sun in the summer.
(1 point)
3. Draw a diagram showing the position of North America with respect to the Sun in the winter.
(1 point)
4. Describe why the leaves of deciduous trees change color in the fall. (2 points)
5. According to the information above, what causes a deciduous tree’s photosynthesis “factory” to
shut down during the winter? (2 points)
6. If the yellow (xanthophyll) and orange (carotene) pigments are present in the leaves of deciduous
trees all year round, why do they usually appear green until fall? (1 point)