Autumn is generally known as the time of year when school starts back up again, the days start getting shorter, the temperature and leaves begin to fall, and crops need to be harvested.  In the Irish Calendar Autumn is seen as the whole months of August, September and October.  Meteorologists recognize the months March, April, and May as Fall in the Southern Hemisphere and September, October, and November as Fall in the Northern Hemisphere.  However, the Autumn Equinox (usually September 22 or 23) is generally seen as the beginning of Fall in the Northern Hemisphere by many Western countries. 

Why Seasons Change

One common misconception about why the Earth experiences seasons is because it gets closer and farther away from the sun as it follows its orbital path.  Another misconception is that the earth tips toward the sun.  This gives the false idea that the Earth is actually swaying back and forth.

The Earth's axis (the center line through the North and South Poles) is not straight up and down, but rather it is tilted by 23.5 degrees and is what is responsible for the change of seasons.  As the Earth orbits the sun, the tilt makes the Northern Hemisphere (the top half of the Earth) closer to the sun in the summer time and farther from the sun in the winter time.  This also happens in the Southern Hemisphere (the bottom half of the Earth) except its seasons are opposite of those in the Northern Hemisphere.  When it is summer in the Northern Hemisphere it is winter in the Southern Hemisphere, and when it is winter in the Northern Hemisphere it is summer in the Southern Hemisphere.  Imagine spending Christmas in Australia going to the beach rather than sledding down a hill in New York!  During summer in the Northern Hemisphere, the tilt of the Earth allows more sunlight to hit the Northern Hemisphere for longer periods of time, thus increasing the temperature.  At the same time, it is winter in the Southern Hemisphere and less sunlight is hitting the Earth in that region.

In the fall and spring, the Earth is in transition.  During the Spring and Fall Equinox (around March 21st and September 23rd, respectively) day and night are equal in length all around the globe.  The beginning of Fall is often seen as the time when we transition from having longer days and shorter nights to shorter days and longer nights. 

Why the Leaves Change Color in the Fall

You may also notice other changes happening around you during this time of the year.  The leaves on many trees in temperate regions (regions that experience wide temperature ranges) start to change color.  What causes this?

Let's begin with the three different types of chemicals that produce different pigments in leaves.  The first chemical is chlorophyll which produces a dominant green pigment. Chlorophyll is present throughout the growing season.  The second type of chemical is the carotenoids which are responsible for the yellow, orange, and some brown colors in leaves.  The third type of chemical is the anthocyanins which are responsible for red, purple, and bluish colors.  We'll talk more about these later.

In the summer, the leaves of deciduous trees (trees that lose their leaves) are full of chlorophyll.  So full, in fact, that the green in the chlorophyll overpowers the other colors present in the leaves, such as yellow and orange.  Chlorophyll is essential for plants.  It turns light into energy, producing sugars and starches as food for the tree.  This allows the tree to grow during the summer. 

When the days start getting shorter due to the earth's tilt and orbit around the sun, the temperature starts to drop and there are now less hours of sunlight in a day.  The lower temperature and fewer hours of sunlight signal the tree to go into storage mode for the winter.  The chlorophyll now starts to break down, causing the green in the leaves to disappear. Carotenoids, the orange and yellow colors, which have been present throughout the entire growing season, can now be seen in the leaves.  Most of the sugars and starches produced by the chlorophyll in the leaves are brought back into the tree.

Anthocyanins, which are very water soluble, are now present in the watery portions of the leaf cells.  The presence of anthocyanins (which is responsible for the red, blue, and purplish colors) is due to the excess sugars that are left behind in the leaf cells after the chlorophyll breaks down.  Bright light also helps to bring out this chemical in the leaves.  The brown color in the leaves, while partially due to the carotenoids present in the leaves, is more often the result of waste from the tree's life processes. An abscission layer of cells builds up where the leaf is connected to the branch of the tree, blocking the transport of sugars to the tree. After a while, the wind or even the weight of the leaves themselves will cause the leaves to fall from the tree.  The fallen leaves are also high in calcium and potassium.  This layer of fallen leaves decomposes, returning these nutrients back to the soil, making the top soil fertile for more plants to grow in the next growing season.