Shaun Tanner has been a meteorologist at Weather Underground since 2004.
By: Shaun Tanner , 7:22 PM GMT on November 03, 2011
Sorry for the long delay in posting a new lesson. Sometimes Mother Nature gets in the way by forcing me to cover a record-setting snowstorm in the Northeast.
You know that Winter is colder than Summer. That is a given. But, have you ever asked yourself why? When I have asked the general public in the past why the Earth has seasons, the response that comes from the majority is, "Earth is closest to the Sun in the Summer." There are two things wrong with this:
1) We are actually closest to the Sun in January...during the Northern Hemisphere Winter. So this blows that whole theory up. If we were closer to the Earth in January, then that would be our Summer. Oops.
2) When it is Summer in the Northern Hemisphere, it is Winter in the Southern Hemisphere. If proximity to the Sun were the reason for the seasons, then the entire planet would be in the same season. Australia would be in Winter the same time as New York City. We know that is not the case.
So what does cause the seasons?
The main cause for the seasons is Earth's tilt. As shown in Figure 1, Earth does not spin along a completely vertical axis. In fact, it is tilted off a vertical axis by 23.5 degrees. How, you might ask, does that created the seasons? Well, it all has to do with direct and indirect sunlight.
I want you to perform an experiment. Go find a flashlight. Any flashlight will work, as long as it is functional. Once you have found yourself a flashlight, go to your nearest wall. Turn on the flashlight and shine it at the wall perpendicularly. What you want to do is shine the light so it takes the most direct path to the wall. Note the area the light covers on the wall. You might even want to take a pencil and draw along the edge of the light on the wall for future reference.
Now, tilt the flashlight so it is at a 45 degree angle from the wall. Now note how much area the light covers. This tilted light covers much more area than the light that took a more direct route. Thus, since the energy from the flashlight has not changed, we can say the energy per square inch in the tiled light is less than in the direct light. You have now completed your seasons experiment.
Figure 1. Tilt of Earth's axis of 23.5 degrees (Source: www.scienceblogs.com).
Now let's take this into Earth's orbit. Earth's tilt is just like the flashlight's tilt, except the Sun is the flashlight and the Earth is the wall. Figure 2 shows how Earth would look in its orbit with the tilt taken into account. You can see how, during Winter in the Northern Hemisphere, the Northern Hemisphere (NH) is tilted away from the Sun while the Southern Hemisphere (SH) is tilted toward the Sun. Because the SH is tilted toward the Sun, it gets more DIRECT sunlight, while the NH gets more INDIRECT sunlight. This translates to more solar heating in the SH and less in the NH from December - March.
The opposite is true during Summer in the NH. The NH is tilted toward the Sun in our Summer, therefore it receives more direct sunlight. There are four important dates when we talk about seasons. When I talk about these, the terms I will use will be specific to the NH. Thus, if I say Winter Solstice, that is the solstice for the NH (in the SH it would be Summer Solstice).
December 21-22 -- The Winter Solstice makes the point at which the the Sun is at its most southern point. At noon on December 21, the Sun is directly over the Tropic of Capricorn (23.5°S latitude -- note that this is also the tilt of the Earth's axis, not a coincidence). This is when the NH goes into Winter as is the shortest "day" of the year. The SH goes into Summer and is the longest "day" of the year there. I say "day" because it actually isn't any longer or shorter than 24 hours, but rather "day" refers to the time from sunrise to sunset. Above the Arctic Circle (66.5°N latitude), there will be least one day of 24 hours worth of nighttime from December 21 through September 22-23. What I am trying to say is that the farther north you go, the more days of completely darkness you will have. The North Pole will have the most days of complete darkness through September 22-23 (more on that to come).
March 21-22 -- The Vernal (Spring) Equinox (Equal) marks the point the Sun has returned from its long trek from the Tropic of Capricorn and is now directly over the equator at noon. This is when the NH goes into Spring and the SH goes into Autumn. Take specific note that the "warmer" seasons are following the Sun. When the Sun approaches the NH, it goes into Spring, when it moves away from the SH, it goes into Autumn. The equinox is a specific point in time, it does not last the entire day. But, everywhere on the planet gets just about 12 hours of daylight and 12 hours on night on that day, give or take a few minutes. Another interesting note is that on the Vernal Equinox, the sun finally rises at the North Pole for the first time in 6 months (since September 22-23), and sets for the first time at the South Pole in 6 months (since September 22-23).
June 21-22 -- This is when the Sun has reached its most northern point, directly overhead the Tropic of Cancer (23.5°N latitude) at noon. This represents the longest "day" of the year in the NH, and the shortest "day" of the year in the SH. The NH goes into Summer, while the SH goes into Winter. The North Pole is still being bathed in sunlight, while the South Pole is in darkest and will be until September 22-23.
September 22-23 -- This is another equinox, so it must mean that the Sun has retreated back to over the equator at noon and both hemispheres have 12 hours of daylight and 12 hours of night, give or take a few minutes. The North Pole goes back into 24 hours of darkness, while the South Pole see the Sun for the first time since March 21-22.
Figure 2. How Earth's tilt causes the seasons. (Source: www.exploratorium.edu)
A couple interesting points regarding seasons
1. We all know that seasons can affect our moods. Long Winter nights can lead to depression, while bouts of depression often dip in the Summer. This describes Seasonal Affective Disorder (SAD) to a "T" and is an actual thing in medical journals. Treatment options of SAD include long daylight walks, more exercise, or even light therapy. My wife went to Sweden several years ago and noted the abundant use of tanning booths in the country. This is because residents of high latitude countries like Sweden use tanning booths as a treatment of SAD during Winter months when daylight hours are scant.
2. Sydney, Australia hosted the 2000 Summer Olympics in September of 2000. Why am I bringing it up? Because it wasn't actually Summer in Sydney when the games were being played. Because Sydney is in the SH, it was actually being played mostly in their Winter. It actually ended October 1, 2000 and thus ended in early Spring.
3. So if the Sun never sets north of the Arctic Circle on any give day, what does it look like all day? Figure 3 shows you just that. Note that it rings the horizon and does not get much higher than a few degrees from where the land meets the sky.
Figure 3. What the Sun looks like north of the Arctic Circle during the Summer.
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