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Are southern hemisphere seasons more severe? | Cosmology & Astronomy | Khan Academy


In previous videos we’ve
already talked about the idea that there are times
in Earth’s orbit when it is closer
to the sun and when it is farther from the sun. And when it is
closer to the sun, so let’s say that this is the
time in the orbit when it’s closer to the sun,
this is the perihelion. And when it’s
farthest from the sun, and I’m exaggerating the
difference, this is aphelion. This is the aphelion
in our orbit, when we are farthest
from the sun. And maybe our orbit looks
something like this. I shouldn’t have the
bulge over there. So maybe our orbit looks
something like this over here. And what I point out in the
first video where we discuss this is that this is not
the cause of the seasons. Even though we are 3% closer
right now, the way our orbit is set up, and we’ll
see in future videos that the difference or
the eccentricity, or how elliptical the orbit is,
does change over time, how much it deviates
from being circular. That’s one way to think
about eccentricity. That does change over time. But right now when we
are closest to the sun we are 3% closer than when
we are farthest from the sun. So 3% closer than at aphelion. And we point out in the first
video when we discussed this that this is not the
cause of the seasons. And in particular, perihelion
, when we were closest to the sun, when we actually
have the most radiation from the sun, that’s actually when
we have the Northern Hemisphere winter. So this occurs right over here. This occurs in January. And aphelion occurs in July. Now, based on this,
this might lead to an interesting question,
because let’s think about January when
we’re at perihelion, and let’s think about July
when we’re at aphelion. Let me draw a quick
globe right over here. And let’s make that the equator. And I’ll draw it
in both situations. So January is obviously when
we have the Northern Hemisphere winter. So I’ll paint it in
blue right over here. It is winter. And July is when we have
the Northern Hemisphere summer, or the Southern
Hemisphere winter. So then we have
winter during July in the Southern Hemisphere. And let me put summer
in a more summery color. I guess that orange is
a pretty good color, but that’s not orange. Here’s orange. All right. That’s orange. And that’s orange. So these are summer. So that’s the summer in the
Southern Hemisphere, which occurs during the winter the
Northern Hemisphere, and vice versa. Summer in the
Northern Hemisphere occurs during winter in
the Southern Hemisphere. And so the question might
be rising in your head, and I did see a few comments
on that first video asking this question, and
it’s a good one. If we are closer to
the sun in January, or we are closest to
the sun in January– this is the perihelion
right over here– and so we are getting more
solar radiation in January, does that moderate the winter? Does that moderate the winter
in the Northern Hemisphere? Or I guess another
way to think about it, does it make the summer
in the Southern Hemisphere when we are closer to
the sun, does it make it more extreme, or hotter? And vice versa, in July, when
we are farthest from the sun, does that moderate the
Northern Hemisphere winter? Because it’s hot
out there, but hey, we’re little bit
farther from the sun. And does it make the Southern
Hemisphere winter colder? So once again does it
make this more extreme? Because it’s already winter
and we’re farther from the sun. So maybe we’re also
getting less radiation. And so there’s a couple
ways to think about it. One, it is true that
when we’re farther we are getting a little bit
less radiation from the sun or we’re getting heated
up a little bit less. But the one reality is that
the Southern Hemisphere climate as a whole is not more
extreme despite getting more solar energy in
the summer and getting less solar energy in the winter. And the reason why it
is not as extreme– Let me draw the
equator here just so we can separate
our hemispheres. The main reason it is believed
that it is not more extreme is that the Southern Hemisphere
has a lot more water in it. So just if you look at the
surface of the Southern Hemisphere you’re looking
at a lot more water than the surface of the
Northern Hemisphere. And this is, of course,
it’s a Mercator projection, and so it distorts things so
that things near the poles get really kind of built
up to look really huge even though they really
aren’t that big. Greenland really isn’t
larger than South America. It just spreads them out so
that you can kind of flatten out the map, so to speak. But the Southern
Hemisphere has more water, and as you may have
learned in chemistry class, water has a higher
specific heat. It takes a higher specific
energy, more heat, to raise water a degree than
it does to raise land a degree. And so water can
absorb more energy. Or when there’s
less energy, water will release more energy
without dropping as much of a temperature. So water has a moderating
influence on the climate. So even though the summers
in the Southern Hemisphere actually are getting
more radiation than the summers in the
Northern Hemisphere, it’s moderated on the
actual temperature because the water
has the ability to absorb more of that
heat without changing the temperature as dramatically. Now, with that said, it is
true that in general Antarctica is colder. Antarctica is colder
than the North Pole. But the main reason
why Antarctica is colder, besides the
fact that it’s on land, as opposed to the
North Pole being in the center of
the Arctic Ocean, is that it’s actually a huge,
very high altitude ice sheet. And so the altitude
for most of Antarctica is around 8,000 feet. So it’s kind of like
an alpine altitude. So the main reason
why it’s colder is possibly being farther
away from the sun in winter might play some role there, but
the main reason why it’s colder is it is just at a
much higher altitude, and it’s to some degree
insulated from the water, or I guess you could
say it’s on the land. So especially during
the long winters it’s going to get
that much colder. But I’ll leave you there,
and to some degree, and this is the other aspect
of it, during the summers– And all of this stuff
is super complicated. So you can’t just draw
out one rule of thumb and say this is the reason. But these are all
the influences– is that if you have a
super large ice sheet it’s also more likely to
reflect more of the energy because it’s white,
as opposed to a darker color like the
ocean or the land. And so you can think about
all of those factors, but the general answer
is it’s a good question. But overall, the climate
in the Southern Hemisphere is not more extreme than
the climate in the Southern Hemisphere, even though
Antarctica is colder.

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Comments
  • From a surfers perspective storms in the southern hemisphere don't get blocked/slowed down/drained of energy as they do in the northern hemisphere when they hit North America or Europe/Asia. Storms can just continue to spin around Antartica. The northern hemisphere Pacific Ocean doesn't produce any significant storms in its summer, but the southern hemisphere Pacific Ocean produces large, wave generating storms all summer long(its summer). Worth mention even though it is a little off topic.

  • You are repeating yourself 4-5 times at each phrase, like we would be mentally retarted, you try to define the same simple trivial things 4 or 5 times. Once is enough! Most of the things you are trying to explain (repeatedly) is stuff every 8 year old kid knows. This is so annoying. This is not rocket science, you could have wrapped up this video in 3 minutes, not 7. More than half this video is an annoying waste of time. 

  • Antarctica cops a double whammy from being not only the southernmost continent but also being a hodgepodge of mountain ranges 2.5km-5km high.

  • I think that if we stayed in the "phelion" longer then we would have a more extreme winter, but we go through it fast enough that it doesn't have time to "freeze" the earth.

  • which all begs the question .. Will there be sometime in the distant future when the northern hemisphere is tilted toward the sun at perihelion ?

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