summer

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.