Epic fantasies typically depict northern areas as cold and southern areas as relatively warm. Is this just to play into reader expectations since that's the way it is on Earth, or is there a scientific reason that a planet's northern hemisphere will tend to be cooler?
No, there's no scientific reason to why a given latitude to the south of the average planet should be any different in average temperature to the same latitude north.
Perhaps this is slightly reinforced by the fact that on Earth there happens to be a lower ratio of land to sea in the southern hemisphere than in the northern hemisphere. Antarctica is surrounded by a vast ocean; there's no one living as close to the south pole as, say, northern Russia or Greenland is to the north pole.
Basically, the perception arises because most SF and fantasy writers come from countries in the northern hemisphere, so from their home town things get colder if you go north and hotter if you go south.
Most of the Earth's current population lives in the northern hemisphere. That means 88% of people think going north from the current position results in a colder climate.
Why has this translated into fantasy? My only guess is that the authors of those fantasies are simply so used to equating north with cold, that it just carried over. There are exceptions to this, of course.
Building on the excellent answers others have given concerning the fact that most fantasy has been written from the Northern Hemispherical perspective (and hence via N-S as an increasing temperature axis), there are actually differences between hemispheres which you could build in to your world. You could also make them more extreme.
First, the earth is nearest to the Sun (perihelion) in early January (4th January in 2015). Since the earth is also moving faster around the Sun at that point this means that:
- the Northern hemisphere gets more solar radiation in its Winter than the Southern gets in its Winter
- Norther Winters are a little shorter and hence their Summers are a little longer than in the Southern hemisphere
The earth's orbit is very close to being a circle (eccentricity 0.0167) so these effects are not that large. It took a long time before ancient astronomers realised that there was anything going on at all. A more eccentric orbit could make these things rather different.
The date of perihelion also changes (because of the precession of the apsides) very slowly — again, that is something that could be different in an artificial world.
However, another bigger effect means that, counter-intuitively, the earth is actually warmer when it is further from the Sun (at aphelion) because that is when the Southern hemisphere (mostly ocean) gets the Sun's radiation rather than perihelion when the Northern hemisphere gets it. There are two neat explanations of this here:
- Today I Found Out: The Earth is Hottest When It Is Furthest From the Sun On Its Orbit, Not When It Is Closest
- Aphelion: shouldn't earth be at its coldest today?
So you could build a world with a different distribution of continents to change these effects as well.
Earth's southern hemisphere isn't warmer than the northern. The closer you get to the north/south poles, the colder it gets, as those areas get less sunlight than other areas, like the equator. I'm guessing the 'epic fantasy' you're referencing is A Song of Ice and Fire. The weather there is funky and fickle, and no one is certain why it is. There are theories, like the planet this world is on has a wacky axis, or that it isn't even on a planet. You can incorporate some of those ideas into your story.
The solar exposure difference between the northern and southern hemisphere is determined by the planet's spin axis and the planets equator (perpendicular to the spin axis), as in this picture:
So you could potentially have a gravitationally locked planet (like Mercury) with a spin axis of -90' (like Uranus) and its northern hemisphere exposed to the system's star all year long and scorched, and the southern hemisphere plunged in the dark and blistering cold, with the only life possible being in a ring 5 degrees below the equator in the southern hemisphere. In reality such a planet would probably not be habitable because of insane weather and thermal stress constantly cracking the surface due to the enormous temperature delta between both hemispheres.
I think it's just easier to say north is colder than south from a northern hemisphere point of view, almost metaphorically, especially in science fiction.
For conventional planets it's completely wrong that the southern hemisphere "is colder".
When you are in one hemisphere or the other,
it gets warmer as you move towards the equator.
So, IF you are in the Northern hemisphere, it gets warmer as you move South.
So, IF you are in the Southern hemisphere, it gets warmer as you move North.