I remember reading somewhere that some intellectuals from the medieval period thought the equator would be uninhabitable because it would be too hot. That made me ponder about a worldbuilding idea - what if there was a planet with an equator that had a climate too hot for humans to live? My question is, if this is possible how could this planet's topography/size/weather patterns differ from our own? Is it possible to make this uninhabitable zone seasonal (so that the humans can perhaps, live underground when the weather warms up)? I want to create a planet that's more or less habitable in all other latitudes, but if that's not plausible, I'd like to hear your reasoning.
A planet that has an unihabitable equator isn't that hard. Just push the planet a little closer to the sun, but stay inside the habitable zone. Our own planet is already hot at the equator. Add another 25% of sunlight and the likelihood of habitability all but drops to zero.
Add to this that the increased water level (melted ice caps) and heat would likely result in some ferocious coriolis winds, making the equator that much more painful to live in.
But if you really want some fun, make the orbit elliptical such that at perihelion the equator is uninhabitable due to heat and at aphelion the poles are uninhabitable due to cold — forcing humanity to be constantly nomadic, moving from pole to equator and back anually to avoid the extreme temperatures.
Note that I'm utterly ignoring mountains, lakes, and altitude, all of which will affect weather considerably. Even under the conditions I've described, the right mountains along the right wind pattern that carries evaporated oceanic water will be habitable (if blisteringly hot) during the summer. Remember that your goal is suspension of disbelief — not necessarily factual reality. But I'd be very curious to see a planet that had such an elliptical orbit that it swung between 125% of "Earth" sunlight and 25% per the graph in the above link. Only considerable technology would allow you to stay in one place annually.
I'd have an equatorial ocean that runs along the equator without major openings to north or south. This would prevent ocean currents from transferring heat away from the equator. Having lots of ocean along the equator would also give the area low albedo and high ability to retain heat.
I think the lands along such equatorial ocean would be very hot and humid.
If that is not bad enough you can totally close off the connections ocean has north or south. The water will then evaporate to large extent creating large areas significantly below sea level. Adiabatic heating will then make these areas very hot. This is presumed to have happened to the Mediterranean.