Quote:
Originally Posted by Daigoro
Does that mean that there'd be a gradient of habitable worlds, with a habitable band located anywhere from the subsolar point to the twilight zone, depending on their average temperature (and hydro load, etc.)? Or is there absolutely going to be an arid band around the subsolar pole?
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I haven't seen or done studies, so I don't actually know.
In the slowly-rotating limit (as illustrated in Merlis & Schneider (op. cit.)) there is a fairly simple pattern of winds and temperatures which seems to be readily understood in terms of a thermally-direct toroidal cell of circulation that is absolutely dictated by the fact that the subsolar region is hotter than what surrounds it. So you're definitely going to get a convergence and humid upwelling at the subsolar point, with a ring of downdrafts some way terminator-wards of it, and a pattern of brisk, relatively dry winds towards the subsolar region. In the case where the torrid zone is too hot for agriculture and permanent human habitation then the band of equable temperatures will be comparatively dry. But that doesn't necessarily mean that all the land will be non-arable. I live in the horse latitudes at 31°5' south, and the area around here is usually pretty green and well-vegetated. The is a bit of semi-arid grassland to the west, and an awful lot of desert west of that, but geographical features interrupt the straightforward patterns of winds and ocean currents — the South Equatorial Current that ought to flow to the west is diverted to the south by a continent in its way, the resulting East Australian current makes the seas here warmer and the air more humid than would be if the entire world were covered by a uniform ocean 5m or 4 km deep.
So I suspect that in the slowly-rotating case you start on a planet 30 K cooler than Earth with an "eyeball" world: only the subsolar region is free from ice. Then as you consider gradually warmer worlds the ice around the subsolar optimum gives way to an expanding ring of arid to semi-arid that is streaked and speckled with fortunate lands where local and geographical effects produce adequate rainfall. Once you get to a point a little warmer than Earth the subsolar region starts to become uninhabitable owing to heat — but there remain some favoured areas in the torrid zone that are cooled by diverted ocean currents etc and remain habitable and wet. At that stage, the zone of habitation is like, say, Australia: largely arid to semi-arid but with some good bits. Eventually the zone of equable temperatures gets pushed towards the terminator. In the twilight zone it isn't as windy, but the low slanting sunlight gets too dim to support agriculture.
Worlds that rotate rapidly enough for the Coriolois effects to be significant (and I don't know how fast that has to be) are more fortunate. The pattern of the winds and rainfall is a lot more complicated, yielding a warm/wet patch that is shaped like a lobster with its tail stretching towards, perhaps even across, the terminator east of the subsolar point. Screw that pattern up with a few mountain ranges and you have an excellent chance of finding pretty much any climate you need, albeit probably in a small area. Take a look at the map of surface temperature on p.4 of the paper by Merlis & Schneider, and the map of potential evaporation minus precipitation on p.5. The fast-rotating cases (on the right of Fig. 1 and Fig. 2 respectively) are charmingly quirky.