Yesterday, PatrickW asked me what climate maps look like, after I suggested that he look into it … and found myself stumped.
While this gives an overview of the rainfall in Australia (and it's a pretty map), it doesn't actually identify how much rainfall is the 'mean', or how that rainfall is divided throughout the three month period from October to December. It doesn't say how many days the rain falls, or how much rain falls on average per rain storm, or whether the rain falls lightly or by means of thunderstorms, or even through ground fog.
Naturally, for the planet Earth, some of this is easy to find out, some of it is harder ... but ultimately, all obtainable. It is possible to devise a random table for anywhere on the planet, and for any day of the year, including the temperature, air pressure, prevailing winds and so on ... but once you leave this planet, then what?
Well, you're fucked, if you want to make anything vaguely as complicated as the map above. That's because of the considerable number of influences on even just the rainfall, as shown above. That very dark red spot in the northwest corner of the map. Why exactly is it so dry, when just to the west or east, the rainfall is or is nearly maximum? I can identify it as the Rudall River and the Great Sandy Desert, likely affected by the Hamersley Range on the west, and the prevailing wind from the Indian Ocean losing moisture as it crosses the high country. But you're going to have a hell of time making any kind of map remotely as complicated as the above - too many factors.
If you're inventing your own world, where do you start? What are the prevailing winds? Are you able to identify the principal zones of convection and subduction at the various latitudes of your world, to at least make an educated guess as to where said winds should originate?
Of course you can't. Because you DON'T care. Why would you? Seems like an enormous waste of effort. Hell, even if you were running the planet Earth you wouldn't bother looking up a single table. I understand your thinking: "Hey, you want to know when it rains? When I say it rains. And when I say it stops, it does. Period."
Well, that's pretty swell. It helps explains tropes like It's Always Spring or the fairly universal Winter snow that occurs everywhere on the world, because the DM is from New York and in New York, it snows in the winter time. It helps provide either four traditional seasons happening everywhere, even in the tropics - or alternately, a tropics setting where there is only one static season that continues all year round. The sun never changes the angle it has in the sky, and sunset always happens at 7 p.m (or 8 p.m., if you don't happen to come from California). And you've saved a lot of time and trouble as a DM. Everyone's a Winnah.
I don't blame the gentle reader ... coming up with actual weather for a completely new planet is pretty nigh impossible, unless you happen to be a meteorologist with a masochistic streak (don't they all have that?). It's an old bugbear of mine (mentioned before) how ridiculous is the placement of deserts or jungles on most purchaseable maps. Bottom of the map, jungle, top of the map, polar regions. Never mind that the total number of 20-mile hexes shown on the map would only reach from the north shore of Lake Superior to Tennessee. Okay, so its a small world with a very dense core (1G).
So I take back the suggestion I made to Patrick. Forget weather. It's at least a year's work, and all you're going to get out of it is a die roll that says, "Fog, 1 degree above zero, wind from the west at 2 knots" ... that your party won't give a damn about anyway.
'Course, I'll still be making my weather tables ...