Axel the Chimeric wrote:

This is something that's been bugging me for a while, for reasons both practical and aesthetic: Would habitats have weather? Now, the obvious answer is that smaller habitats with few open spaces, such as tin can habs or beehives, don't have weather patterns; there's simply nowhere for them to form. However, in the enormous cylinder habitats, I have to wonder, especially given how they have day and night cycles. This, combined with the fact that there are habitats described as being modeled on rainforests, suggests that rain exists in habitats, but it's never clarified if this is from natural weather systems, artificial ones, or simply sprinklers in a central spire to simulate rain.

The shuttle assembly building apparently is large enough to get condensation that forms small indoors clouds... For smaller habitats the weather will be controlled by sprinkler systems and similar. You need a certain size in order to get spontaneously formed weather. This is determined by the Rayleigh number of the habitat. When this is more than ~1000 convection will start happening. Let's see now... for air around 20 C, kinematic viscosity is 1.923e-6 m/s^2, thermal diffusivity 19e-6 m^2/s, thermal expansion 3.43e-3 per Kelvin. Let's assume a 0.5 g habitat with temperature differences on the order of 1 degrees. Then I get Ra = x^3*(1*0.5*9.82*3.43e-3)/(1.923e-6*19e-6)=x^3*460e6 where x is the size. That is definitely enough to get convection currents even one the one meter scale! (I should have known that, just looking at the dust in the air in my room) So air will tend to flow if there are fairly small temperature differences in the habitat. But to get a cloud you need more. In the atmosphere cloud sizes are presumably affected by the scale height, which is 8 km on Earth and 16 km on this habitat - most habitats are small enough that there is no pressure difference between the axis and the "ground". Also important are the temperature differences again: will there be parts of the habitat where moisture will condense freely in the air? This depends on whether the temperature goes below the dewpoint of water, which in turn depends on how much water there is in the air. For a comfortable habitat this ought to be around 10 C, but in very moist habitats it can be higher. I suspect it is hard to get "proper" clouds. In particular, rain requires that a droplet falls through enough cloud so that it can accumulate moisture to become large - given EP habitat sizes even the best "natural" cloud will likely produce little more than a drizzle on its own. My guess is that a habitat not built to have a weather will still have some related behaviour - air will tend to flow, moisture condenses on cold surfaces and drips down. This is likely undesirable. So a well-built habitat will try to make sure these things at least happens where you want them. They will have a "cyborg weather" which is partially spontaneous, partially controlled by sprinklers, heating/cooling systems and air conditioning.