> My understanding of the greenhouse effect is that the atmosphere acts like
> diode. A one way valve. Radiation (visible light) is able to pass thru
> (incoming) and heats up the surface...which then radiates heat (outgoing)
> which is unable to leave due to the atmosphere being opaque to infared
> So given that....how is Venus suffering from "runaway greenhouse"?
> Given an opaque atmosphere how is it getting ANY heat?
The models get pretty complex on this, but the basic idea is that Venus' atmosphere is not 100% opaque. Visible light coming in from space tends to penetrate much deeper than IR light trying to make it back into space. This gives you a greenhouse effect, because it is easier for energy to get in than to get out. Most of the energy would be deposited in the middle layers of the atmosphere rather than at the surface, so you have a lot of complex modeling to do to figure out things like the surface temperature.
When last I checked (about 3 years ago) we still didn't have a complete model of the Venusian atmosphere, but the partial ones were showing a decent level of agreement with our observations. There are probably thing going one here that we haven't caught on to yet, but the basic concept of massive greenhouse warming looks sound.
Billy Brown, MCSE+I