I had some dry ice to use up before it melted. For those who've missed the concept, dry ice is solid carbon dioxide. It is at about -80 degrees Celsius. However, when it melts it turns straight into a gas. You don't get liquid carbon dioxide at normal pressures. We say that the dry ice sublimes - the technical term for turning straight from a solid to a gas. A favourite trick is to pour water onto the dry ice. The water might be at 5 degrees Celsius at this time of year but that is considerably warmer than the dry ice. The dry ice sublimes and releases the carbon dioxide gas. Look at the picture. Carbon dioxide is invisible so why is there a white gas? The answer is that the carbon dioxide gas is still very cold. This make the water vapour in the air condense to form clouds. We've made fog! It was how they used to make "smoke" on stage.
The fog is much colder than the air around it. It contracts and is therefore more dense so it sinks. That explains the "waterfall" of fog over the lip of the tray. I have seen cloud do this between valleys on days when there is a cloud inversion.
