For starters, it's clear that there's significant subsurface ice in the area where Phoenix operated. One trench, dubbed "Snow White" by mission controllers, had a clear layer of ice within it, and others had slightly more diffuse signs. The white chunks that were observed sublimating from a trench were confirmed to be ice by an onboard experiment that noticed an endothermic transition occurring at about 2°C, after which its instruments picked up indications of water vapor.
Beyond confirming the presence of water, various instruments gave some indication of what that water was doing. Electrical conductivity in soil samples increased at night, suggesting that the water sublimated off in the sunlight and was returning to the Martian soil at night. The cameras detected clouds and low-lying fog, indicating that the water was taking part in normal atmospheric processes; these were forming at approximately -65°C, much too warm for them to be comprised of frozen carbon dioxide.
But the really amazing data came from the LIDAR instrument, which was able to track the formation of the clouds at the atmosphere's boundary layer. Cloud formation became more pronounced as the summer gradually faded and the atmosphere cooled at night, and the scientists were eventually able to detect cirrus-like clouds as they dropped "tilted vertical sheets" of ice particles back to the surface of the planet. In short, they watched it snow.
link: Snowfall on Mars? NASA's Phoenix Lander recorded it - Ars Technica
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