Short answers, pulled from the story.
Water vapor contributes more to Earth's total greenhouse effect than carbon dioxide and methane combined because its hydroxyl bond strongly absorbs infrared radiation. It acts as a feedback gas: as carbon dioxide warms the atmosphere, more water evaporates, amplifying the warming. IPCC AR6 projects an increase of around seven percent of total atmospheric water vapor for every one degree Celsius of warming.
Approximately 99.13 percent of all atmospheric water vapor is contained in the troposphere, the lowest layer of the atmosphere. The mean residence time of a water molecule in the troposphere is about nine to ten days before it falls back to the surface as precipitation.
Water vapor has been detected in the atmospheres of all seven other planets in the Solar System, in Earth's Moon, in moons of other planets, and in the atmospheres of dwarf planet Ceres and several comets. Beyond the Solar System, it has been confirmed in the atmosphere of exoplanet HD 209458 b in the constellation Pegasus, and in the atmospheres of HAT-P-11b and K2-18b.
Dry air at standard temperature and pressure has a density of 1.27 grams per liter, while water vapor at standard temperature has a density of only 0.0048 grams per liter. This is because the molar mass of water is 18.02 grams per mole, far lower than the average molar mass of air at approximately 28.57 grams per mole, making moist air lighter and more buoyant than dry air.
Antarctica receives the lowest rate of precipitation of any continent on Earth, so instead of melting conventionally, ice across large areas slowly sublimes directly from solid to vapor over millennia. This process leaves behind all non-volatile materials, exposing meteorites on the surface in unparalleled numbers and in excellent states of preservation.
The mean global content of water vapor in the atmosphere is roughly sufficient to cover the planet's surface with a layer of liquid water about 25 millimeters deep. By comparison, mean annual precipitation for the planet is about one meter, implying that the atmosphere cycles through its entire water supply roughly forty times per year.