The gas that is most variable within the troposphere is water vapour. The amount of water vapour in the troposphere can vary greatly from place to place and over time. It can range from a few percent near the surface of the Earth to near saturation in some areas. This variability is due to the fact that water vapour is constantly being evaporated from the surface of the Earth and into the atmosphere, and then condensing back to liquid water in the form of clouds and precipitation. The amount of water vapour in the air can have a significant impact on the weather, as well as on the Earth's climate. For example, high levels of water vapour in the atmosphere can lead to increased cloud cover and precipitation, while low levels of water vapour can lead to drought conditions.