Depending upon its temperature, water comes in three different forms: solid (ice), liquid (rain drops), and gas (invisible to human eyes). As water grows hotter it eventually changes from a liquid into a gas. This gas is called water vapor. Water vapor is a very important part of Earth's atmosphere because it traps heat near the surface and keeps our planet warm. Water vapor is also important because as it rises into the atmosphere it cools and turns back into water droplets. As more water droplets appear, they eventually form a cloud. Some clouds produce rain and snow, bringing fresh water back to the surface. So scientists monitor water vapor because it influences Earth's weather patterns, and because it is a very important part of our world's climate system. These maps show satellite measurements of water vapor for a given day, or over a span of days