Infrared sensors on satellites measure water vapor in a layer of the atmosphere about 6-10 km above Earth's surface. At this altitude, strong currents such as jet streams control the movement of weather systems. Jet streams show up as elongated dark regions bordered by lighter regions in the images. Dark areas have little water vapor (dry air), while white areas represent moist weather systems. This animation shows water vapor images recorded every 6 hours over a period of four days.