Meteorologists monitor the atmosphere above the surface by using a radio-equipped meteorological instrument package – called radiosondes – carried aloft by a helium-filled weather balloon.
The radiosondes measure vertical profiles of air temperature, relative humidity and pressure from the ground all the way up to about 19 miles. Temperature and relative humidity are measured electronically; a small aneroid barometer measures pressure. At low air pressures in the stratosphere, the balloon expands so much that it explodes and the radiosonde drifts back to the ground underneath a small parachute.
Wind speed and direction can also be determined by tracking the position of the balloon. When winds are also measured, the observation is called a rawinsonde. Rawinsonde measurements are made worldwide at several hundred locations twice each day. Rawinsondes are the workhorses of the weather data network above the ground.
However, they are usually launched only from land-based weather stations, which leaves out the 70 percent of the atmosphere that lies above the oceans. Therefore, modern-day meteorologists also use satellite observations to observe the upper atmosphere. Special satellite instruments measure temperature and humidity averaged through a layer of the atmosphere. The movement of clouds in a sequence of satellite pictures is used to infer wind speed and direction. The Global Positioning System satellite network used in cars to tell you where you are as you drive around, is also used to tell how much water vapor is in the atmosphere.
