The ASOS is the primary surface-weather-observing network for the United States. It is designed to update observations nonstop, every minute without exception, and is located at more than 900 airports across the country. According to the NWS Forecast Office, the ASOS reports sky conditions (cloud height and cloud amount, for example) up to 12,000 feet, surface visibility, basic current weather information (type and intensity of rain, snow and freezing rain), pressure and altimeter settings at sea level, air and dew-point temperatures and wind direction and speed. The system is not designed to report tornadoes, funnel clouds, ice crystals or blowing obstructions (snow and sand, for example) and cannot see around the horizon.
Radar, which stands for RAdio Detection And Ranging, has been used to detect precipitation since the 1940s. Enhancements have been implemented to enable forecasters to more precisely examine storms. The Doppler radar is the modern version of the 1940s radar and can detect intra-cloud motions in high resolution. It is the premier source for detecting sever weather and the NWS relies on it heavily for weather-warning operations. It operates by emitting extremely short bursts of nearly light-speed radio waves from the rotating radar antenna. The radio waves are reflected (called the "echo") and received between the bursts of radio waves. The distance from the radar and intensity of the echo is recorded and analyzed.
The NWS says that radiosondes have been employed for more than 60 years to gather weather data from the entire depth of the atmosphere. The small, expendable device, hung from a balloon, measures pressure, temperature and relative humidity. The data are sent to a ground receiver via a battery-powered transmitter. Wind speed is indirectly measured from tracking the position of the radiosonde in flight. The balloon measures five feet tall by four feet wide at liftoff, but it expands to the height of a two-story building when it reaches 100,000 feet. The balloon can ascend to over 115,000 feet before it bursts and allows a small parachute to slow the descent of the radiosonde. The radiosonde drifts to the ground where it can be mailed back to the NWS by citizens who come across the device.
