Observations  Surface Aviation Weather Observations  Air Route Traffic Control Center (ARTCC)  Upper Air Observations  The data gathered from surface and upper altitude  observations form the basis of all weather forecasts,  advisories, and briefings. There are four types of weather  observations: surface, upper air, radar, and satellite.  Surface aviation weather observations (METARs) are a  compilation of elements of the current weather at individual  ground stations across the United States. The network is  made up of government and privately contracted facilities  that provide continuous up-to-date weather information.  Automated weather sources, such as the Automated Weather  Observing Systems (AWOS), Automated Surface Observing  Systems (ASOS), as well as other automated facilities, also  play a major role in the gathering of surface observations.  Surface observations provide local weather conditions  and other relevant information for a specific airport. This  information includes the type of report, station identifier,  date and time, modifier (as required), wind, visibility,  runway visual range (RVR), weather phenomena, sky  condition, temperature/dew point, altimeter reading, and  applicable remarks. The information gathered for the surface  observation may be from a person, an automated station, or  an automated station that is updated or enhanced by a weather  observer. In any form, the surface observation provides  valuable information about individual airports around the  country. Although the reports cover only a small radius, the  pilot can generate a good picture of the weather over a wide  area when many reporting stations are viewed together.  The Air Route Traffic Control Center (ARTCC) facilities  are responsible for maintaining separation between flights  conducted under instrument flight rules (IFR) in the en  route structure. Center radars (Air Route Surveillance Radar  (ARSR)) acquire and track transponder returns using the same  basic technology as terminal radars. Earlier center radars  displayed weather as an area of slashes (light precipitation)  and Hs (moderate rainfall). Because the controller could not  detect higher levels of precipitation, pilots had to be wary  of areas showing moderate rainfall. Newer radar displays  show weather as three shades of blue. Controllers can select  the level of weather to be displayed. Weather displays of  higher levels of intensity make it difficult for controllers to  see aircraft data blocks, so pilots should not expect air traffic  control (ATC) to keep weather displayed continuously.  Observations of upper air weather are more challenging  than surface observations. There are several methods by  which upper air weather phenomena can be observed:  The Aircraft Meteorological Data Relay (AMDAR) is  an international program utilizing commercial aircraft to  provide automated weather observations. The AMDAR  program provides approximately 220,000-230,000 aircraft  observations per day on a worldwide basis utilizing aircraft  onboard sensors and probes that measure wind, temperature,  humidity/water vapor, turbulence and icing data. AMDAR  vertical profiles and en route observations provide significant  benefits to the aviation community by enhancing aircraft  safety and operating efficiency through improved weather  analysis and forecasting. The AMDAR program also  contributes to improved short and medium term numerical  weather forecasts for a wide range of services including  Pilots also provide vital information regarding upper air  weather observations and remain the only real-time source  of information regarding turbulence, icing, and cloud  heights. This information is gathered and filed by pilots  in flight. Together, PIREPs and radiosonde observations  provide information on upper air conditions important for  flight planning. Many domestic and international airlines  have equipped their aircraft with instrumentation that  automatically transmits in flight weather observations  through the DataLink system.  Since the pressure decreases as the balloon rises in the  atmosphere, the balloon expands until it reaches the limits  of its elasticity. This point is reached when the diameter has  increased to over 20 feet. At this point, the balloon pops and  the radiosonde falls back to Earth. The descent is slowed by  means of a parachute. The parachute aids in protecting people  and objects on the ground. Each year over 75,000 balloons  are launched. Of that number, 20 percent are recovered and  returned for reconditioning. Return instructions are printed  on the side of each radiosonde.  The balloon flight can last as long as 2 hours or more and  can ascend to altitudes as high as 115,000 feet and drift as  far as 125 miles. The temperatures and pressures experienced  during the flight can be as low as -130 °F and pressures as  low as a few thousandths of what is experienced at sea level.  radiosonde observations, pilot weather reports (PIREPs),  Aircraft Meteorological Data Relay (AMDAR) and the  Meteorological Data Collection and Reporting System  (MDCRS). A radiosonde is a small cubic instrumentation  package that is suspended below a six foot hydrogen- or  helium-filled balloon. Once released, the balloon rises at a rate  of approximately 1,000 feet per minute (fpm). As it ascends,  the instrumentation gathers various pieces of data, such as air  temperature, moisture, and pressure, as well as wind speed  and direction. Once the information is gathered, it is relayed  to ground stations via a 300 milliwatt radio transmitter.  13-2 