Airports
Surface weather observations have traditionally been taken at airports due to safety concerns during takeoffs and landings. The ICAO defines the International Standard Atmosphere (also known as ICAO Standard Atmosphere), which is the model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earth's atmosphere. This is useful in calibrating instruments and designing aircraft, and is used to reduce a station's pressure to sea level pressure where it can then be used on weather maps.
In the United States, the FAA mandates the taking of weather observations for safety reasons. To help facilitate the purchase of an automated airport weather station, such as ASOS, the FAA allows federal dollars to be used for the installation of certified weather stations at airports. The airport observations are then transmitted worldwide using the METAR observing code. METAR reports typically come from airports or permanent weather observation stations. Reports are generated once an hour; however, if conditions change significantly, they may be updated in special reports called SPECIE's.
Read more about this topic: Surface Weather Observation