 The ICAO defines the International Standard Atmosphere, which is the model of the standard variation of pressure, temperature, density, and viscosity with altitude in the Earth's atmosphere, and is used to reduce a station pressure to sea level pressure.Airport observations can be transmitted worldwide through the use of the METAR observing code.Personal weather stations taking automated observations can transmit their data to the United States mesonet through the use of the Citizen Weather Observer Program (CWOP), or internationally through the Weather Underground Internet site.