Surface weather observations are the fundamental data used for safety as well as
climatological reasons to
forecast weather and issue
warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated
weather stations, or in a hybrid scheme using weather observers to augment the otherwise automated weather station. The
ICAO defines the
International Standard Atmosphere (ISA), 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
Citizen Weather Observer Program (CWOP), the UK
Met Office through their Weather Observations Website (WOW), or internationally through the
Weather Underground Internet site. A thirty-year average of a location's weather observations is traditionally used to determine the station's
climate.