Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide.Office of the Federal Coordinator of Meteorology. Surface Weather Observation Program. Retrieved on 2008-01-12. 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.Weather Underground. Personal Weather Station. Retrieved on 2008-03-09. A thirty-year average of a location's weather observations is traditionally used to determine the station's climate.MetOffice. Climate Averages. Retrieved on 2008-03-09. In the US a network of Cooperative Observers make a daily record of summary weather and sometimes water level information.
History Reverend John Campanius Holm is credited with taking the first systematic weather observations in Colonial America. He was a chaplain in the Swedes Fort colony near the mouth of the Delaware River. Holm recorded daily observations without instruments during 1644 and 1645. While numerous other accounts of weather events on the East Coast were documented during the 17th Century. President George Washington kept a detailed weather diary during the late 1700s at Mount Vernon, Virginia. The number of routine weather observers increased significantly during the 1800s. In 1807, Dr. B. S. Barton of the University of Pennsylvania requested members throughout the Union of the Linnaean Society of Philadelphia to maintain instrumented weather observing sites to establish a climatological history.During the early 1900s, numerous observer stations moved from farms to residential districts of towns, where service was available to mail the observation forms. By 1926, more than 5000 observing locations were located throughout the U.S., West Indies, and the Caribbean.In 1939, the Bureau of Aeronautics in the U. S. Navy began to actively develop automated weather stations.
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 elevation/altitude in the Earth's atmosphere. This is useful in calibrating instruments and designing aircraft,ICAO, Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet)) , Doc 7488-CD, Third Edition, 1993, and is used to reduce a station's pressure to sea level pressure (SLP) where it can then be used on weather maps.Patricia M. Pauley. An Example of Uncertainty in Sea Level Pressure Reduction. Retrieved on 2008-03-29.
In the United States, the FAA mandates the taking of weather observations at larger airports 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.Allweatherinc. Why buy an AWOS? Retrieved on 2008-01-12. 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 SPECIs.National Climatic Data Center. METAR Home Page. Retrieved on 2008-01-12.
Data reported Surface weather observations can include the following elements:
The Station Identifier, or Location identifier, consists of four characters for METAR observations,Texas A&M University. Coding the Type of Report, Station Identifier, Date/Time, and Report Modifier groups. Retrieved on 2008-04-06. with the first representing the region of the world the station lies within. For example, the first letter for areas in and around the Pacific Ocean is P, and for Europe is E. The second character may represent the country/state the location lies within. For Hawaii, the first two letters are "PH" while for Great Britain, the first two letters of the station identifier are "EG". Canada and the contiguous United States are an exception, with the first letters C and K representing the regions, respectively. The final two or three letters normally represent the name of the location or airport. Visibility, measured in meters for most sites worldwide, except in the United States where statute miles are reported.National Weather Service. Frequently Asked Questions about METAR/SPECI and TAF. Retrieved on 2008-04-06. Runway visibility, measured in meters in many locations worldwide, or feet within the United States. Temperature is a measure of the kinetic energy of a sample of matter. Temperature is the unique physical property that determines the direction of heat flow between two objects placed in thermal contact. If no heat flow occurs, the two objects have the same temperature;Glossary of Meteorology. Temperature. Retrieved on 2008-04-06. otherwise heat flows from the hotter object to the colder object. Temperature, within meteorology, is measured with thermometers exposed to the air but sheltered from direct solar exposure.Glossary of Meteorology. Air Temperature. Retrieved on 2008-04-06. In most of the world, the degree Celsius scale is used for most temperature measuring purposes. However, the United States is the last major country in which the degree Fahrenheit temperature scale is used by most lay people, industry, popular meteorology, and government. Despite this, METAR reports from the United States also report the temperature (and dewpoint, see below) in degrees Celsius. Dew point is the temperature to which a given parcel of air must be cooled, at constant atmospheric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point. When the dew point temperature falls below freezing it is called the frost point, as the water vapor no longer creates dew but instead creates frost or hoarfrost by deposition.Glossary of Meteorology. Dewpoint. Retrieved on 2008-04-06. The dew point is associated with relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature. If the relative humidity is 100%, the dew point is equal to the current temperature. Given a constant dew point, an increase in temperature will lead to a decrease in relative humidity. At a given barometric pressure, independent of temperature, the dew point determines the specific humidity of the air. The dew point is an important statistic for general aviation pilots, as it is used to calculate the likelihood of carburetor icing and fog. When used with the air temperature, a formula can be used to estimate the height of cumuliform, or convective, clouds.Glossary of Meteorology. Dewpoint Formula. Retrieved on 2008-04-06. Wind is determined using anemometers and wind vanes, or aerovanes, located a standard above ground level (AGL). Average wind speed is measured using a two-minute average in the United States,Office of the Federal Coordinator for Meteorology. Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports September 2005 Appendix A: Glossary. Retrieved on 2008-04-06. and a 10-minute average elsewhere.Hurricane Research Division. Frequently Asked Questions Subject D4) What does "maximum sustained wind" mean? How does it relate to gusts in tropical cyclones? Retrieved on 2008-04-06. Wind direction is measured using degrees, with north representing 0 or 360 degrees, with values increasing from 0 clockwise from north. Wind gusts are reported when there is variation of the wind speed of more than between peaks and lulls during the sampling period. Sea level pressure (SLP) is the pressure at sea level or (when measured at a given elevation on land) the station pressure reduced to sea level assuming an isothermal layer at the station temperature. This is the pressure normally given in weather reports on radio, television, and newspapers or on the Internet. When barometers in the home are set to match the local weather reports, they measure pressure reduced to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the normal range of fluctuations in pressure is the same for everyone. The pressures which are considered high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tools.Patricia M. Pauley. An Example of Uncertainty in Sea Level Pressure Reduction. Retrieved on 2008-04-14. Altimeter setting is a term and quantity used in aviation. The regional or local air pressure at mean sea level is called the altimeter setting, and the pressure which will calibrate the altimeter to show the height above ground at a given QNH airfield.USA Today. Understanding Air Pressure. Retrieved on 2008-04-14. Present weather, which present restrictions to visibility or presence of thunder or squalls, are reported in observations to indicate to aviation any possible threats during landings and takeoffs from airports. Types included in surface weather observations include precipitation, obscurations, other weather phenomena such as, well-developed dust/sand whirls, squalls, tornadic activity, sandstorms, volcanic ash, and dust storms.Texas A&M University. Present Weather Group w'w'(ww). Retrieved on 2008-04-14. Intensity of precipitation is primarily measured for meteorological concerns. However, it can be of concern to aviation as heavy precipitation can limit visibility. Also, intensity of freezing rain can determine how hazardous it is for pilots to fly nearby certain locations since it can be an in-flight hazard by depositing ice on the wings of aircraft, which can be detrimental to flight.Ben C. Bernstein, Thomas P. Ratvasky, Dean R. Miller, and Frank McDonough. Freezing Rain as in In-Flight Icing Hazard. Retrieved on 2008-04-14. Precipitation amount over the past 1, 3, 6 or 24 hours is of particular interest to meteorologists in verifying forecast amounts of precipitation and determining station climatologies. Snowfall amount during the past 6 hours is taken for meteorological and climatological concerns. However, it may also be reported hourly using "SNOINCR" remarks to provide air field technicians information on how frequently snow must be plowed from runways and taxiways. Snow depth is measured for meteorological and climatological concerns once a day. However, during periods of snowfall, it is measured each six hours to determine amount of recent snowfall.NOAA Meteorological Assimilation Data Ingest System. How to Take Snow Measurements. Retrieved on 2008-04-14.
Example of a METAR surface weather observation METAR LBBG 041600Z 12003MPS 090V150 1400 R04/P1500N R22/P1500U +SN BKN022 OVC050 M04/M07 Q1020 NOSIG 9949//91=National Climatic Data Center. Key to METAR Surface Weather Observations. Retrieved on 2008-03-09.