Surface weather observations are the fundamental data used for safety as well asclimatological reasons toforecast weather and issuewarnings worldwide.[1] They can be taken manually, by a weather observer, by computer through the use of automatedweather stations, or in a hybrid scheme using weather observers to augment the otherwise automated weather station. TheICAO defines theInternational Standard Atmosphere (ISA), which is the model of the standard variation ofpressure,temperature,density, andviscosity withaltitude in theEarth's atmosphere, and is used to reduce a station pressure to sea level pressure. Airport observations can be transmitted worldwide through the use of theMETAR observing code.Personal weather stations taking automated observations can transmit their data to the United Statesmesonet through theCitizen Weather Observer Program (CWOP), the UKMet Office through their Weather Observations Website (WOW),[2] or internationally through theWeather Underground Internet site.[3] A thirty-year average of a location's weather observations is traditionally used to determine the station'sclimate.[4] In the US a network ofCooperative Observers make a daily record of summary weather and sometimes water level information.
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 developautomated weather stations.[5]
In the United States, theFAA mandates the taking of weather observations at larger airports for safety reasons. To help facilitate the purchase of anautomated airport weather station, such as ASOS, the FAA allows federal dollars to be used for the installation of certified weather stations at airports.[8] The airport observations are then transmitted worldwide using theMETAR observing code. METAR reports typically come fromairports 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.[9][10][11][12]
Surface weather observations can include the following elements:
TheStation Identifier, orLocation identifier, consists of four characters for METAR observations,[13] 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.[14]
Runway visibility, measured in meters in many locations worldwide, or feet within the United States.[14]
Temperature is a measure of thekinetic 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;[15] otherwise heat flows from the hotter object to the colder object. Temperature, withinmeteorology, is measured withthermometers exposed to the air but sheltered from direct solar exposure.[16] In most of the world, thedegree Celsius scale is used for most temperature measuring purposes. However, the United States is the last major country in which thedegree Fahrenheit temperature scale is used by most lay people, industry, popularmeteorology, and government.[14] 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 constantatmospheric pressure, forwater vapor tocondense into water. The condensed water is calleddew. The dew point is asaturation point. When the dew point temperature falls below freezing it is called thefrost point, as the water vapor no longer creates dew but instead createsfrost orhoarfrost bydeposition.[17] The dew point is associated with relativehumidity. A highrelative 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 thespecific humidity of the air. The dew point is an important statistic forgeneral aviation pilots, as it is used to calculate the likelihood ofcarburetor icing andfog. When used with the air temperature, a formula can be used to estimate the height of cumuliform, or convective, clouds.[18]
Wind is determined usinganemometers andwind vanes, oraerovanes, located a standard 10 metres (33 ft) above ground level (AGL). Averagewind speed is measured using a two-minute average in the United States,[19] and a 10-minute average elsewhere.[20]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 10 knots (5.1 m/s) between peaks and lulls during the sampling period.[19]
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 inweather 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 thenormal range of fluctuations in pressure is the same for everyone. The pressures which are consideredhigh pressure orlow pressure do not depend on geographical location. This makesisobars on a weather map meaningful and useful tools.[21]
Altimeter setting is a term and quantity used inaviation. The regional or local air pressure at mean sea level is called thealtimeter setting, and the pressure which will calibrate the altimeter to show the height above ground at a givenQNHairfield.[22]
Present weather, which present restrictions to visibility or presence ofthunder orsqualls, 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.[23]
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.[24]
Precipitation amount over the past 1, 3, 6 or 24 hours is of particular interest tometeorologists 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.[25]
Personal weather stations, maintained by citizens rather than government officials, do not use METAR code. Software allows information to be transmitted to various sites, such as the Weather Underground globally,[3] or the CWOP within theUnited States,[27] which can then be used by the appropriate meteorological organizations either to diagnose real-time conditions, or be used within weather forecast models.
Data collected by land locations coding in METAR are conveyed worldwide via phone lines or wireless technology. Within many nations' meteorological organizations, this data is then plotted onto a weather map using thestation model. A station model is a symbolic illustration showing theweather occurring at a givenreporting station.[28] Meteorologists created the station model to plot a number of weather elements in a small space onweather maps.[29] Maps filled with dense station-model plots can be difficult to read, but they allow meteorologists, pilots, and mariners to see important weather patterns.
Weather maps are used to display information quickly showing the analysis of various meteorological quantities at various levels of the atmosphere, in this case the surface layer.[30] Maps containing station models aid in the drawing ofisotherms, which more readily identifies temperature gradients,[31] and can help in the location ofweather fronts. Two-dimensionalstreamlines based on wind speeds show areas of convergence and divergence in the wind field, which are helpful in determining the location of features within the wind pattern. A popular type of surface weather map is thesurface weather analysis, which plotsisobars to depict areas ofhigh pressure andlow pressure.
For over a century, reports from the world's oceans have been received real-time for safety reasons and to help with general weather forecasting. The reports are coded using thesynoptic code, and relayed viaradio orsatellite to weather organizations worldwide.[32]Buoy reports are automated, and maintained by the country that moored the buoy in that location. Larger moored buoys are used near shore, while smaller drifting buoys are used farther out at sea.[33]
Due to the importance of reports from the surface of the ocean, thevoluntary observing ship program, known as VOS, was set up to train crews how to take weather observations while at sea and also to calibrate weather sensors used aboard ships when they arrive in port, such asbarometers andthermometers.[34] TheBeaufort scale is still generally used to determine wind speed from manual observers out at sea. Ships withanemometers have issues with determining wind speeds at higher wind speeds due to blockage of the instruments by increasing high seas.
Climate, (fromAncient Greekklima) is commonly defined as the weather averaged over a long period of time.[35] The standard averaging period is 30 years for an individual location,[4] but other periods may be used. Climate includes statistics other than the average, such as the magnitudes of day-to-day or year-to-year variations. TheIntergovernmental Panel on Climate Change (IPCC) glossary definition is:
Climate in a narrow sense is usually defined as the "average weather", or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.[36]
The main difference between climate and everyday weather is best summarized by the popular phrase "Climate is what you expect, weather is what you get."[37] Overhistoric time spans there are a number of static variables that determine climate, including: latitude, altitude, proportion of land to water, and proximity to oceans and mountains. Degree of vegetation coverage affects solar heat absorption, water retention, andrainfall on a regional level.
^Training Guide in Surface Weather Observations(PDF). U.S. DEPARTMENT OF COMMERCE NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION National Weather Service Office of Systems Operations Systems Integration Division Observing Systems Branch. May 1998.
^U.S. DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION ORDER JO 7900.5E. U.S. DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION. 15 January 2020.{{cite book}}: CS1 maint: date and year (link)
