Surface weather observation
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. [http://www.ofcm.gov/fmh-1/pdf/B-CH2.pdf 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 ICAOdefines the International Standard Atmosphere, which is the model of the standard variation of pressure, temperature, density, and viscositywith altitudein 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 METARobserving code. Personal weather stations taking automated observations can transmit their data to the United Statesmesonet through the use of the Citizens weather observer program, or internationally through the Weather Underground web site.Weather Underground. [http://www.wunderground.com/weatherstation/index.asp 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. [http://www.metoffice.gov.uk/climate/uk/averages/index.html Climate Averages.] Retrieved on 2008-03-09.]
Surface weather observations have traditionally been taken at
airports due to safety concerns during takeoffs and landings. The ICAOdefines the International Standard Atmosphere(also known as ICAO Standard Atmosphere), which is the model of the standard variation of pressure, temperature, density, and viscositywith altitudein the Earth's atmosphere. This is useful in calibrating instruments and designing aircraft,ICAO, " [http://www.icao.int/cgi/goto_m.pl?isbn/index.html Manual of the ICAO Standard Atmosphere (extended to 80 kilometres (262 500 feet))] ", Doc 7488-CD, Third Edition, 1993, ISBN 92-9194-004-6] and is used to reduce a station's pressure to sea level pressurewhere it can then be used on weather maps. [Patricia M. Pauley. [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0434(1998)013%3C0833%3AAEOUIS%3E2.0.CO%3B2 An Example of Uncertainty in Sea Level Pressure Reduction.] Retrieved on 2008-03-29.]
In the United States, the
FAAmandates 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. [Allweatherinc. [http://www.allweatherinc.com/pdf/why_buy_an_awos.pdf Why buy an AWOS?] Retrieved on 2008-01-12.] The airport observations are then transmitted worldwide using the METARobserving 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 SPECI's. [ National Climatic Data Center. [http://www.ncdc.noaa.gov/oa/wdc/metar/index.php METAR Home Page.] Retrieved on 2008-01-12.]
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. [http://www.met.tamu.edu/class/METAR/metar-pg5.html 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 identifie 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. [http://www.nws.noaa.gov/oso/oso1/oso12/faq.htm Frequently Asked Questions about METAR/SPECI and TAF.] Retrieved on 2008-04-06.]
*Runway visibility, measured in
meters in many locations worldwide, or feetwithin the United States. National Weather Service. [http://www.nws.noaa.gov/oso/oso1/oso12/faq.htm Frequently Asked Questions about METAR/SPECI and TAF.] Retrieved on 2008-04-06.]
Temperatureis a measure of the kinetic energyof 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. [http://amsglossary.allenpress.com/glossary/search?id=temperature1 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. [http://amsglossary.allenpress.com/glossary/search?id=air-temperature1 Air Temperature.] Retrieved on 2008-04-06.] In most of the world, the degree Celsiusscale is used for most temperature measuring purposes. However, the United States is the last major country in which the degree Fahrenheittemperature scale is used by most lay people, industry, popular meteorology, and government. National Weather Service. [http://www.nws.noaa.gov/oso/oso1/oso12/faq.htm Frequently Asked Questions about METAR/SPECI and TAF.] Retrieved on 2008-04-06.] Despite this, METAR reports from the United States also report the temperature (and dewpoint, see below) in degrees Celsius.
Dew Pointis the temperatureto which a given parcel of airmust be cooled, at constant barometric pressure, for water vaporto condense into water. The condensed water is called dew. The dew point is a saturationpoint. 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 frostor hoarfrost by deposition. [Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=dewpoint1 Dewpoint.] Retrieved on 2008-04-06.] The dew point is associated with relative humidity. A high relative humidityindicates 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 humidityof the air. The dew point is an important statistic for general aviationpilots, as it is used to calculate the likelihood of carburetor icingand fog. When used with the air temperature, a formula can be used to estimate the height of cumuliform, or convective, clouds. [Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=dewpoint-formula1 Dewpoint Formula.] Retrieved on 2008-04-06.]
*Wind speed and direction is determined using
anemometers located a standard convert|10|m|ft above ground level. Average wind speed is measured using a two-minute average in the United States,Office of the Federal Coordinator for Meteorology. [http://www.ofcm.gov/fmh-1/pdf/M-APNDXA.pdf 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. [http://www.aoml.noaa.gov/hrd/tcfaq/D4.html 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 convert|10|kn|m/s between peaks and lulls during the sampling period.Office of the Federal Coordinator for Meteorology. [http://www.ofcm.gov/fmh-1/pdf/M-APNDXA.pdf Federal Meteorological Handbook No. 1 - Surface Weather Observations and Reports September 2005 Appendix A: Glossary.] Retrieved on 2008-04-06.]
Sea level pressureis 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. [http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0434(1998)013%3C0833%3AAEOUIS%3E2.0.CO%3B2&ct=1 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 airfield. [ USA Today. [http://www.usatoday.com/weather/wbarocx.htm Understanding Air Pressure.] Retrieved on 2008-04-14.]
*Present weather, which present restrictions to visibility or presence of
thunderor 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 duststorms. [ Texas A&M University. [http://www.met.tamu.edu/class/METAR/metar-pg9-ww.html 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. [http://gltrs.grc.nasa.gov/reports/2000/TM-2000-210058.pdf Freezing Rain as in In-Flight Icing Hazard.] Retrieved on
Precipitationamount over the past 6 or 24 hours is of particular interest to meteorologists in verifying forecast amounts of precipitation and determining station climatologies.
Snowfallamount during the past 6 hours is taken for meteorological and climatological concerns.
*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. [
NOAAMeteorological Assimilation Data Ingest System. [http://www.madis-fsl.org/snow_measurements.html How to Take Snow Measurements.] Retrieved on 2008-04-14.]
Example of a METAR surface weather observation
METAR LBBG 041600Z 12003MPS 310V290 1400 R04/P1500N R22/P1500U +SN BKN022 OVC050 M04/M07 Q1020 NOSIG 9949//91= [
National Climatic Data Center. [http://www.ncdc.noaa.gov/oa/climate/conversion/swometardecoder.html Key to METAR Surface Weather Observations.] Retrieved on 2008-03-09.]
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 Weather Underground globally,Weather Underground. [http://www.wunderground.com/weatherstation/index.asp Personal Weather Station.] Retrieved on
2008-03-09.] or CWOPwithin the United States, [Russ Chadwick. [http://wxqa.com/ Citizen Weather Observer Program.] Retrieved on 2008-03-09.] which can then be used by the appropriate meteorological organizations either to diagnose real-time conditions, or be used within weather forecast models.
Use on weather maps
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 the
station model. A station model is a symbolic illustration showing the weatheroccurring at a given reporting station. [Steve Ackerman and Tom Whittaker. [http://cimss.ssec.wisc.edu/wxwise/station/page2.html Station Model.] Retrieved on 2008-03-27.] Meteorologists created the station model to plot a number of weather elements in a small space on weather maps. [Illinois Central College. [http://faculty.icc.edu/easc111lab/labs/labj/prelab_j.html LAB J: Weather Maps and Humidity.] Retrieved on 2008-03-27.] 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. [Encarta. [http://encarta.msn.com/dictionary_/chart.html Chart.] Retrieved on 2007-11-25.] Maps containing station models aid in the drawing of isotherms, which more readily identifies temperature gradients, [DataStreme. [http://dstreme.comet.ucar.edu/extras/sample.act.html AIR TEMPERATURE PATTERNS.] Retrieved on 2007-11-25.] and can help in the location of weather fronts. Two-dimensional streamlines 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 the surface weather analysis, which plots isobars to depict areas of high pressureand low pressure.
hip and buoy reports
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 the synoptic code, and relayed via
radioor satelliteto weather organizations worldwide. [ National Weather Service. [http://www.vos.noaa.gov/ObsHB-508/ObservingHandbook1_2004_508_compliant.pdf National Weather Service Observing Handbook 1: Marine Surface Weather Observations.] Retrieved on 2008-01-13.] 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. [National Data Buoy Center. [http://www.ndbc.noaa.gov/mooredbuoy.shtml Moored Buoy Program.] Retrieved on 2008-01-13.]
Due to the importance of reports from the surface of the ocean, the
voluntary 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 as barometers and thermometers. [National Data Buoy Center. [http://www.vos.noaa.gov/vos_scheme.shtml The WMO Voluntary Observing Ships (VOS) Scheme.] Retrieved on 2008-01-13.] The beaufort scaleis still generally used to determine wind speed from manual observers out at sea. Ships with anemometershave issues with determining wind speeds at higher wind speeds due to blockage of the instruments by increasing high seas.
Use in establishing climate of a location
Ancient Greek"klima") is commonly defined as the weather averaged over a long period of time. [Glossary of Meteorology. [http://amsglossary.allenpress.com/glossary/search?id=climate1 Climate.] Retrieved on 2008-03-09.] The standard averaging period is 30 years for an individual location,MetOffice. [http://www.metoffice.gov.uk/climate/uk/averages/index.html Climate Averages.] Retrieved on 2008-03-09.] 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. The Intergovernmental 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." [ Intergovernmental Panel on Climate Change. [http://www.grida.no/climate/ipcc_tar/wg1/518.htm Appendix I: Glossary.] Retrieved on 2007-06-01.]
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." [National Weather Service Office Tucson, Arizona. [http://www.wrh.noaa.gov/twc/ Main page.] Retrieved on
2007-06-01.] Over historic 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, and rainfallon a regional level.
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