   #copyright

Weather forecasting

2007 Schools Wikipedia Selection. Related subjects: Climate and the Weather;
Storms

   Modern weather predictions aid in timely evacuations and potentially
   save lives and property damage
   Modern weather predictions aid in timely evacuations and potentially
   save lives and property damage
   Weather map of Europe, 10 December 1887
   Weather map of Europe, 10 December 1887

   Weather forecasting is the application of science and technology to
   predict the state of the atmosphere for a future time and a given
   location. Humankind has attempted to predict the weather since ancient
   times. Today, weather forecasts are made by collecting quantitative
   data about the current state of the atmosphere and using scientific
   understanding of atmospheric processes to project how the atmosphere
   will evolve. The chaotic nature of the atmosphere, the massive
   computational power required to solve the equations that describe the
   atmosphere, and incomplete understanding of atmospheric processes mean
   that forecasts become less accurate as the range of the forecast
   increases.

History of weather forecasting

   For millennia people have tried to forecast the weather. In 650 BC, the
   Babylonians predicted the weather from cloud patterns. In about 340 BC,
   Aristotle described weather patterns in Meteorologica. Chinese weather
   prediction lore extends at least as far back as 300 BC.

   Ancient weather forecasting methods usually relied on observed patterns
   of events. For example, it might be observed that if the sunset was
   particularly red, the following day often brought fair weather. This
   experience accumulated over the generations to produce weather lore.
   However, not all of these predictions prove reliable and many of them
   have since been found not to stand up to rigorous statistical testing.

   It was not until the invention of the telegraph in 1837 that the modern
   age of weather forecasting began. Before this time, it had not been
   possible to transport information about the current state of the
   weather any faster than a steam train. The telegraph allowed reports of
   weather conditions from a wide area to be received almost
   instantaneously by the late 1840's. This allowed forecasts to be made
   by knowing what the weather conditions were like further upwind.

       Imagine a rotating sphere that is 12,800 kilometers (8000 miles) in
       diameter, has a bumpy surface, is surrounded by a 40-kilometer-deep
   mixture of different gases whose concentrations vary both spatially and
          over time, and is heated, along with its surrounding gases, by a
   nuclear reactor 150 million kilometers (93 million miles) away. Imagine
    also that this sphere is revolving around the nuclear reactor and that
      some locations are heated more during one part of the revolution and
     other locations are heated during another part of the revolution. And
   imagine that this mixture of gases continually receives inputs from the
         surface below, generally calmly but sometimes through violent and
        highly localized injections. Then, imagine that after watching the
    gaseous mixture, you are expected to predict its state at one location
             on the sphere one, two, or more days into the future. This is
      essentially the task encountered day by day by a weather forecaster.
    —On the difficulty of weather forecasting, Bob Ryan, Bulletin of the
                                    American Meteorological Society, 1982.

   The two men most credited with the birth of forecasting as a science
   were Francis Beaufort (remembered chiefly for the Beaufort scale) and
   his protégé Robert Fitzroy (developer of the Fitzroy barometer). Both
   were influential men in British Naval and Governmental circles, and
   though ridiculed in the press at the time, their work gained scientific
   credence, was accepted by the British Navy and formed the basis for all
   of today's weather forecasting knowledge.

   Great progress was made in the science of meteorology during the 20th
   century. The possibility of numerical weather prediction was proposed
   by Lewis Fry Richardson in 1922, though computers fast enough to
   complete the vast number of calculations required to produce a forecast
   before the event had occurred did not exist at that time. Practical use
   of numerical weather prediction began in 1955, spurred by the
   development of programmable electronic computers.

Modern day weather forecasting system

   Components of a modern weather forecasting system include:
     * Data collection
     * Data assimilation
     * Numerical weather prediction
     * Model output post-processing
     * Forecast presentation to end-user

Data collection

   Observations of atmospheric pressure, temperature, wind speed, wind
   direction, humidity, precipitation are made near the earth's surface by
   trained observers, automatic weather stations or buoys. The World
   Meteorological Organization acts to standardize the instrumentation,
   observing practices and timing of these observations worldwide.
   Stations either report hourly in METAR reports, or every six hours in
   SYNOP reports.

   Measurements of temperature, humidity and wind above the surface are
   found by launching radiosondes ( weather balloon). Data are usually
   obtained from near the surface to the middle of the stratosphere, about
   30,000 m (100,000 ft). In recent years, data transmitted from
   commercial airplanes through the AMDAR system has also been
   incorporated into upper air observation, primarily in numerical models.

   Increasingly, data from weather satellites are being used due to their
   (almost) global coverage. Although their visible light images are very
   useful for forecasters to see development of clouds, little of this
   information can be used by numerical weather prediction models. The
   infra-red (IR) data however can be used as it gives information on the
   temperature at the surface and cloud tops. Individual clouds can also
   be tracked from one time to the next to provide information on wind
   direction and strength at the clouds steering level. Polar orbiting
   satellites provide soundings of temperature and moisture throughout the
   depth of the atmosphere. Compared with similar data from radiosondes,
   the satellite data has the advantage that coverage is global, however
   the accuracy and resolution is not as good.

   Meteorological radar provide information on precipitation location and
   intensity. Additionally, if doppler radar are used then wind speed and
   direction can be determined.

Data assimilation

   During the data assimilation process, information gained from the
   observations is used in conjunction with a numerical model's most
   recent forecast for the time that observations were made (since this
   contains information from previous observations) to produce the
   meteorological analysis. This is the best estimate of the current state
   of the atmosphere. It is a three dimensional representation of the
   distribution of temperature, moisture and wind.

Numerical weather prediction (NWP)

   Numerical weather prediction models are computer simulations of the
   atmosphere. They take the analysis as the starting point and evolve the
   state of the atmosphere forward in time using understanding of physics
   and fluid dynamics. The complicated equations which govern how the
   state of a fluid changes with time require supercomputers to solve
   them. The output from the model provides the basis of the weather
   forecast.

Model output post processing

   The raw output is often modified before being presented as the
   forecast. This can be in the form of statistical techniques to remove
   known biases in the model, or of adjustment to take into account
   consensus among other numerical weather forecasts.

   In the past, the human forecaster used to be responsible for generating
   the entire weather forecast from the observations. However today, for
   forecasts beyond 24hrs human input is generally confined to
   post-processing of model data to add value to the forecast. Humans are
   required to interpret the model data into weather forecasts that are
   understandable to the end user. Additionally, humans can use knowledge
   of local effects which may be too small in size to be resolved by the
   model to add information to the forecast. Increasing accuracy of
   forecast models continues to decrease the need for post-processing and
   human input, mostly in areas with a low variation in terrain.

Presentation of weather forecasts

   The final stage in the forecasting process is perhaps the most
   important. Knowledge of what the end user needs from a weather forecast
   must be taken into account to present the information in a useful and
   understandable way.

Public information

   One of the main end users of a forecast is the general public.
   Thunderstorms can cause strong winds, dangerous lightning strikes
   leading to deaths and power outages, and widespread hail damage. Heavy
   snow or rain can bring transportation and commerce to a stand-still, as
   well as cause flooding in low-lying areas. Excessive heat or cold waves
   can kill or sicken those without adequate utilities. The National
   Weather Service provides forecasts and watches/warnings/advisories for
   all areas of the United States to protect life and property and
   maintain commercial interests. Environment Canada is responsible for
   dispensing similar forecasts and warnings to the public in Canada.
   Traditionally, television and radio weather presenters have been the
   main method of informing the public, however increasingly the internet
   is being used due to the vast amount of information that can be found.

Air traffic

   The aviation industry is especially sensitive to the weather. Fog
   and/or exceptionally low ceilings can prevent many aircraft landing and
   taking off. Similarly, turbulence and icing can be hazards whilst in
   flight. Thunderstorms are a problem for all aircraft, due to severe
   turbulence and icing, as well as large hail, strong winds, and
   lightning, all of which can cause fatal damage to an aircraft in
   flight. On a day to day basis airliners are routed to take advantage of
   the jet stream tailwind to improve fuel efficiency. Air crews are
   briefed prior to take off on the conditions to expect en route and at
   their destination.

Marine

   Commercial and recreational use of waterways can be limited
   significantly by weather in that wind direction and speed, wave
   periodicity and heights, tides, and precipitation can each influence
   the safety of marine transit. Consequently, a variety of codes have
   been established to efficiently transmit detailed marine weather
   forecasts to vessel pilots via radio, for example the MAFOR (marine
   forecast).

Utility companies

   Electricity and gas companies rely on weather forecasts to anticipate
   demand which can be strongly affected by the weather. In winter, severe
   cold weather can cause a surge in demand as people turn up their
   heating. Similarly, in summer a surge in demand can be linked with the
   increased use of air conditioning systems in hot weather. By
   anticipating a surge in demand, utility companies can purchase
   additional supplies of power or natural gas before the price increases,
   or in some circumstances, supplies are restricted.

Private sector

   Increasingly, private companies pay for weather forecasts tailored to
   their needs so that they can increase their profits or avoid large
   losses. For example, supermarket chains may change the stocks on their
   shelves in anticipation of different consumer spending habits in
   different weather conditions. State Departments of Transportation and
   private road maintenance companies also use their forecasts to
   demonstrate a 'best effort' in defending against lawsuits as a result
   of traffic accidents.

Military applications

   Similarly to the private sector, Military weather forecasters present
   weather conditions to the war fighter community. Equally provide
   pre-flight weather briefs and flight weather briefs from take off to
   terminal location. Including updates throughout the flight path. Also,
   military weather forecasters provide real time resource protection
   services for military installations, not covered by the National
   Weather Service. Three military branches have independent weather
   forecasting techniques tailored for their specific needs. For example,
   Naval Forecasters cover the waters and ship weather forecasts; Air
   Force Forecasters cover air operations in both wartime and peacetime
   operations and provide Army support; Coast Guard Forecasters provide
   ship forecasts for ice breakers and other various operations within
   their realm; And Marine Forecasters forecast for their troops and local
   aviation assets. There is a silent cooperative agreement between
   civilian forecasters and military forecasters, both working in unison
   for the improvement of weather forecasting in general.

Persistence forecasting

   The simplest method of forecasting the weather, persistence relies upon
   today's conditions to forecast the conditions tomorrow. This can be a
   valid way of forecasting the weather when it is steady state, such as
   during the summer season in the tropics. This method of forecasting
   strongly depends upon the presence of a stagnant weather pattern. It
   can be useful in both short range forecasts and long range forecasts.

Nowcasting

   The forecasting of the weather in the 0-12 hour timeframe is often
   referred to as nowcasting. It is in this range that the human
   forecaster still has an advantage over computer NWP models. In this
   time range it is possible to forecast smaller features such as
   individual shower clouds with reasonable accuracy, however these are
   often too small to be resolved by a computer model. A human given the
   latest radar, satellite and observational data will be able to make a
   better analysis of the small scale features present and so will be able
   to make a more accurate forecast for the following few hours.

   Below is a sample nowcast, issued by the National Weather Service in
   Mount Holly, New Jersey:
000
FPUS71 KPHI 240805
NOWPHI

SHORT TERM FORECAST
NATIONAL WEATHER SERVICE MOUNT HOLLY NJ
405 AM EDT FRI JUN 24 2005
DEZ002>004-MDZ015-019-020-NJZ013-014-020-022>027-241200-
ATLANTIC NJ-ATLANTIC COASTAL CAPE MAY NJ-CAPE MAY NJ-CAROLINE MD-
COASTAL ATLANTIC NJ-COASTAL OCEAN NJ-DELAWARE BEACHES DE-
EASTERN MONMOUTH NJ-INLAND SUSSEX DE-KENT DE-OCEAN NJ-
QUEEN ANNE'S MD-SOUTHEASTERN BURLINGTON NJ-TALBOT MD-
WESTERN MONMOUTH NJ-
INCLUDING THE CITIES OF...ATLANTIC CITY AND DOVER
405 AM EDT FRI JUN 24 2005

.NOW...

AREAS OF FOG AND LOW CLOUDS WILL BE OVER SOUTHERN DELAWARE AND
PORTIONS OF THE NORTHEASTERN MARYLAND SHORE EARLY THIS MORNING, AS
WELL AS ALONG THE NEW JERSEY COAST. THE PATCHY DENSE FOG MAY REDUCE
THE VISIBILITY TO A QUARTER MILE OR LESS AT TIMES. IF YOU WILL BE
DRIVING THIS MORNING, BE SURE TO LEAVE PLENTY OF ROOM BETWEEN YOUR
VEHICLE AND THE ONE AHEAD OF YOU. YOUR VISIBILITY COULD DROP QUICKLY
IF YOU DRIVE INTO A DENSE PATCH OF FOG. WATCH ESPECIALLY FOR
PEDESTRIANS. THE FOG SHOULD DISSIPATE AN HOUR OR TWO AFTER SUNRISE.
$$

Medium Range Forecasting

Analog technique

   A more complicated way of making a forecast, it requires remembering a
   previous weather event which is expected to be mimicked by an upcoming
   event. What makes it a difficult technique to use is that there is
   rarely a perfect analog for an event in the future. Some call this type
   of forecasting pattern recognition, which remains a useful method of
   observing rainfall over data voids such as oceans, as well as the
   forecasting of precipitation amounts and distribution in the future. A
   variation on this theme is used in Medium Range forecasting, which is
   known as teleconnections, when you use systems in other locations to
   help pin down the location of another system within the surrounding
   regime. One method of using teleconnections are by using ENSO-related
   phenomena.

Ensemble forecasting

   Although a forecast model will predict realistic looking weather
   features evolving realistically into the distant future, the errors in
   a forecast will inevitably grow with time due to the chaotic nature of
   the atmosphere. The detail that can be given in a forecast therefore
   decreases with time as these errors increase. There becomes a point
   when the errors are so large that the forecast is completely wrong and
   the forecast atmospheric state has no correlation with the actual state
   of the atmosphere.

   However, looking at a single forecast gives no indication of how likely
   that forecast is to be correct. Ensemble forecasting uses lots of
   forecasts produced to reflect the uncertainty in the initial state of
   the atmosphere (due to errors in the observations and insufficient
   sampling). The uncertainty in the forecast can then be assessed by the
   range of different forecasts produced. They have been shown to be
   better at detecting the possibility of extreme events at long range.

   Ensemble forecasts are increasingly being used for operational weather
   forecasting (for example at ECMWF, NCEP, and the Canadian forecasting
   centre).

Websites providing forecasts

Meteorological agencies

   These are academic or governmental meteorology organizations. Most
   provide at least a limited forecast for their area of interest on their
   website.
     * The World Meteorological Organization
     * NOAA weather page

                NOAA satellite images
                National Weather Service

     * Met Éireann (Ireland)
     * The Met Office of the UK
     * European Centre for Medium Range Weather Forecasting (ECMWF)
     * Environment Canada Weather Office
     * Australian Bureau of Meteorology
     * New Zealand MetService
     * Meteo Suisse (Swiss Weather Agency, in English
     * Finnish Meteorological Institute
     * Royal Meteorological Institute of Belgium
     * Korea Meteorological Administration
     * Hong Kong Observatory

Commercial organisations

   These are high profile commercial sites.
     * Freese-Notis Weather Site — Meteorologists with doppler radar
       images, data analysis, forensics and a wide variety industry
       specific US and international weather forecasts, agriculture and
       energy being strong areas of performance plus supplies advanced web
       site content
     * Weather Underground — Provides a wide variety of US and
       international weather information
     * Unisys Weather Site — Provides meteorological data, analysis, and
       forecasts for the US along with ancillary information for aircraft
       or flight systems related information domestically and overseas
     * Verizon's Weather Forecasts - Provides weather briefs & extended
       outlook for the U.S. Also powers weather on 20,000 other websites
     * MeteoGroup Europe's largest independent weather forecasting
       company. Provides weather for Transport, Energy, Agriculture,
       Maritime, Media, Leisure, Health & Environment, Insurance and Water
       Management.
     * Accuweather — Weather forecasts and weather-related news (US and
       international) known for television grade services
     * The Weather Channel — Weather forecasts and weather-related news
       mainly for the US, but also has world forecasts.
     * TheWeatherOutlook — Weather forecasts for the UK.
     * BBC Weather — Weather forecasts for the UK and climate overviews
       for the whole world. Links to contemporary climate change news for
       astute observers of the controversial topic
     * Wetterzentrale - A German language site displaying a large range of
       high quality weather charts.
     * The Weather Network - Canadian weather and forecasts.
     * Weatherzone - Provides a wide variety of Australian weather
       information.
     * Weather.com.au - Australian weather forecasts.
     * Weatheronline - Meteorological data, analysis, and forecasts for
       the UK and the whole world. Provides radar images, climate data and
       weather-related news.
     * MSN Weather (Data provided by FORECA) - Worldwide Weather Forecasts
       (Hourly, Extended Ten-day and Historical Averages)

   Retrieved from " http://en.wikipedia.org/wiki/Weather_forecasting"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
