Weather forecasting
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Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather for as long as there are records.[citation needed] 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 difference in time between the present moment and the time for which the forecast is being made (the range of the forecast) increases.
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[edit] History
For millennia people have tried to forecast the weather. In 650 BC, the Babylonians predicted the weather from cloud patterns as well as astrology.[1] In about 340 BC, Aristotle described weather patterns in Meteorologica. Chinese weather prediction lore extends at least as far back as 300 BC.[2]
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.[3]
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 1840s.[4] This allowed forecasts to be made by knowing what the weather conditions were like further upwind. 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 Royal Navy, and formed the basis for all of today's weather forecasting knowledge.[5]
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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 did not exist to complete the vast number of calculations required to produce a forecast before the event had occurred. Practical use of numerical weather prediction began in 1955,[7] spurred by the development of programmable electronic computers.
[edit] Modern day techniques
[edit] Data collection
Surface weather 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 meters (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 because of 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 infrared (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 weather radar is used then wind speed and direction can be determined.[8]
[edit] 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.
[edit] Numerical weather prediction
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.[9]
[edit] 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 24 hours 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.
[edit] 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.
[edit] Public information
Most end users of forecasts are members of the general public, who have interest in such information for several reasons. Thunderstorms can create strong winds and dangerous lightning strikes that can lead to deaths, power outages,[10] 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 sicken or kill those with inadequate utilities, and droughts can impact water usage and destroy vegetation. Many use forecasts simply to schedule outdoor recreation or to assist in determining daily attire.
Several countries employ government agencies to provide forecasts and watches/warnings/advisories to the public in order to protect life and property and maintain commercial interests. Examples include the United States' National Weather Service (NWS)[11] and Canada's Meteorological Service of Canada (MSC).[12] Traditionally, newspaper, television, and radio have been the primary outlets for presenting weather forecast information to the public. Increasingly, however, the internet is being used due to the vast amount of specific information that can be found. In all cases, these outlets update their forecasts on a regular basis, in accordance with their respective publication schedules.
[edit] Air traffic
The aviation industry is especially sensitive to the weather. Fog and/or exceptionally low ceilings can prevent many aircraft from landing and taking off. Similarly, turbulence and icing can be hazards whilst in flight. Thunderstorms are a problem for all aircraft because of severe turbulence and icing, as well as large hail, strong winds, and lightning, all of which can cause severe 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. Aircrews are briefed prior to take off on the conditions to expect en route and at their destination. Additionally, runway orientation is often changed to take advantage of a headwind to reduce the distance required for takeoff.
[edit] 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).
[edit] Agriculture
Farmers rely on weather forecasts to decide what work to do on any particular day. For example, drying hay is only feasible in dry weather.
[edit] 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.
[edit] 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.
[edit] Military applications
Similarly to the private sector, military weather forecasters present weather conditions to the war fighter community. They 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.
[edit] United States
Within the United States, four branches of the armed forces have independent weather forecasting techniques tailored for their specific needs: 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; United States Coast Guard forecasters provide ship forecasts for ice breakers and other various operations within their realm; and Marine forecasters provide support for ground- and air-based United States Marine Corps operations. All four military branches take their initial meteorology training at Keesler Air Force Base. Military and civilian forecasters actively cooperate in analyzing, creating and critiquing weather forecast products.
[edit] 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.[13]
[edit] 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.[14]
[edit] Severe weather alerts and advisories
A major part of modern weather forecasting is the severe weather alerts and advisories which the National Weather Services issues in the case that severe or hazardous weather is expected. Some of the most commonly known of severe weather advisories are the severe thunderstorm and tornado warning, as well as the severe weather or the tornado watch. Other forms of these advisories include winter weather, high wind, flood, tropical storm, hurricane, and fog. Severe weather advisories and alerts are broadcast through the media, including radio.
[edit] Medium range forecasting
[edit] 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.[15] Some call this type of forecasting pattern recognition, which remains a useful method of observing rainfall over data voids such as oceans,[16] 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.[17] One method of using teleconnections are by using ENSO-related phenomena.[18]
[edit] 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 and the inexactness of the initial observation set which was used to begin the model run. 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 center). [19]
[edit] See also
- Citizens weather observer program
- Crometeo
- Downscaling
- Environment Canada
- Forecasting
- geodesic grid
- National Collegiate Weather Forecasting Competition
- National Weather Service
- Trend estimation
- Tropical cyclone forecasting
- Weather control
- Weather wars
- WxChallenge
[edit] References
- ^ Mistic House. Astrology Lessons, History, Predition, Skeptics, and Astrology Compatibility. Retrieved on 2008-01-12.
- ^ University of California Museum of Paleontology. Aristotle (384-322 B.C.E.). Retrieved on 2008-01-12.
- ^ Jerry Wilson. Skywatch Signs of the Weather. Retrieved on 2007-04-15.
- ^ Encyclopedia Brittanica. Telegraph. Retrieved on 2007-05-05.
- ^ Eric D. Craft. An Economic History of Weather Forecasting. Retrieved on 2007-04-15.
- ^ Bob Ryan. On the difficulty of weather forecasting. Bulletin of the American Meteorological Society, 1982.
- ^ Paul N. Edwards. Atmospheric General Circulation Modeling. Retrieved on 2007-02-16.
- ^ University of Washington. An improving forecast. Retrieved on 2007-04-15.
- ^ United Kingdom Met Office. Numerical weather prediction. Retrieved on 2007-02-16.
- ^ University of Illinois at Urbana-Champaign. Lightning. Retrieved on 2007-02-16.
- ^ National Weather Service. About NOAA's National Weather Service. Retrieved on 2007-02-16.
- ^ Environment Canada. Main website. Retrieved on 2007-02-16.
- ^ University of Illinois at Urbana-Champaign. Persistence Forecasting: Today equals Tomorrow. Retrieved on 2007-02-16.
- ^ E-notes.com. Weather and Climate | What Is Nowcasting? Retrieved on 2007-02-16.
- ^ Other Forecasting Methods: climatology, analogue and numerical weather prediction. Retrieved on 2006-02-16.
- ^ Kenneth C. Allen. Pattern Recognition Techniques Applied to the NASA-ACTS Order-Wire Problem. Retrieved on 2007-02-16.
- ^ Weather Associates, Inc. The Role of Teleconnections & Ensemble Forecasting in Extended- to Medium-Range Forecasting. Retrieved on 2007-02-16.
- ^ Thinkquest.org. Teleconnections: Linking El Niño with Other Places. Retrieved on 2007-02-16.
- ^ Klaus Weickmann, Jeff Whitaker, Andres Roubicek and Catherine Smith. The Use of Ensemble Forecasts to Produce Improved Medium Range (3-15 days) Weather Forecasts. Retrieved on 2007-02-16.
[edit] External links
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[edit] Websites providing forecasts
[edit] 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
- yr.no - forecasts for 7 million places worldwide, with data from the Norwegian Meteorological institute and ECWMF
- 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
- French National Meteorological Service
- Royal Meteorological Institute of Belgium
- Korea Meteorological Administration
- Hong Kong Observatory
- Italian Air Force (Aeronautica Militare) Meteorological Service
- South African Weather Service
[edit] Weather data feeds external links
- UK and 4300 cites worldwide RSS pay to access
- accuweather.com API not publically exposed, must contact to have access
- weatherbug.com Terms of service may limit usage
- yahoo gets data from weather.com Terms of service may limit usage
- weather365.net, weathercarts and forecasts worldwide, partly public, terms of service may limit usage
[edit] Other external links
- Economic history and impact of weather forecasting from EH.NET
- About Live Weather Report Do yourself Weather Forecast
- Ten Day Weather Forecasts Ten Day Weather Forecasts
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