Weather forecasts are made by collecting quantitative data about the current state of the atmosphere at a given place and using meteorology to project how the atmosphere will change. Once calculated by hand based mainly upon changes in barometric pressure, current weather conditions, and sky condition or cloud cover, weather forecasting now relies on computer-based models that take many atmospheric factors into account.
Human input is still required to pick the best possible forecast model to base the forecast upon, which involves pattern recognition skills, reconnections, knowledge of model performance, and knowledge of model biases. The inaccuracy of forecasting is due to the chaotic nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, the error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes.
The use of ensembles and model consensus help narrow the error and pick the most likely outcome. There is a vast variety of end uses to weather forecasts.
Weather warnings are important forecasts because they are used to protecting life and property. Forecasts based on temperature and precipitation are important to agriculture, and therefore to traders within commodity markets.
Temperature forecasts are used by utility companies to estimate demand over coming days. On an everyday basis, many use weather forecasts to determine what to wear on a given day.
Since outdoor activities are severely curtailed by heavy rain, snow and wind chill, forecasts can be used to plan activities around these events, and to plan ahead and survive them. Weather forecasting is a part of the economy, for example, in 2009, the US spent approximately $5.1 billion on weather forecasting, producing benefits estimated at six times as much.
In 904 CE, In Wahshiyya's Batman Agriculture, translated into Arabic from an earlier Aramaic work, discussed the weather forecasting of atmospheric changes and signs from the planetary astral alterations; signs of rain based on observation of the lunar phases ; and weather forecasts based on the movement of winds. 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. Of these predictions prove reliable, and many of them have since been found not to stand up to rigorous statistical testing.
The Royal Charter sank in an 1859 storm, stimulating the establishment of modern weather forecasting. It was not until the invention of the electric telegraph in 1835 that the modern age of weather forecasting began. Before that, the fastest that distant weather reports could travel was around 160 kilometers per day (100 mi/d), but was more typically 60–120 kilometers per day (40–75 mi/day) (whether by land or by sea).
By the late 1840s, the telegraph allowed reports of weather conditions from a wide area to be received almost instantaneously, allowing forecasts to be made from knowledge of weather conditions further upwind. The two men credited with the birth of forecasting as a science were an officer of the Royal Navy Francis Beaufort and his protégéRobert Fitzroy.
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. Beaufort developed the Wind Force Scale and Weather Notation coding, which he was to use in his journals for the remainder of his life.
He also promoted the development of reliable tide tables around British shores, and with his friend William Whew ell, expanded weather record-keeping at 200 British Coast guard stations. Robert Fitzroy was appointed in 1854 as chief of a new department within the Board of Trade to deal with the collection of weather data at sea as a service to mariners.
All ship captains were tasked with collating data on the weather and computing it, with the use of tested instruments that were loaned for this purpose. Weather map of Europe, December 10, 1887. A storm in 1859 that caused the loss of the Royal Charter inspired Fitzroy to develop charts to allow predictions to be made, which he called “forecasting the weather ", thus coining the term weather forecast “.
Fifteen land stations were established to use the telegraph to transmit to him daily reports of weather at set times leading to the first gale warning service. His warning service for shipping was initiated in February 1861, with the use of telegraph communications.
The first daily weather forecasts were published in The Times in 1861. In the following year a system was introduced of hoisting storm warning cones at the principal ports when a gale was expected.
The Weather Book” which Fitzroy published in 1863 was far in advance of the scientific opinion of the time. As the electric telegraph network expanded, allowing for the more rapid dissemination of warnings, a national observational network was developed, which could then be used to provide synoptic analyses.
It was not until the 20th century that advances in the understanding of atmospheric physics led to the foundation of modern numerical weather prediction. In 1922, English scientist Lewis Fry Richardson published Weather Prediction By Numerical Process”, after finding notes and derivations he worked on as an ambulance driver in World War I.
He described therein how small terms in the prognostic fluid dynamics equations governing atmospheric flow could be neglected, and a finite difference scheme in time and space could be devised, to allow numerical prediction solutions to be found. Richardson envisioned a large auditorium of thousands of people performing the calculations and passing them to others.
However, the sheer number of calculations required was too large to be completed without the use of computers, and the size of the grid and time steps led to unrealistic results in deepening systems. The first computerized weather forecast was performed by a team composed of American meteorologists July Carney, Philip Thompson, Larry Gates, and Norwegian meteorologist Reign Fjørtoft, applied mathematician John von Neumann, and ENAC programmer Clara Dan von Neumann.
Practical use of numerical weather prediction began in 1955, spurred by the development of programmable electronic computers. In 1911, the Met Office began issuing the first marine weather forecasts via radio transmission.
These included gale and storm warnings for areas around Great Britain. In the United States, the first public radio forecasts were made in 1925 by Edward B.
Hideout, on Week, the Edison Electric Illuminating station in Boston. Sites launch radiosondes, which rise through the depth of the troposphere and well into the stratosphere.
Additionally, if a pulse Doppler weather radar is used then wind speed and direction can be determined. Modern weather predictions aid in timely evacuations and potentially save lives and prevent property damageCommerce provides pilot reports along aircraft routes, and ship reports along shipping routes.
Research flights using reconnaissance aircraft fly in and around weather systems of interest such as tropical cyclones. Reconnaissance aircraft are also flown over the open oceans during the cold season into systems that cause significant uncertainty in forecast guidance, or are expected to be of high impact 3–7 days into the future over the downstream continent.
The irregularly spaced observations are processed by data assimilation and objective analysis methods, which perform quality control and obtain values at locations usable by the model's mathematical algorithms (usually an evenly spaced grid). The data are then used in the model as the starting point for a forecast.
These equations are initialized from the analysis data and rates of change are determined. The rates of change predict the state of the atmosphere a short time into the future.
The equations are then applied to this new atmospheric state to find new rates of change, and these new rates of change predict the atmosphere at a yet further time into the future. The length of the time step chosen within the model is related to the distance between the points on the computational grid, and is chosen to maintain numerical stability.
Time steps for global models are on the order of tens of minutes, while time steps for regional models are between one and four minutes. The global models are run at varying times into the future.
The Met Office's Unified Model is run six days into the future, the European Center for Medium-Range Weather Forecasts model is run out to 10 days into the future, while the Global Forecast System model run by the Environmental Modeling Center is run 16 days into the future. The visual output produced by a model solution is known as a prognostic chart, or pro.
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. This guidance is presented in coded numerical form, and can be obtained for nearly all National Weather Service reporting stations in the United States.
As proposed by Edward Lorenz in 1963, long range forecasts, those made at a range of two weeks or more, are impossible to definitively predict the state of the atmosphere, owing to the chaotic nature of the fluid dynamics equations involved. In numerical models, tiny errors in initial values double roughly every five days for variables such as temperature and wind velocity.
Additional transport equations for pollutants and other aerosols are included in some primitive-equation message models as well. Therefore, numerical methods obtain approximate solutions.
This can be a valid way of forecasting the weather when it is in a steady state, such as during summer in the tropics. This method of forecasting strongly depends upon the presence of a stagnant weather pattern.
Therefore, when in a fluctuating weather pattern, this method of forecasting becomes inaccurate. Rapid pressure rises are associated with improving weather conditions, such as clearing skies.
Mares tail shows moisture at high altitude, signalling the later arrival of wet weather. Along with pressure tendency, the condition of the sky is one of the more important parameters used to forecast weather in mountainous areas. High thin cirrostratus clouds can create halos around the sun or moon, which indicates an approach of a warm front and its associated rain.
Morning fog portends fair conditions, as rainy conditions are preceded by wind or clouds that prevent fog formation. Cloud-free skies are indicative of fair weather for the near future.
The forecasting of the weather within the next six hours is often referred to as now casting. In this time range it is possible to forecast smaller features such as individual showers and thunderstorms with reasonable accuracy, as well as other features 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. However, there are now expert systems using those data and message numerical model to make better extrapolation, including evolution of those features in time.
In the past, the human forecaster was responsible for generating the entire weather forecast based upon available observations. Humans are required to interpret the model data into weather forecasts that are understandable to the end user.
Humans can use knowledge of local effects that may be too small to be resolved by the model to add information to the forecast. While increasing accuracy of forecast models implies that humans may no longer be needed in the forecast process at some point in the future, there is currently still a need for human intervention.
The analog technique is a complex way of making a forecast, requiring the forecaster to remember a previous weather event that 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.
It 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 similar technique is used in medium range forecasting, which is known as reconnections, when systems in other locations are used to help pin down the location of another system within the surrounding regime.
An example of reconnections are by using El Niño-Southern Oscillation (ENSO) related phenomena. An example of a two-day weather forecast in the visual style that an American newspaper might use.
Most end users of forecasts are members of the public. Thunderstorms can create strong winds and dangerous lightning strikes that can lead to deaths, 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 sicken or kill those with inadequate utilities, and droughts can impact water usage and destroy vegetation.
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. 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.
Examples include the National Oceanic and Atmospheric Administration's National Weather Service (News) and Environment Canada's Meteorological Service (MSC). Traditionally, newspaper, television, and radio have been the primary outlets for presenting weather forecast information to the public.
Increasingly, 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.
Other forms of these advisories include winter weather, high wind, flood, tropical cyclone, and fog. Severe weather advisories and alerts are broadcast through the media, including radio, using emergency systems as the Emergency Alert System, which break into regular programming.
There are a number of sectors with their own specific needs for weather forecasts and specialist services are provided to these users. Fog or exceptionally low ceilings can prevent many aircraft from landing and taking off.
Thunderstorms are a problem for all aircraft because of severe turbulence due to their updrafts and outflow boundaries, icing due to the heavy precipitation, 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 takeoff on the conditions to expect en route and at their destination. Additionally, airports often change which runway is being used to take advantage of a headwind.
This reduces the distance required for takeoff, and eliminates potential crosswinds. Commercial and recreational use of waterways can be limited significantly by wind direction and speed, wave periodicity and heights, tides, and precipitation.
Consequently, a variety of codes have been established to efficiently transmit detailed marine weather forecasts to vessel pilots via radio, for example the Major (marine forecast). Typical weather forecasts can be received at sea through the use of RTT, Nave and Radio fax.
Farmers rely on weather forecasts to decide what work to do on any particular day. Prolonged periods of dryness can ruin cotton, wheat, and corn crops.
While corn crops can be ruined by drought, their dried remains can be used as cattle feed substitute in the form of silage. Frosts and freezes play havoc with crops both during the spring and fall.
Orange groves can suffer significant damage during frosts and freezes, regardless of their timing. Weather forecasting of wind, precipitations and humidity is essential for preventing and controlling wildfires.
Different indices, like the Forest fire weather index and the Haines Index, have been developed to predict the areas more at risk to experience fire from natural or human causes. Conditions for the development of harmful insects can be predicted by forecasting the evolution of weather, too.
An air handling unit is used for the heating and cooling of air in a central location (click on image for legend). 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 through the use of brownouts and blackouts.
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.
Weather forecasts can be used to invest in the commodity market, such as futures in oranges, corn, soybeans, and oil. United Kingdom Armed Forces The UK Royal Navy, working with the UK Met Office, has its own specialist branch of weather observers and forecasters, as part of the Hydrographic and Meteorological (HM) specialization, who monitor and forecast operational conditions across the globe, to provide accurate and timely weather and oceanographic information to submarines, ships and Fleet Air Arm aircraft.
Royal Air Force A mobile unit in the RAF, working with the UK Met Office, forecasts the weather for regions in which British, allied servicemen and women are deployed. A group based at Camp Bastion provides forecasts for the British Armed Forces in Afghanistan.
United States Armed ForcesUS Navy Emblem of JTC Joint Typhoon Warning CenterSimilar to the private sector, military weather forecasters present weather conditions to the war fighter community. Military weather forecasters provide preflight and in-flight weather briefs to pilots and provide real time resource protection services for military installations.
The United States Navy provides a special service to both themselves and the rest of the federal government by issuing forecasts for tropical cyclones across the Pacific and Indian Oceans through their Joint Typhoon Warning Center. Air Force forecasters cover air operations in both wartime and peacetime operations and provide Army support; United States Coast Guard marine science technicians provide ship forecasts for icebreakers 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 enlisted meteorology technical training at Kessler Air Force Base. Military and civilian forecasters actively cooperate in analyzing, creating and critiquing weather forecast products.
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