Weather forecasting

2008/9 Schools Wikipedia Selection. Related subjects: Climate and the Weather; Storms

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)