Forecasting  

Powerful new tools available to meteorologists enable them to produce more and more accurate predictions. Reliable forecasts can now be made for24 hours  ahea, and work is underway to refine the long range forecasts that look up to a month ahead.
    All this effort is not to make sure that you picnic. It is a matter of life,death, or large sums of money for all sorts of people. These include not only fishermen and sea captains wondering whether or not to put to sea, but also airline pilots and farmers who want a fine, settled period of weather to sow and harvest their crops. Even ice cream makers vary the number of cornets they make according to the weather forecast.
    Berofe meteorologists can make a forecast, they need to have an extremely accurate picture of what the weather is doing over as large an area as possible. So weather observations are collected from: Synoptic observatioons
All levels of the atmosphere must be observed because a shift in the upper atmosphere winds of 2,000 km can trigger a radical change in the weather. That upper atmosphere change might, in turn, be a reaction to a weather event in the opposite hemisphere two or three days previously. So weather information must be collected all round the world to give a complete picture.
    The accuracy and type of observations made are crucial, as well the time when they are made. In order to get a snapshot of what is going on at any one instant, observations are made synoptically. This means that they are taken at the same time worldwide, with the main observations coming at 00.00, 06.00, 12.00 and 18.00 Coordinated Universal Time (UTC) or, as it is oterwise known, Greenwich Mean Time (GMT). The data are then sent via communications satellite, land line or radio to one or all of the world's meteorological centres.
    At a ground station, a trained observer measures wind speed anddirection, as well as visibility, atmospheric pressure, air temperature and dew point temperature. The observer also reports cloud types and amounts, and general weather conditions. He or she also measures the amount of rainfall, hours of sunshine and temperature maximums and minimum on a daily basis.
    Despite their name, ground stations can also be on water - around the world there are thousands of stations all making identical weather reports. For example, in the British Isles alone, there about 65 weather stations that report back every hour of the day and night to the British Meteorological Office Headquarters at Bracknell, Berkshire. Another 35 or so weather stations report back every hour during daylight.
 
Weather ships
 Weather forecasters used to rely on weather ships for mid-ocean observations, which are very importantbecause this is where many weather systems are born. These are now being replaced by automatic buoys. The bouys transmit weather data back to a data collection station on land. There is also a moving network of about 7,000 merchant ships that carry meteorological apparatus and report back. Of these ships, about 2,500 send their reports to Bracknell.
    Usefull as all the observations taken at surface level are, they must be complemented by measurements of weather conditions higher up in the atmosphere. This is where ballons (called radiosondes) come in. Around the world there is a network of stations where helium or hydrogen-filled instrument carrying ballons are released every six or 12 hours.
    The balloons rise 15 km in the air before they burst. The instruments measure pressure, temperature and humidity at frequent intervals during the ascent, and transmit the data to a ground station or satellite, which then relays it to Earth.
    The wind speed and directions are determined by radar traking of the balloon as it climbs. From launch to maximum height takes about an hour. There are not as many balloon-launching stations as the meteorologists would like - they are expensive to maintain - especially in the oceans.
    An alternative scheme called the Automated Shipboard Aerological System (ASAP) is already being employed. Merchants ship launch balloon's movements after launch and, therefore, the wind direction and speed are tracked using a satellite navigation system.

Aircraft
Jet airliners fly at a height of 10 - 13 km, which is one of the most important levels of the atmosphere.This is where the jet streams blow from west to east at speeds up to 300 km/h. Changes in a jet stream trigger major weather changes. As an example, it was a very fast jet stream that caused the 1987 hurricane-force winds in southern Britain and western France. To observe these changes, many aircraft are now being fitted with automatic instruments packages that collect and report back temperature, wind speed and direction, wind turbulance and the plane's positions and height.
    The data are transmitted automatically every hour to a weather centre via communications satellites. The system, called Aircraft to Satellite Data Relay (ASDAR), is providing weather forecasters with much valuable information about the upper atmosphere. The system is also to make readings as a plane climbs or descends, filling out the picture sent back by radio-sounds.
 
Satellites
Satellites have made a dramatic impact on weather forecasting because they can send back large enough pictures of cloud formations to to show complete weather systems. A sequence of pictures taken at different times enable forecasters to see how weather systems are moving and moving and developing.
    Currently, two sorts of meteorological satellites, such as Meteosat series, cover about a fifth of the world's surface on a permanent basis. Polar orbiting satellites, such as the Tiros series, send back data from narrow strips of land from north pole to south pole. Satellites carry radiometers to measure electromagnetic radiation. This radiation comprises visible light, infra-red (heat) radiation and microwave radiation. They also use infra-red techniques to make assessments of temperature and humidity at various levels in the atmosphere.
    Satellites are particularly important to weather forecasters because they provide information from areas where balloon coverage is poor. The information gathered by satellite is relayed back ground receiving stations for analysis.
 
Accurate forecasts
Some countries maintain a network of weather radar stations. These give an accurate - and instantaneous - picture of how belts of rain (associated with war and cold fronts) are moving across country. With this information, forecasters can compile accurate short-range forecasts. Such forecasts have a very practical purpose. they can, for example influence whether or not a local council will salt the roads to prevent ice forming.
    A worldwide network of meteorological communications centres are linked by very fast telecommunications circuits so they can exchange these vast amount of coded data to process that it can only be done by the most powerful computers.
    Weather forecasting systems are based on computer programs that are mathematical 'model' of the way the weather works. These use the synoptic readings taken at 00.00 UTC and 12.00 UTC to determine the current situation. The model is then usec to forecast the weather situation as it will be minutes, hours and days ahead using equations developed by expert meteorologists.

Computer charts
One mathematical model running on the COSMOS computing systems works out the weather of all the points on an imaginary grid, with point spaced about 150 km apart at ground level. It also works out the weather for 1 5 heights in the atmosphere above each grid point using radiosonde and aircraft data.
    Meteorologists add any other information they think appropriate from other sources such as satellite observations and aircraft readings. The information is then presented on charts drawn by the computer. These charts show the atmospheric pressure at the different levels of the atmosphere, and are worked out up to six days ahead.
    At the end of the computer run, charts and forecasts are examined by a human meteorologist who interprets the information in the light of experience. He or she also takes local geographical features such as mountain ranges and lying land into account because these can make the difference between low cloud and rain.
    After all this might number crunching and analysis, the forecasts and charts are made available to TV and radio stations, airports, public services, police, local authorities, motoring organizations and anyone else who needs them for commercial purposes and is therefore prepared to pay for them. For some users, specialized forecasts are prepared and tailored to their needs.

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