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.