Met Office College - Course Notes
Weather forecasting
Contents
1. Introduction
2. Observations
2.1 Surface observations
2.2 Upper-air observations
2.3 Radars
2.4 Satellites
3. Analysis
4. Forecast
4.1 The computer model
4.2 Role of the forecaster
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Principal, Met Office College
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1. Introduction
There are three stages in the process of forecasting the weather:

Make observations

Perform an analysis

Produce a forecast
Knowledge of what the weather is at present is essential before a
forecast can be made of what it is likely to be in the future. Therefore,
regular, reliable and accurate measurements are required. These have to
be communicated rapidly around the world using a telecommunications
system dedicated to weather information.
The observations are used to “analyse” the weather patterns at a
particular time.
Once an analysis has been carried out, a forecast of the weather pattern
can be made. Usually the forecast is made with the aid of a computer.
The forecaster uses the output from the computer to decide what the
weather will be and to provide services to a wide range of customers
(Fig 1).
2. Observations
Data sources are many and various. They include ships, aircraft, oil rigs,
buoys and balloons, as well as manned land stations around the world.
Automation is often used to assist or replace the human observer and
can provide information from inhospitable and remote areas.
Information from remote-sensing equipment, both ground-based and
satellite-borne, increasingly supplements and complements the
conventional systems.
2.1 Surface observations
Traditionally meteorologists have relied upon observations taken near
the earth’s surface using instruments (e.g. barometers, thermometers
and rain gauges) and visual observations (e.g. cloud and weather type).
These surface observations are made at approved sites over the land and
from ships at sea.
Standard types of instruments are used, and observations are usually
made at least every three hours (hourly in many cases). Over land in the
UK there is a network of key observing stations, which are needed to
define the broad scale weather patterns. These are staffed by
professional meteorologists who make observations every hour.
However, local small-scale detail is also needed and this demands
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Weather Forecasting
observations from a denser network. Some of these additional sites are
staffed by Met Office observers, but many are run by auxiliary observers
(such as coastguards). Also there are an increasing number of fully
automated sites. In all there are several hundred observing sites in the
UK providing surface observations.
For weather observations at sea, the Met Office is indebted to the crews
of hundreds of vessels and rigs of the UK Voluntary Observing Fleet.
This is part of a larger scheme involving thousands of ships world-wide.
To fill in some of the gaps, moored buoys are being positioned to the
west of Ireland; other buoys drift in the Atlantic.
2.2 Upper-air observations
Upper-air information is gathered using balloon-borne instruments
called radio-sondes, which provide information about the pressure,
temperature, humidity and wind up through the atmosphere to a height
of at least 20 km. Radio-sondes are released twice a day at the same time
(midday and midnight GMT) all over the world.
The UK has a network of radio-sonde stations around the coast, from
Camborne in Cornwall to Lerwick in the Shetland Islands. In addition,
some forecast offices have radio-sonde equipment to allow
measurements to be made when required due to local conditions, such
as when it is foggy.
At sea, several merchant ships are equipped with automatic systems,
which release radio-sondes at regular intervals during each voyage.
Aircraft reports of wind and temperature along their routes provide
valuable additional data of upper-air conditions.
2.3 Radars
A network of weather radars can be used to display the distribution and
intensity of rainfall over a region. The UK network covers all of the
British Isles, and a larger network shows the rainfall over much of
western Europe. These radars also provide rainfall information for river
management and flood warnings. Radar pictures are often shown in
television weather broadcasts.
2.4 Satellites
Since the first meteorological satellite was placed in orbit in 1960,
satellites have become an essential tool for weather forecasters. The
satellites used by meteorologists fall into two categories.

Polar-orbiting satellites pass around the earth from pole to pole at a
height of about 870 km. It takes approximately 1 hour and 42
minutes for the satellite to complete its orbit, by which time the earth
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has “moved on” by about 25 degrees. Consequently each “pass”
provides information about a different strip of the atmosphere.

Geostationary satellites remain stationary with respect to the earth.
This is achieved by having an orbit above the equator at a height of
about
36,000 km. At this height it takes the satellite exactly 24 hours to
complete one orbit, so that it always views the same part of the
globe. Geostationary satellites are positioned at intervals around the
equator, together providing data from all around the globe. The
European satellite, Meteosat, is positioned on the Greenwich
Meridian (0° longitude) directly above the Gulf of Guinea.
Polar-orbiting satellites provide pictures of clouds and information
about the temperature through the atmosphere. Geostationary satellites
Aircraft
Satellites
Radiosondes
Later
data
Land
stations
Satellite
imagery
Ships
Radar
imagery
Drifting
buoys
Human
intervention
Global forecast
services
National and international
forecast services up to seven
days ahead
Aviation
Shipping
Industry
Commerce
Media
Agriculture
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Weather Forecasting
produce time sequences of cloud images. From these, winds and storm
tracks can be inferred by looking at how clouds move between pictures.
Figure 1. The flow of information from observations to forecasts.
3. Analysis
Figure 2. Forecaster’s analysis of mean-sealevel pressure (isobars) and weather fronts for
00:00 hrs on 25 October.
A Global Telecommunications System (GTS)
has been set up to transfer weather
observations (and forecasts) around the
world. This international circuit comprises a
sequence of high-speed computer-tocomputer links, using communication
satellites and land lines.
The telecommunications centre in the Met
Office Headquarters has the role of passing
data between Washington (USA) and
continental Europe via Paris (France) and
Offenbach (Germany). It also collects
observations from the UK and distributes
them world-wide via the GTS. A complete set
of observations from the UK is available just a
few minutes past the hour of observation.
Figure 3. 24-hour forecast of isobars and rain
from the computer model for 00:00 hrs on
26 October. The round symbols indicate
continuous rain and the triangular symbols
indicate showers.
Figure 4. 24-hour forecast of isobars and weather
fronts based on the computer model forecast for
00:00 hrs on 26 October.
Observations taken from the GTS are stored on
computer and are then analysed in two ways:

The observations at a specific time are
plotted on a chart. The chart is then
analysed by a forecaster (with the aid of
satellite and radar information) in order
to understand what is going on in the
atmosphere. For example, lines of
constant pressure (isobars) are drawn to
help the forecaster identify important
weather features such as depressions,
anticyclones and fronts (see Figure 2).
The forecaster can also use this type of
chart to make short-term local forecasts.

The observations are used to define the
starting conditions for a computer
forecast (see Figures 3 and 4), which can
go as far as about a week ahead.
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There are two difficulties in using
observations. Firstly, the observations may
be inaccurate (e.g. instruments may not be
functioning properly or there may be
transmission errors) and secondly there are
some regions (e.g. over the oceans) where
there are few observations. The use of
computers allows forecasters to minimise
these difficulties, and make maximum
effective use of whatever data are available.
4. Forecast
Using computers to forecast the weather has played a key role in
improving the accuracy and detail of weather forecasts, and in
lengthening the period for which useful guidance can be given. The
calculations involved are both numerous and complex and they must be
performed quickly so that forecasts are available in good time.
Consequently some of the most powerful computers in the world are
used.
4.1 The computer model
Weather forecasts are based on the solution of a set of mathematical
equations, which describe the physical processes in the atmosphere. To
solve these complex equations it is first necessary to divide the
atmosphere up into little boxes, each with a grid point at the centre. The
properties of the atmosphere are then represented by what is happening
at each of the points. The array of grid points, the system of equations
and the method of solving the equations is referred to as the model.
Forecast models typically have 20 to 50 levels in the vertical, with a
horizontal grid point spacing of 10 to 100 km. To cover the whole world
at this resolution needs a very large number of grid points, hence the
need for massive computing power!
The observations taken at a particular time can be used to compute the
grid point values of pressure, temperature, humidity and wind. This set
of values (the computer analysis) then represents the atmosphere at the
start of the forecast.
Using the mathematical equations, a forecast can be made of the
pressure, temperature, humidity and wind a few minutes later. Once all
the new values have been calculated, the process starts again with
another short-period forecast being made. By repeating this procedure
many times over, a forecast out to several days can be built up. The
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Radiation from
the sun
Radiation from
the atmosphere
Evaporation and heat exchange
Formation of
rain and snow
Mountain
effects
Formation of
clouds
Radiation from
the earth
Sea
Friction
Land
“super-computer” at the Met Office only takes a few minutes to produce
a six-day global forecast.
As well as giving forecasts of pressure, temperature, humidity and
wind, the computer models also make forecasts of cloud and rain. The
computer forecasts of surface pressure, temperature, wind, cloud and
rain are often shown on the television.
Different computer models are used for different types of forecast. As
well as the global forecast model, the UK Met Office uses a highresolution model to forecast local scale variations in and around the
British Isles. This meso-scale model is especially good at taking into
account the local effect of hills and the contrast between land and sea.
Another variant of the model is used for climate change predictions
many years into the future.
Figure 5. Some of the physical processes represented in computer models used
to forecast the weather.
4.2 Role of the forecaster
Despite greater computer power, improvements to the computer models
and other technological advances, there is still an important role for the
forecaster.
For the general development of weather systems, the model provides
insight into how the atmosphere is behaving and developing; but it is
only a guide. Good as it is, forecasters have to make allowances for the
model’s known problem areas – the handling of small-scale features, for
example. Forecasters can also modify forecasts to take into account any
late observations and the latest radar and satellite pictures.
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In providing specific services, each individual customer presents special
problems for the local forecaster based at an airfield or Civil (Weather)
Centre. Experience and local knowledge add the fine detail to the
computer forecast so that the best advice for a specific location (e.g. for
an oilrig) or specialised requirements (e.g. for power industries) can be
given.
There is no doubt, the contribution of human and the computer together
produces the best results.
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