Tonight the first of a new series of weather satellites takes to the skies. . . and its proud watchers and users will be meeting in Dublin next week

Dublin next week will be awash not (only) with rain but with weather people. You will not find there a full cross-section of the breed, but rather several hundred of an interesting sub-species known for its particular interest in weather satellites.

They will be here for the 2002 EUMETSAT Meteorological Satellite Conference, from September 2nd-6th in Dublin Castle.

This annual gathering is held in a different European country every year, and is perhaps the most prestigious of the many which address the various uses to be made of data from meteorological satellites.

Last year it took place in Antalya, Turkey. The time before it was Bologna, Italy. This year's conference will be hosted by Met Éireann.

EUMETSAT, the European Meteorological Satellite Organisation, is a co-operative venture by 18 European countries.

They decided some 20 years ago to pool their financial resources and take on a commitment which none of them individually could ever contemplate: they would launch weather satellites for the use of forecasters in Europe and elsewhere.

The organisation comprises all the EU member-states, together with Norway, Switzerland and Turkey, and has the additional support of several "co-operating states" from the emerging nations of central and eastern Europe.

EUMETSAT's governing body is its council, on which sit the directors of the national weather services of all its member-states. The day-to-day management of the organisation is in the hands of its director general, Dr Tillmann Mohr, from Germany, and three directors comprising a Frenchman, a Dane and an Irishman not unknown in Irish weather circles but whose identity modesty forbids me to reveal.

Obviously, the primary objective of the 2002 conference, as usual, will be to allow specialists in satellite meteorology from all over the world to exchange views and information.

But the serious business this year is likely to be conducted in an atmosphere of special celebration; shortly before midnight Irish time tonight a new series of geostationary weather satellites, the eagerly-awaited Meteosat Second Generation (MSG), will be inaugurated with the launch of MSG-1 from the European Space Centre in Kourou, French Guyana, South America.

A geostationary satellite is one which has been launched so that it ends up 36,000 km above the equator, at which height the satellite's speed in orbit exactly matches the speed of the Earth rotating on its axis underneath. To an observer on the ground, therefore, such a satellite would appear to be fixed in space and it looks down constantly on the same segment of the globe.

EUMETSAT currently operates two geostationary satellites. Meteosat-7 is directly over western Africa, and provides the images of Europe you see on your television screen each evening. An older spacecraft, Meteosat-5, looks down continually on the Indian Ocean, capturing much of Asia. Three other geostationary satellites, maintained by other agencies, complete the team of five, dotted like well-spaced beads on a celestial necklace high above the equator. Together they provide images of the entire globe.

Pictures from a geostationary satellite allow forecasters to see where fronts and depressions are located, and this is particularly useful in areas of the world where there are few conventional ground-based observations, like over the oceans. And with the satellite "fixed in space", geostationary images readily lend themselves to the making of a "movie" of any developing weather situation.

Yet they also provide a wealth of other information. Wind speeds in the upper atmosphere can be calculated by observing the distance travelled in, say, 30 minutes by distinctive features of the cloud pattern.

Rainfall intensity can be estimated from the general brightness and texture of the cloud, and surface and upper-level temperatures can be obtained from infra-red images, produced by instruments sensitive to thermal radiation rather than to the visible light of the conventional pictures. All this data are extracted from the images using automated methods, and used as raw material for computer models of the atmosphere to produce the daily weather forecast.

The present Meteosat satellites have served us well but they are almost primitive compared to MSG. MSG-1 will record data in 12 radiation channels rather than the present three; it will provide images every 15 minutes rather than every half an hour; and its instruments are much more sensitive than those in use at present, and therefore capable of undertaking much more detailed measurements.

In addition, in a new departure, MSG-1 will monitor the "radiation budget" of the Earth, the balance between solar energy received and the heat reradiated by our planet back to space. This last is typical of new satellite capabilities which have emerged in recent years in response to the needs of those studying climate change and global warming.

Climatologists have little interest in the hour-by-hour condition of the atmosphere. Their concern is with its average state over weeks, months and years, and with other data some of which is amenable to satellite collection. These include snow-cover, the amount of ice in polar regions, and the amount, type and condition of the vegetation cover.

But the geostationary orbit has its limitations. Northern countries (like Ireland) are viewed by geostationary satellites at a very oblique angle because of the curvature of the Earth. Moreover, owing to its relatively great distance from the ground, temperature values and other data from a geostationary spacecraft are less accurate than scientists would wish.

"Polar-orbiting" satellites cope with some of these difficulties. They travel around the globe from pole to pole, following as it were the lines of longitude only 800 km or so above the surface.

They provide undistorted pictures in the northern latitudes, and because of their lower altitude they can more easily monitor the vertical distribution of atmospheric temperature, providing for large areas the same kind of information traditionally gathered by radiosondes sent aloft attached to hydrogen-filled balloons.

NOAA, the US National Oceanic and Atmospheric Administration, has operated polar-orbiting weather satellites for many years, and EUMETSAT and NOAA are currently co-operating in an ambitious joint programme for future generations of these spacecraft.

This is the background against which the 2002 EUMETSAT Meteorological Satellite Conference will take place in Dublin.

If there are significant hitches with the launch of MSG-1 tonight the gathering will be subdued. But if Ariane-5 performs its task as planned, and MSG-1 reaches it destined orbit in good health, the Dublin 2002 conference will radiate a glow of renewed confidence in the future of European satellite meteorology.