British engineers have finished assembling a satellite that experts believe could have a transformative impact on our weather forecasts. The Aeolus spacecraft will fire a laser into the atmosphere to make 3D maps of wind behaviour across the planet. The data will be incorporated into the models that project weather patterns a few days ahead.
It is information that should give more warning of approaching storms. It ought also to remove some of the surprises associated with weather systems that end up behaving in a very different way to how they were forecast.
"We pride ourselves on being able to forecast the weather a little bit more than a week ahead with acceptable quality," said Prof Erland Källén from the European Centre for Medium-Range Weather Forecasts (ECMWF).
"But of course there are situations when our forecasts are poor and in many cases we have tracked that down to likely errors in the initial state – that is, we have a lack of observational information and, in particular, of winds in the tropics," he told BBC News.
Meteorologists have multiple ways of measuring the wind, from whirling anemometers and balloons to satellites that infer wind behaviour by tracking cloud movement or by sensing the choppiness of the seas.
But these are all somewhat limited indications, telling us what is happening in particular places or at particular heights.
The European Space Agency’s (Esa) Aeolus satellite is a completely new approach.
With its ultraviolet laser, it will aim to build a truly global view of how wind blows on Earth from the surface of the planet all the way up through the troposphere and into the stratosphere (from 0km to 30km).
It will achieve this by measuring how the pulses of light from its laser are scattered back off air molecules and water droplets, even particles of dust. The return signal will betray not only the altitude of wind streams but also gather some information about their speed and direction.
"The molecules in the air and the clouds move with the wind and that motion causes a shift in the frequency of the return signal," explained Prof John Remedios, the director of UK’s National Centre for Earth Observation.
"The laser sends out a signal at one frequency and you get it back at a slightly different frequency. It’s called the Doppler effect and you’ll be familiar with it from the usual story of how an ambulance siren changes as it passes you in the street."