By Dr David Whitehouse BBC News Online science editor

The US space agency's Terra satellite has measured how much sunlight the Earth reflects back into space.

The "Earthshine" data should give scientists a better understanding of how the atmosphere works.

The Moderate Resolution Imaging Spectroradiometer (Modis) instrument on Terra routinely measures this so-called albedo.

Researchers say the new data from Terra are just what they would expect as the satellite passes over the wide range of geological features found across the planet's surface.

"Zooming in on Africa's Sahara Desert and the Arabian Peninsula, for instance, Modis observed considerable variability in reflectance across the region - from the darkest volcanic terrains to the brightest sand sheets," said Elena Tsvetsinskaya of Boston University.

Vital role

Albedo information is important because most weather forecasting models treat this region as uniform. But, in reality, reflectance variations produce heating differences that result in atmospheric motions that influence clouds and rain.

By combining Modis data with geologic information, Tsvetsinskaya and colleagues have provided weather and climate analysts with a new map of albedo across Northern Africa and the Arabian Peninsula so that they can fine-tune their forecasts.

Red areas reflect more light into space than green areas

Albedo variations play a vital role in the Earth's climate. For example, the bright snow- and ice-covered landscapes of Canada and Siberia during winter reflect most solar radiation straight back into space, keeping the surface cold.

As the snow melts in spring, the forests are exposed to the sunlight. The vegetated surface is much darker and absorbs light, which warms the surface.

Scientists recently discovered that once the snow melts in the Earth's northern forests, the surface releases a significant amount of heat into the overlying atmosphere that influences global weather systems.

The Modis albedo measurements allow scientists to monitor how the Earth's reflectivity changes on a global scale.

"This will help us determine how the Earth's climate is changing, both globally and locally," said Crystal Schaaf of Boston University.

Launched on 18 December, 1999, Terra is the flagship of Nasa's Earth Observing System series of satellites.

Its data may allow scientists to better understand and predict how various surface features absorb and reflect solar radiation, which influence both short-term weather patterns and longer-term climate trends.