ISS004-E-11807 (15 May 2002) --- This digital photograph, taken through the windows of the International Space Station on May 15, 2002, shows condensation trails over the Rhône Valley in the region west of Lyon, France. Condensation trails-or contrails-are straight lines of ice crystals that form in the wake of jet liners where air temperatures are lower than about -40 degrees Centigrade. Scientists have observed that newer contrails are thin whereas older trails have widened with time as a result of light winds. Because of this tendency for thin contrails to cover greater areas with time, it is estimated that these “artificial clouds” cover 0.1 per cent of the planet’s surface. Percentages are far higher in some places, say the scientists, such as southern California, the Ohio River Valley and parts of Europe, as illustrated here. The climatic impact of such clouds is poorly understood, which is why scientists continue to study them using images such as this.