Images captured by NASA's Terra satellite on Wednesday and Thursday show smoke plumes over both Spain and the western Mediterranean.
NASA said data collected from its own satellites and other observation stations suggested the smoke came from wildfires in Canada.
They also it was possible but "less likely" that the some had drifted over Spain from wildfires as far afield as the US state of Colorado, or from other areas in the southwestern US.
According to the space agency, smoke has a "faintly yellow-tan or milky gray color" when it is compared with the "brighter" white clouds.
Smoke can travel vast distances once it leaves the boundary layer, or that part of the atmosphere closest to the ground.