Contrail shadows - See Les Cowley's "Atmospheric Optics" site on contrail shadows: http://www.atoptics.co.uk/atoptics/contr1.htm http://www.atoptics.co.uk/atoptics/contr2.htm Figure-diagram: "[T]he [contrail cloud's] shadow falls on a thin lower layer of cirrus cloud." http://www.atoptics.co.uk/atoptics/images1/shim18.gif - Clear Skies, Kurt