ROUGH CUT (NO REPORTER NARRATION)<br/> <br />Visitors to the Grand Canyon on Thursday (December 11) were treated to a rare sight -- puffy white clouds filling the massive opening.<br/> <br />Time lapse video showed the dense layer of clouds rising from the bottom and nearly reaching the rim of the canyon.<br/> <br />Meteorologists say the unusual occurrence is a result of a process called cloud inversion which happens once every few years. The effect is a result of warm air and a lack of wind sitting on top of the clouds, keeping them near the earth's surface and preventing them from rising into the sky.
