Cap Cloud over the Sierra Nevadas . ... Credit Copyright: Guido MontaЯИs Explanation: One might say this was a bell weather day for the Sierra Nevada mountains . ... Such a Cap cloud is formed by air forced upwards by a mountain peak , with the air then cooling , saturating with moisture, and finally having its molecular water condense into cloud droplets . Such a bell -shaped cloud structure is unusual as air typically moves horizontally, making most clouds nearly flat across at the bottom. ...