Cirrus spissatus occurs as dense and thickening cirrus that will partly or completely obscure the Sun (or Moon). It appears as dark, grey patches when seen against the light.
Fair weather will persist as long as they appear in irregular patchy patterns, but if they are moving in from westerly directions they are a sign of an approaching warm front and, possibly, rain within 12 to 36 hours.
They also occur at the spreading top of thunderstorm anvils, especially under high wind-shear conditions and are also known as false cirrus then.
