photo #7789577

A yachting friend of Neil Comley (Cloud Appreciation Society Member 45,090) suggested that the high streaked cloud Cirrus can be ‘the edge of the wedge’, an early harbinger of wet weather to come. ‘How reliable is my cloud oracle?’ Neil asked us, pointing out that these Cirrus uncinus clouds he spotted above the Nahoon River in East London, South Africa eventually gave way to overcast skies and a couple of days of gentle showers. Neil’s friend is right that Cirrus clouds can precede the arrival of a weather front – but they’re a reliable indicator only if the Cirrus are invading the sky, becoming more and more plentiful over time, and if their streaks have a hooked appearance like the Cirrus uncinus Neil spotted here. (Uncinus is Latin for ‘hooked’.) The cloud can be the first sign of moisture high in the atmosphere and the pattern of elevated winds that precede a warm front. They are the edge of a wedge in that the thin clouds join as the weather passes over, combining into a continuous high layer that then thickens as the cloud base lowers until a deep and grey cloud layer eventually arrives later in the day. This is the layered precipitation cloud Nimbostratus, also known as the thick end of the wedge.