Strange Cloud Streaks ~ Jan Willem de Wit (blog) saw this peculiar sight at Gouda in The Netherlands. Looking south from the town in late morning the underside of the clouds had these light stripes. Cloud images ©Jan Willem de Wit, shown with permission
The scene had meteorologists (and myself) puzzled. Atmospheric optics expert Peter Paul Hattinga Verschure came up with the explanation...

The camera was pointing approximately southwards and the sun was 17° high 26° east of south.

The area in that direction from the town of Gouda has a great number of drainage channels running roughly in a SE-NW direction (see map). These stretch up to 20km from the town. More than half of The Netherlands is below sea level and water management and drainage is a very serious business.

The smooth water in the channels (there was almost no wind) acted as mirrors reflecting sunlight upwards onto the low cloud base to form an image of the channels on the cloud surface. These are the bright stripes.

The water channel grids to the SE of Gouda

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Ian Loxley of the Cloud Appreciation Society classifies the clouds as probably Cumulonimbus pannus
Image Abe Maaijen