Sunlit clouds. Aerial photograph of sunlight reflecting off cirrus clouds at sunset. Cirrus clouds are elongated tufted clouds, also known as mare's tails, that are found at high altitudes over 5000 metres. They consist of tiny ice crystals, which are blown by the wind into their characteristically wispy shapes. A large concentration of cirrus clouds may indicate an approaching weather front, and that the weather is likely to change within 24 hours of these clouds forming. Photographed over the Greek island of Santorini in the Mediterranean Sea.