Beautiful streaks of clouds appeared over London on Monday evening. Contrails from highflying aircraft slowly dissipated into clouds barely distinguishable from surrounding wisps of cirrus and veils of cirrostratus clouds.

But most eye-catching of all were a couple of chequered patches made up of clouds arranged in beautiful ripples, like the waves from a stone hitting water. These were cirrocumulus undulatus, known as a mackerel sky.

Although mackerel skies often give warning of stormy weather on the way, these patches of cirrocumulus were too small to worry about and carried no great weather forecast. Instead, they and the other high-altitude clouds were a sign of moisture high in the atmosphere. The wavy patterns showed strong high-level winds making undulations in the clouds.

As all these clouds drifted lazily past the setting sun, a large smudge of multicoloured light emerged through the shroud of thin cloud. This was a sundog, an optical effect caused by ice crystals in the clouds bending and splitting the sunlight into the colours of the rainbow like glass prisms.

Keep an eye out for similar psychedelic colours when the sun is low and veiled by wispy high-level clouds.