photo #6716581

During the Class of 2024 graduation ceremony at Brookline High School, Massachusetts, US, Roven Ramiro (Cloud Appreciation Society Member 61,530) saw the clouds fly celebration colours. This optical effect is known as a circumhorizon arc. It forms when ice crystals in a cloud like this streak of Cirrus are in the shape of tiny hexagonal plates aligned horizontally as they fall through the sky like autumn leaves. If the crystals are clear, they act like microscopic prisms that refract the sunlight as it passes through their faces, separating it into a spectrum of hues. When the crystals are in the part of the sky off towards the horizon and the Sun is very high up above, the angles can be just right for their collective glints to combine into a flat, horizontal band of colours. When only a patch of the right ice crystals are in the right part of the sky, just a patch of circumhorizon arc appears. The Sun also needs to be really high in the sky, at an angle of more than 58 degrees from the horizon, which means that circumhorizon arcs will only form in the middle of the day in the summer for many regions of the world. If you ever spot one, it’s certainly cause for celebration.