By Betty Robinson, bettyrrobinson.ca
Light is such an interesting area of study! There is no end to the technologies we can develop using our understanding of the properties of light.
But it’s also just fun to look at. In the second book in my Elizabeth series—Elizabeth’s Enlightening Day at the Zoo, out later this summer—Elizabeth and her friend Zahra see examples of iridescence on bird feathers and butterfly wings. Iridescence can also be seen in clouds.
Clouds are made of water. The water could be in the form of liquid droplets, ice crystals, or both. The ice crystals are usually found in the higher part of the troposphere*, where it’s cold. And the sizes of the water particles can vary.
In the middle troposphere, where it’s warmer than in the upper part, as clouds start to form the water droplets are about the same size, and quite small. This is one important factor that is favourable for us to see cloud iridescence. Another important factor is nearness to the Sun. Most cloud iridescence happens with small droplets near the Sun. The sunlight is diffracted through the droplets, and we see the iridescent colours. And as the cloud continues to change its shape and grow, the iridescence also changes, which is why the colours are somewhat scattered and disorganized in iridescent clouds.
Cloud iridescence. The iridescent colours in clouds are scattered and disorganized. Image by Karthik Easvur, https://commons.wikimedia.org/wiki/File:Rainbow_cloud_1.jpg
Compare the scattered iridescence to the more organized colours we can see in a circumhorizon arc. Circumhorizon arcs can happen when the Sun is high in sky, close to 60 degrees. These arcs are in cirrus clouds, which are higher in the atmosphere than iridescent clouds. At that height, the clouds contain ice crystals. The ice crystals can make the arcs look a little fuzzy.
Circumhorizon arc, by Dehk, https://commons.wikimedia.org/wiki/File:Circumhorizontal_arc_in_Michigan.jpg
* We live in the troposphere, which extends up to about 20 kilometres (about 12.5 miles) above Earth’s surface.