A purple cloud is somewhat rare but not exceptionally rare.
Purple clouds are the result of scattering of sunlight by atmospheric particles, most particularly during sunset or sunrise, when the sun's light travels through more of the atmosphere.
The presence of moisture as well as other particles, like pollution or dust can also enhance the purple hues of purple clouds.
Not every sunrise or sunset will make or produce a vibrant purple sky, but the purple clouds are a natural phenomenon which can actually be seen and observed relatively frequently.
The presence of dust, pollution and moisture in the atmosphere can affect how the light is scattered.
And the particles can also enhance the scattering of the violet or blue light and make the purple clouds more prominent.
The blue color of the sky is caused by the scattering of sunlight by air molecules.
And when the sun is low on the horizon, the light then travels through more of the atmosphere, and shorter wavelengths, such as violet and blue are also scattered more effectively and sometimes it results in the purple hues or colors.
Purple skies and purple clouds are most commonly seen and observed during sunset times and sunrise times, because of the sun's light passing through the atmosphere at a low angle, maximizing the scattering effect.