Iridescence is best close to the sun. Be sure to shade the sun from both eyes. Viewing the reflection of the sky in a dark mirror is an alternative.
Cloud water droplets are 1 to 100 micron (1/1000 – 1/10 mm) diameter. The smallest are only a few visible light wavelengths across. They significantly diffract incident light waves. Waves scattered from the droplet surface are the main source of a corona or iridescence close to the sun. In effect, spherical waves radiate outwards from each point on the surface. The waves overlap and interfere and light is reinforced in some angles and negated in others. The angles are wavelength dependent giving rise to the iridescent colours.
A cloud with droplets of similar size everywhere across it produces a circular diffraction patter – a corona. Cloud with spatially varying droplet sizes gives the less structured colours of iridescence. The two phenomena are the same.