Iridescent clouds over Lamas, San Martin Peru. (credit: David Alvarado)

DISCUSSION:  On January 18, photographer David Alvarado captured an incredible view of iridescent clouds over Lamas, San Martin Peru.  These clouds were composed of vivid colors, and demonstrate the process of diffraction in the atmosphere.  In these photos, almost all colors of the visible spectrum of light can be seen with blue and green being the more prominent hues.  Iridescent clouds are caused by a process known as irisation, or what is more commonly referred to as diffraction.  While light is commonly described by being composed of several photons, light can also be described as a wave.   In fact, this wave-particle duality is of extreme importance in the field of physics. 

Diffraction is the process in which a wave, in this case light from our Sun, encounters an obstacle like water vapor, or small ice crystals.  The process of diffraction can be visualized the same way as when water strikes a pier along a coastline.  The water cannot pass through the pier so they bend around the pier.  As the light interacts with these nearly microscopic particles, the light scatters into the colors that our eyes perceive.  The scattering of light is due to constructive interference, or when two waves align to produce a wave with larger amplitudes.  Usually, the process of irisation occurs with clouds that form higher in the troposphere.  The troposphere extends upward to near 10 to 12 kilometers or 6.2 to 7.4 miles and is where most of weather occurs on Earth.  Some examples of clouds that form at these altitudes are altocumulus, cirrocumulus, and cirrus.  These cloud types are thinner optically, allowing for the light to encounter less drops as necessary.  Irisation is also more commonly observed in the early stages of cloud formation because the water vapor or ice crystals have a similar shape and size. 

The main difference between iridescent clouds and those produced by solar and lunar haloes is droplet and/or ice crystal size.  Iridescent clouds form from smaller particles where as haloes are created from larger particle sizes.  Iridescent clouds can also form on the top of thunderstorms which are commonly referred to as pileus caps.  During a thunderstorm, the convective cumulus cloud rises and pushes air upward with it.  Moisture that rapidly condenses at this altitude produces tiny cloud particles of efficient optical thickness and size to facilitate the diffraction of light.   The diffraction of light is seen as beautiful colors similar to a rainbow.  

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©2018 Meteorologist Allan Diegan