NLCs with a Difference ~ The Internet is awash with noctilucent cloud images from recent spectacular displays over Europe. These pictures are different and tell us about the particles that make up Earth's highest clouds.   Roel Wijtmans (Photosynthesis in Nature) took them near Sundsvall in eastern Sweden on 11th July two hours before local sunrise.  The left-hand picture is in visible light.  The right-hand is in infra-red.  We see no NLCs in IR!     Images ©Roel Wijtmans, shown with permission
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NLCs are high (50-53 mile) and cold (>-123 Celsius). They have no intrinsic visible light luminosity and we see them only by the high altitude sunlight that they scatter.

Roels's images show that they scatter visible light but scatter near infra-red far less effectively. This gives a clue to size of the cloud particles. Particles that scatter short wavelengths much more effectively than longer wavelengths are Rayleigh scatterers. Rayleigh particles are smaller than visible light wavelengths. Air molecules are Rayleigh scatterers and their scattered light deficient in red gives us blue skies. Fine smoke is similarly blue at right angles to the light.

In fact, NLCs are composed of miniscule ice crystals only 0.02- 0.1 micron (1micron = 1/1000 mm) across. Excellent Rayleigh scatterers. Some of their blue colour is from another process, absorption by ozone in the stratosphere far below them.