Precipitation

During arctic winter the phenomenon of "Diamond Dust" often occurs. This is a snow-like precipitation from blue, totally cloudless sky. Sky.jpg shows the blue sky during the precipitation event that took place for several hours. Very fine, sinking ice-crystals can be noticed on the other images. These are no snow flakes, i.e. agglomerates from needle-like ice-crystals, but single crystals. The precipitation is so fine that one barely can recognize it - except in the back light. Dust2.jpg and Dust5.jpg were not taken with the back light of the sun and therefore only few ice-crystals can be noticed. The hexagonal plate structure of the ice-crystals becomes clear because of the light reflection on some of the images.
Dust1.jpg, Dust3.jpg and Dust4.jpg are somehow misleading because of optical deceit by the back light.

The pictures suggest a dense, intensive precipitation, which - in reality - is so not noticeable and does not look that way. Dust2.jpg shows more realistic what an observer might see.

Because it was almost totally calm during the precipitation event it can be ruled out that these were ice-crystals which have been suspended from the ground. This precipitation comes "out of the blue" under conditions of extreme dryness of the air and moderately low temperatures (here around -15 deg C). (There was an inversion in a height of 100 m above the observer which was noticeable by a power plant plume.
Therefore it can be assumed that the air above was even warmer).

Under these conditions water vapour deposits onto slowly depositing aerosol particles by diffusion. Within several hours those precipitation particles are generated by very slow "deposition - diffusion growth". They reached the ground with a velocity of around 1.8 m/s, i.e. they passed the observer in 1 second. This results in a particle diameter of around 2 mm.

Dust1-5.jpg and Sky.jpg: S. Borrmann, Kiruna, North Sweden, APE-ENVISAT-Validation Field Campaign, 5 March 2003, 8:00 to 08:20 a.m.