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   Random or poorly aligned?  

  Halos formed by column crystals with large tilts. The tilts are uniformly distributed and the numbers are the maximum tilt. The sun was 30� in the simulation.  

Randomly oriented crystals are often said to be the source of 22� halos. They could produce them, but they are not essential.

What are needed are "poorly aligned" crystals, ones with a rather large distribution of tilts about their equilibrium positions.

The HaloSim simulations at right show the effect of increasingly poor alignment of column crystals. These tend to drift downwards in clouds with their prism axes nearly horizontal.   

Maximum tilts of 30 - 45� give a halo seen very often, circular but brighter at the top and bottom.

Slightly greater tilts give reasonably uniform halos.

When the sun is higher the tilts needed for nearly uniform halos get smaller.