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Needles
Needles are thin, columnar slivers of ice. Ice needles, like all ice crystals, have a hexagonal cross-section although because they are so thin, it is difficult to detect this pattern with the naked eye. Ice needle formation is favoured at temperatures of 21 to 25 degrees Fahrenheit.
Columns
Ice crystals can also form hexagonal columns, which are often hollow at either end and so are called "hollow columns." Air resistance causes these columns to orient themselves with their long axes horizontal to the ground as they fall through the air.
Plates
These ice crystals appear as hexagonal plates of varying thickness. How thick the crystals are depends on the temperature. Superficially, plates resemble columns; however, plates are much thinner than columns and have a larger surface area, which causes plates to drift like leaves toward the ground, with their large faces horizontal.
Ice Crystals and Snow Crystals
Ice crystals are not quite the same as snowflakes, which form from ice crystals under suitable meteorological conditions. Both have hexagonal structures, but snowflakes tend to be more complex, forming delicate, fern-like branches. Snowflakes form at around 5 degrees Fahrenheit and under humid conditions--if the temperature is much more or less than that or if the air is not very humid, ice crystals will form instead.
Ice Crystals and Halos
When the sun reflects and refracts off of the ice crystals that make up cirrus clouds, it can create a halo. A halo may form a circle around the Sun or the Moon, or it may be more intricate, with many webbed arches appearing across the sky. The type of ice crystal best suited for halo formation is the hexagonal prism. This phenomenon is more common in Antarctica, where the cold temperatures create very clean prisms.