SNOWFLAKE MORPHS. “Thin plates and stars grow around -2 C (28 F), while columns and slender needles appear near -5 C (23 F). Plates and stars again form near -15 C (5 F), and a combination of plates and columns are made around -30 C (-22 F).”
http://www.pbs.org/newshour/rundown/the-science-of-snowflakes/