When most people in South-East England drew back their curtains this morning, the first question on their lips was probably: 'What are the temperature and humidity conditions for the different types of snowflake morphology?'
Fortunately, Kenneth Libbrecht of Caltech provides the answer:
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)...snow crystals tend to form simpler shapes when the humidity (supersaturation) is low, while more complex shapes at higher humidities. The most extreme shapes -- long needles around -5C and large, thin plates around -15C -- form when the humidity is especially high.