On weather related topics....One of my favorite blogs is the Updraft Blog by Minnesota Public Radio, which is about Minneapolis, MN, but has some really cool technical/science/weather stuff on a regular basis. Recently they posted some cool pictures of snow crystals and a graph of how different snow crystals form at different temperatures and water/air saturation levels.
The following images are from the awesome Updraft Blog, which got the images courtesy of Caltech and snowcrystals.com
It turns out different snow crystal types form at different temperature and humidity levels.
From the Updraft Blog:
The morphology diagram tells us a great deal about what kinds of snow crystals form under what conditions. For example, we see that 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).
Furthermore, we see from the diagram that 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.
Why snow crystal shapes change so much with temperature remains something of a scientific mystery. The growth depends on exactly how water vapor molecules are incorporated into the growing ice crystal, and the physics behind this is complex and not well understood. It is the subject of current research in my lab and elsewhere.