The shape of snow crystals depends on the temperature and humidity of the atmosphere in which they have grown. In this Nakaya diagram, the vertical axis shows the density of water vapor in excess of saturation with respect to ice. Reading from left to right at low supersaturation, one sees abrupt morphological changes (dynamical phase transitions) from hexagonal plate to prism to plate and back to prism. The curved line shows the saturation with respect to liquid water as a function of temperature.