What conditions are needed for snow and what conditions are needed for sleet/freezing rain?
Of course the temperature in the cloud where snow forms must be at or below the freezing point of water, 32F. The temperature may be right at 32F because when water freezes it gives up a lot of (latent) energy resisting further temperature drop. In the summer, snow may form as crystal snow flakes growing as moisture condenses an freezes into symmetrical figures dictated by the orientation of growing previous water molecules from a seed. These can fall from the cloud and melt and arrive as rain heated above freezing by the air it passes through. If a strong updraft (relatively warm air rising) carries the rain back up it may refreeze as hail. Large hail stones have been through several up and down cycles and often show layers when cracked open. If the snow falls and only partially melts you may observe sleet (which is a mixture of rain and snow). Some times rain in a cloud is pure enough and agitated enough to resist freezing even slightly below the freezing point. When these super-cool
