What phenomenon occurs when the temperature remains below freezing throughout the atmosphere?

When the temperature remains below freezing throughout the atmosphere, the environment is conducive to the formation of snow. Snow is created when water vapor in the atmosphere undergoes deposition and crystallizes into ice crystals, which then stick together and fall to the ground as snowflakes.

For snow to occur, the necessary condition is that the air temperature from the clouds to the surface must be at or below freezing, allowing the ice crystals to remain intact as they descend. This contrasts with the other options: rain occurs when temperatures are above freezing, sleet occurs when raindrops freeze on their way down, and freezing rain happens when rain falls through cold air and freezes upon contact with surfaces. Thus, the conditions for sustained sub-freezing temperatures throughout the atmosphere distinctly lead to snow formation.