When the dew point is below the freezing point of water (0° C), the equilibrium is between water vapor and ice. If saturated air that is already at 0° C is chilled further, ice crystals will form...
The Dew Point is the temperature at which the air is at 100% Relative Humidity. If the temperature fall below the Dew Point, then there is more moisture in the air than it can hold, and water...