Drizzle drops are already larger than cloud or fog drops, but smaller than rain drops. Drizzle drops are usually from 0.2 to 0.5 mm in diameter.
Sometimes the whole sky is covered by a gray cloudy sheet. These are stratus clouds. They form when warmer air creeps into a wedge of colder air. When rising, water vapor condenses into droplets of water, forming a cloud. This is a typical situation on a warm front.
Usually such clouds are very low, their lower border is only 300 meters above the ground. They are thin, therefore, despite the gloomy picture, a rainfall from them is unlikely to fall. But drizzle will fall.
Drizzle drops appear as a result of the merging of cloud drops and fall down before they grow into rain drops.
In winter, drizzle can cause the formation of ice. If drizzle drops are very chilled and fall to the surface at a temperature below zero, then they will freeze immediately.