Freezing drizzle can coat roads and sidewalks with a thin layer of smooth ice, causing bad accidents, but it is often difficult to forecast. Freezing drizzle is similar to freezing rain except that the droplets are smaller. By definition, drizzle droplets are less than half a millimeter in diameter.
Larger freezing rain droplets usually form as snowflakes and then melt in a warm layer on the way down before freezing upon contact with a frozen ground. Freezing drizzle, on the other hand, can form in temperatures below freezing.
Because the droplets are so small, they are made of supercooled water. This means they remain in liquid form even in freezing temperatures. The environment for freezing drizzle is a moist, stable atmosphere with very little rising motion, which can be difficult to identify. The tiny, supercooled droplets will immediately freeze on contact with any surface. Usually, the glaze is light but given enough time, can accumulate to the point at which roads and sidewalks become completely slick.
