This morning much of Mid-Michigan was covered with fog. The fog was caused by warm air moving over the snow covered ground. The warm air causes moisture some of the snow to turn directly into a vapor (skipping the liquid stage). This process is know as sublimation. When the air is completely saturated, meaning it can hold no more vapor, the water begins to condense or turn into liquid droplets (fog). This type of fog is known as advection fog.