Fog formed when water vapor is added to air that is much colder than the vapor's source; most commonly, when very cold air drifts across relatively warm water. No matter what the nature of the vapor source (warm water, industrial combustion exhaust, exhaled breath), its equilibrium vapor pressure is greater than that corresponding to the colder air; thus, the water vapor, upon becoming mixed with and cooled by the cold air, rapidly condenses. It should be noted that this mechanism never allows the fog to actually reach the vapor source. Also, upon further mixing in sufficiently turbulent or convective flow (only a slight degree is needed), the fog particles evaporate at a more or less well defined upper limit of the fog. Note, also, that although advection of air is necessary to produce steam fog, it differs greatly from an advection fog in the usual sense, which is caused by warm, moist air moving over a cold surface. Steam fog is commonly observed over lakes and streams on cold autumn mornings as well as in polar regions. It is sometimes confused with ice fog, but its particles are entirely liquid. At temperatures below −29°C (−20°F), these may freeze into droxtals and create a type of ice fog that may be known as frost smoke.