However, the term is usually a synonym for shallow radiation fog in some cases the depth of the fog is on the order of tens of centimetres over certain kinds of terrain with the absence of wind.Īdvection fog layer in San Francisco with the Golden Gate Bridge and skyline in the backgroundĪdvection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. Ground fog is fog that obscures less than 60% of the sky and does not extend to the base of any overhead clouds.
Examples of this phenomenon include tule fog. Radiation fog is most common in autumn and early winter. Radiation fog occurs at night, and usually does not last long after sunrise, but it can persist all day in the winter months, especially in areas bounded by high ground. In perfect calm, the fog layer can be less than a meter thick, but turbulence can promote a thicker layer. The cooling ground then cools adjacent air by conduction, causing the air temperature to fall and reach the dew point, forming fog. Radiation fog is formed by the cooling of land after sunset by infrared thermal radiation in calm conditions with a clear sky. The marine layer, and any fog-bank it may contain, will be "squashed" when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering.įog can form in a number of ways, depending on how the cooling that caused the condensation occurred. The inversion boundary varies its altitude primarily in response to the weight of the air above it, which is measured in terms of atmospheric pressure. The thickness of a fog layer is largely determined by the altitude of the inversion boundary, which in coastal or oceanic locales is also the top of the marine layer, above which the air mass is warmer and drier. Drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. This can occur when the fog layer is lifted and cooled sufficiently, or when it is forcibly compressed from above by descending air. Drizzle occurs when the humidity of fog attains 100% and the minute cloud droplets begin to coalesce into larger droplets. At 100% relative humidity, the air cannot hold additional moisture, thus, the air will become supersaturated if additional moisture is added.įog commonly produces precipitation in the form of drizzle or very light snow. However, fog can form at lower humidities, and can sometimes fail to form with relative humidity at 100%. This occurs from either added moisture in the air, or falling ambient air temperature. įog normally occurs at a relative humidity near 100%.
Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Water vapor normally begins to condense on condensation nuclei such as dust, ice, and salt in order to form clouds. Some examples of ways that water vapor is condensed include wind convergence into areas of upward motion precipitation or virga falling from above daytime heating evaporating water from the surface of oceans, water bodies, or wet land transpiration from plants cool or dry air moving over warmer water and lifting air over mountains. įog begins to form when water vapor condenses into tiny water droplets that are suspended in the air. Those outside the camera lens's depth of field appear as orbs.įog forms when the difference between air temperature and dew point is less than 2.5 ☌ (4.5 ☏). A close-up view of water droplets forming fog.
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