Meteorological physics Background

Stratus and also Fog are phenomena the are basic to differentiate for a person observer in ~ ground level. The an interpretation of each has been formulated together follows:

The reason why these two phenomena are disputed together is the reality that for satellite sensors viewing castle from over the differentiation is no at all simple. The only real difference between Fog and Stratus is the different altitude of the cloud base, which because that Stratus lies a few hundred meters over ground, vice versa, in Fog the cloud base descends to ground level. A current solution for automatically diagnosis of even if it is the cloud is at the soil (fog) or lifted off the soil (stratus) was presented in previous chapter (seeCloud structure In Satellite Images).

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This chapter mainly concentrates on advection Stratus/fog and Radiation Fog.

Favoured synoptic setting for St/Fog

Fog (or low Stratus) is formed, as soon as moist air near ground level starts come condensate. This condensation have the right to be produced in the environment by three mechanisms:By ascent and resultant cooling of an waiting parcel.By radiative heat loss.By the mixing of 2 parcels of slightly unsaturated air at first having different temperatures.The necessary condition for both the advection and also radiation St/fog formation is a sufficient moisture contents in the lowest layers that the atmosphere. Otherwise the problems differ substantially for these Fog types.

Radiation Fog formation frequently calls because that clear skies, ample humidity in the surface ar layer and light winds. Wanted locations come fulfil these problems can be found adjacent to high pressures end land (mostly during the winter season) with linked weak press gradient. Wetness the soil considerably increases the possibilities of Radiation Fog, because that which reason a an extremely favoured instance for Fog development is the skies clearing and also wind diminish in the evening after a merganser day. Too strong wind will most likely develop Stratocumulus than Stratus.

For the Advection Fog the important components are the advection of moisture and temperature by the wind. Depending upon the wind speed and fetch end water the interaction between the cooler surface ar layer and also the overlying air outcomes in low-level Stratus cloudiness and also even Fog. As an typical moderate (4-7 m/s) surface ar winds are common for Advection Fog occurrences. Moist and also warm southwesterly or southerly wait streams end cool waters such together over cold ocean currents can create an extremely extensive Fog sheets. Advection Fog is likewise frequently observed in wintertime short pressure warm sector areas over the continents.

Radiation Fog

Diagram reflecting the synoptic conditions for night-time Radiation Fog with front and also side view panels. The typical verticaldistribution the temperature, dew allude temperature and also wind room presented in the prior panel.

Radiation Fog forms, as soon as the ground cools quickly after sunset by long-wave radiation to space. The cooling of the air near the ground renders it progressively thermally stable, which weakens and eventually stops turbulence over ground. Together radiative cooling continues, overabundance water vapour starts to condense right into Fog droplets. As shortly as the deepening Fog layer becomes optically thick (some 10s of metres), the upper part of it i do not care the reliable interface come space.

As the Fog it s okay thicker, that becomes an ext uniform in the vertical, through a well-defined top. Depending upon the season and synoptic conditions the Radiation Fog may or might not build into a maturation Fog. For example in summer, Fog layers are most often thin early to quick nights. In the winter, however, under stationary synoptic situations Radiation Fog or lifted Fog (Stratus) deserve to last because that days.

The dissipation the Radiation Fog have the right to take ar through the adhering to mechanisms:The dispersing of a cloud sheet end the Fog, i beg your pardon reduces the radiative cooling in ~ the Fog top layer.Atmospheric layers through high amounts of water vapour can aid to dissipate fog by long wave radiation.Heat indigenous the ground have the right to lift the Fog into low cloud.Increasing shear in ~ Fog top caused by boosting wind over the Fog increases the entrainment of dried air right into the cloud and speeds up the rate ofdissipation.In mature Fog the stable of droplets come the soil or top top vegetation contributes considerably to the dissipation.Solar radiation (excluding mid-winter) have the right to penetrate to the soil in areas where Fog is thinner (such together elevated places) and heat the ground and evaporate the Fog. Consequently, warm will be convected right into the cloud. During summertime due to short nights the Fog has actually no time come thicken long enough, prior to the morning sun already starts heating the ground and also breaks the Fogs right into patches and also later maybe right into fair-weather cumulus.

Advection Fog/St

Diagram mirroring the synoptic conditions for Advection Fog v front and side see panels. The common vertical distribution oftemperature, dew allude temperature and wind room presented in the front panel.

When moist and also warm wait is advected through the wind, unstable mixing in the boundary layer moistens the waiting from below. This results in advection stratus clouds. When Stratus to meet cooler waters (with sea surface temperatures reduced than wait dewpoint temperature) or cold (possibly snow-covered) land, turbulence and also thermal convection decrease significantly, but the water vapour-driven convection indigenous the surface proceeds to mix the overlying air. This leader to cooling that the unsaturated air and also lowering the the cloud base, eventually also to the formation of Advection Fog.

Important components for the development of Advection Fog are the fetch over the sea (or end land through wet snow) and also wind speed. A lengthy fetch together with not as well high a wind speed permits the cloud to sink lower and eventually type Fog. End land locations the eye temperature is an important factor: snow having actually below-zero temperature can quickly absorb humidity from overlying air. This is as result of the smaller sized water vapour pressure for ice surfaces contrasted to liquid water surfaces. Fog development over dry eye is; therefore, much an ext common than over wet snow, together wet eye is a resource of moisture.

Advection Fog have the right to be intended to be persistent, because the synoptic situation does not readjust rapidly. In wintertime warm sectors the middle and also upper troposphere typically remain reasonably dry, i beg your pardon favours considerable low level cloud decks come persist. Either a frontal i and readjust of air-mass, boosting wind/turbulence or the advection the mid-level or low-level cloud over the St/fog height are necessary for dissipation of the Fog to take it place. The advection the sea Fog end warmer water can likewise lead come lifted stratus and also possibly dispersal the cloud.

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Advection Fog has the same propensity to shrink from the outer edges as Radiation Fog, however the advection the the clouds needs to be taken right into account. Brighter clues indicating thicker and more persistent cloud might be lugged downwind, if the upwind edge of clouds will certainly clear an ext rapidly.