FOG AND STRATUS - KEY PARAMETERS

by FMI


A moist boundary layer generally indicates the presence of low-level cloudiness. A moist lower troposphere (represented by e.g. 925 hPa relative humidity, cloud liquid water or other relevant parameter) and relatively dry middle troposphere (represented by e.g. 500 hPa relative humidity) are typically present in a situation where there is warm sector advection St/fog.

The use of cross sections of relative humidity helps in finding moist lower level layers between the main pressure levels.

Radio soundings show the vertical distribution of humidity best.

Radio soundings also clearly show the warm advection as veering of the wind with height. Wintertime warm advection often produces a significant elevated temperature inversion.

Diagram illustrating the vertical distribution of humidity for advection fog
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03 December 1998/12.00 UTC - radio sounding Joikoinen; black: temperature, red: dewpoint, numbers at left - wind direction (degrees) and speed (m/s) separated by comma
The good vertical resolution of humidity of radio soundings can be also used for the detection of shallower Radiation Fog. Low level moisture can be restricted to a very thin layer near the surface, while the air above is reasonably dry.
Diagram illustrating the vertical distribution of humidity for radiation fog
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05 December 1998/12.00 UTC - radio sounding Joikoinen; black: temperature, red: dewpoint, numbers at left - wind direction (degrees) and speed (m/s) separated by comma

Temperature/thickness advection

Advection Fog is observed within or to the rear of areas of moderate or strong warm advection (typically within a warm sector).
03 December 1998/12.00 UTC - black: height contours 700 hPa, red: positive thickness advection, blue: negative thickness advection, low cloud observations

Low-level (surface) winds

In the case of Radiation Fog the surface and low level winds are weak. For advection St/fog the surface and low level winds are moderate to strong with often a strong shear in the boundary layer (see the radio sounding examples above).

Surface temperature

Moist air flow over colder sea surface may produce Stratus or Fog. The likelihood for fog increases if the dewpoint temperature of the air is close to or above the surface temperature. The same mechanism works over areas of cold water (lakes) or cold land. The presence of melting snow enhances the development of St/fog over land.

SUB-MENU OF FOG AND STRATUS
METEOROLOGICAL PHYSICAL BACKGROUND
TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS