FOG AND STRATUS - TYPICAL APPEARANCE IN VERTICAL CROSS SECTIONS

by FMI


Radiation Fog

05 November 1999/13.01 UTC - NOAA RGB image (channel 3, 4 and 5); position of vertical cross section indicated
In this case, radiation St/fog developed after the passage of a Warm Front within a high pressure ridge. Meteosat images for this case can be found in the chapter "Cloud structure in satellite image".

The following cross sections are taken across the Radiation Fog:

05 November 1999/12.00 UTC - surface pressure, colours: temperature 850 hPa; position of vertical cross section indicated
05 November 1999/12.00 UTC - Vertical cross section; white: potential temperature, green: relative humidity above 75%, yellow: relative humidity below 75%
The image above left shows high pressure centred just north of the Black Sea. A Warm Front extends from south Russia towards the Arctic Sea. The Warm Front is weak in the southern part, while in the northern part the front is much more pronounced. The Radiation Fog is associated with the ridge of high pressure extending north from the Black Sea. The horizontal temperature gradient over the St/fog area is weak; surface winds of only a few m/s were observed within the St/fog. The resulting warm advection is very weak.

The isentropes show no frontal characteristics within the St/fog (image above right). The air is also quite stable above the boundary layer, while within boundary layer it is not. This may be a result of weak heat flux from the ground which is warming up the stratus layer from below and shifting the temperature profile towards a pseudo-adiabatic lapse rate. The vertical humidity profile shows the typical moist layer at lower levels and relatively dry middle and upper troposphere. The thickness of the St/fog layer is approximately 1-1,5 km.

03 December 1998/13.28 UTC - NOAA RGB image (channel 3, 4 and 5); position of vertical cross section indicated
This example shows cross sections of the previously discussed advection fog/St case of 03 December 1998 over Northern Europe. The following cross sections are taken across the Stratus.
03 December 1998/12.00 UTC - surface pressure, colours: temperature 850 hPa; position of vertical cross section indicated
03 December 1998/12.00 UTC - Vertical cross section; white: Potential temperature, green: relative humidity above 75%, yellow: relative humidity below 75%
The image above left clearly shows a warm air-mass, which is advected from the southwest over Scandinavia. Intensive warm advection is taking place east of Scandinavia (which can be seen as isobars pointing perpendicularly across the isotherms). Intensive cold advection can be seen over the Norwegian Sea. Temperatures at 850 hPa in Scandinavia are well above zero. Surface winds are moderate or fresh within the warm sector.

The image above right shows the typical configuration of isentropes and moisture within the Warm Front near the right (northern) edge of the cross section, whereas within the warm sector itself (Stratus area), the isentropes don't show any frontal characteristics. Air is stable above stratus layer, whereas within the Stratus layer the air is less stable (the radio sounding presented in the previous subchapter shows this better). At the southern (left) edge of the cross section the air is also very stable in the boundary layer due to a strong surface-based temperature inversion, which is caused by longwave cooling of the ground in the absence of clouds. This can be verified by looking at the satellite image, which shows clear conditions near the southern part of the cross section axis. The vertical humidity profile again shows the typical moist layer at lower levels (1-2 kilometres) and relatively dry middle and upper troposphere.


SUB-MENU OF FOG AND STRATUS
KEY PARAMETERS
WEATHER EVENTS