• Temperature Advection is a primary parameter for determining a CF in CA; but it often varies with height. Several types can be discriminated:
  • The zero line of temperature advection is far in advance throughout all levels (i.e. there is cold advection everywhere). This is the most frequent situation, especially with fronts which are characterised by only low level clouds.

    	13 January 1999/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

  • The zero line of temperature advection approaches the front near the surface, above it is more in advance and is backbent at higher levels. Cold advection is farthest in advance at levels around 700 hPa. As there is WA below and CA above this is a typical characteristic for an unstable stratification.

    	28 January 1999/06.00 UTC - Vertical cross section; black: isentropes (ThetaE), red thick: temperature advection - WA, red thin: temperature advection - CA, orange thin: IR pixel values, orange thick: WV pixel values

  • The zero line of temperature advection is curved the other way round (nearest point at about 700 hPa, quite far in advance at the surface and at high levels). This is only found in a few cases.

All typical parameters seen in vertical cross sections are now illustrated with help of one case; the position of the vertical cross section line is indicated below.

This vertical cross section shows the typically Cold Front - like inclined zone of isentropes (crowding zone at the frontal surface), and widespread CA throughout the front and surrounding region, although there are some areas of WA embedded.

• Advection of the specific humidity is positive (moist) in front of the front and often negative (dry) within and behind. The maxima are usually found at levels of 850 and 700 hPa. Due to the decrease of temperature with height, the absolute values are also decreasing.
  (Note: In the following chapters negative humidity advection is called dry advection and positive humidity advection is called moist advection.).

The position of the zero line in relation to the frontal cloud band and TA can be used as a tool to determine whether further development is possible (see below)

The combination of temperature advection with humidity advection shows the following situation: Far in front of the CF in CA there is WA together with moist advection; then, closer to the CF in CA, there is a zone of cold but still moist advection which is most pronounced in levels between 800 and 500 hPa. Below the frontal surface CA and Dry Advection prevails. Therefore a main characteristic of a CF in CA is the difference in moisture between the two air masses involved, which might be even more significant than temperature (see Cold Front In Warm Advection - Typical appearance in vertical cross section ).

• Upward motion dominates the front at the leading edge. To the rear, there is less upwards motion or even downward motion.

The vertical cross sections are an effective tool for the forecaster: fronts which show the combination of cold advection throughout all levels and a cloud band which is dominated by negative humidity advection are usually (in 85%) decaying even if there is quite strong upwards motion at the time of observation.

The decay of a front under the influence of cold and dry advection, as described above, is illustrated below.

The front is completely under cold and dry advection. The sequence of the two images show the decay of frontal cloudiness.