• Warm advection (WA):
  The field of temperature advection is characterized by a juxtaposition of a WA maximum within the cloud bulge and a CA maximum in the dry air behind. This indicates the circulation of air masses within the deepening low (see Meteorological physical background ). In reality the WA maximum can be rather weak while usually a pronounced CA maximum exists behind the cloud bulge. The zero line of the temperature advection passes through the bulge and should mark the centre of the Wave.
• Positive vorticity advection (PVA):
  The cloud bulge is superimposed by a PVA maximum in the higher levels of the troposphere (500 and 300 hPa), indicating the deepening of a low.
  Both (WA and PVA) are responsible for the production of the increased cloudiness of the Wave bulge.
• Absolute topography at 1000 and 500 hPa:
  In the absolute topography at lower levels of the troposphere (for instance at 1000 hPa) a low pressure area can be observed which often manifests only as a pronounced trough but sometimes shows already a weak closed circulation intensifying during the life cycle. In upper levels, as for instance at 500 hPa, usually a large scale trough can be observed with cloudiness in the south-western stream.
• Isentropic potential vorticity (IPV):
  According to Hoskins (1986) anomalies of IPV representing very dry stratospheric air can be regarded as being connnected with cyclogenesis. The approach of an IPV anomaly at higher levels in the direction of a low level baroclinic zone may cause cyclogenesis. From present knowledge the Waves described in this chapter are not a typical example for such a development, but very often an anomaly develops upstream on the rear side of the Wave at higher levels around approximately 300 hPa; this is accompanied by the development of a black area in the WV image.

This case shows two Waves: a southern one at approximately 52N/20W (see Meteorological physical background) and a northern one between Greenland and Iceland. For both waves a very classical distribution of the TA field can be recognized with much more cold advection for the northern Wave. The surface layer shows pronounced troughs of which the northern one is already deeper. The field of PVA shows a pronounced maximum for the northern Wave but two separated maxima for the southern one. The latter are correlated with two small scale troughs within the upper levels of 500 hPa.
The last two images contain isentropic potential vorticity lines (in IPV units) on two different isentropic surfaces. The first one (310K) shows in the area of the southern Wave tropospheric air at heights between 500 and 400 hPa, while for the northern Wave stratospheric air has protruded downward as far as 300 hPa. On the 320K surface the same can be seen for the southern Wave.