Key Parameters

  • Height contours 500 hPa:
    In the initial stage there is an upper trough. The bottom of the trough forms an inverse omega shape and is detached from the main stream forming a separate low. In the final stage the low weakens and merges with another trough or low.
  • Temperature 500 hPa:
    The air within the Upper Level Low is colder than in the surroundings. The isotherms shows a life cycle similar to that of the geopotential.
  • Height contours 1000 hPa:
    Usually loose field with no remarkable features. In some cases some weak cyclonic circulation may appear.
  • Equivalent thickness:
    There is a thickness ridge ahead of the low and a trough or a distinct minimum behind or in the centre of the low.
  • Thermal front parameter:
    There are two baroclinic zones:
    • A frontal-like cloud band ahead of the low
    • A Baroclinic Boundary (see Baroclinic Boundary ) behind the low.
  • Potential vorticity:
    Within the Upper Level Low there is colder air with lower tropopause than in its surroundings. Consequently, in the area of the low there is a local maximum of PV.

Height contours and temperature 500 hPa

1. Trough stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
14 September 2005/06.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

2. Tear-off stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
14 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

3. Cut-off stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

4. Final stage

Cyan: height contours 500 hPa, dark green: temperature 500 hPa
16 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; cyan: height contours 500 hPa, dark green: temperature 500 hPa

Height countours at 1000 hPa

Magenta: height contours 500 hPa, cyan: height contours 1000 hPa
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; magenta: height contours 500 hPa, cyan: height contours 1000 hPa

Equivalent thickness and thermal frontal parameter

green: equivalent thickness, blue: front
15 September 2005/12.00 UTC - Meteosat 8 IR 10.8 image; green: equivalent thickness, blue: thermal frontal parameter

Potential vorticity

magenta: PV
15 September 2005/12.00 UTC - Meteosat 8 WV 6.2 image; magenta: height of PV=1 unit