Key Parameters

  • Surface Wind
    Surface winds have to be moderate to strong, in all cases the surface winds are between 10-20 kt. Furthermore, the surface wind is aligned with the left hand side of the Cloud Streets.
  • Height Contours 1000 hPa/Geostrophic wind
    The geostrophic wind is usually aligned with the right hand side of the Cloud Streets, this can also be shown by the height contours at 1000 hPa because the geostrophic wind is orientated parallel to the height contours.
  • Instability/laps rate
    In the layer in which the Cloud Streets form the whole profile has an unstable character
  • Height of the Unstable Layer
    Generally, the depth of the unstable layer has a height of 1.5-2.0 km, and is usually capped by a sharp inversion. The best way to identify this is by using radiosonde plots.
  • Wind profile in unstable layer
    The wind direction often shows a curved profile: in the lowest part of the unstable layer the winds are veering, from the mid-level the winds are backing again. The variation in direction is, however, not very large and usually the winds in the unstable layer vary no more than 20-30 degrees. The wind speed should increase with height to a maximum of at least 20 kt in the middle or upper part of the convection layer. Above these levels the wind may decrease or increase. Numerical prediction models do not represent this small scale variation of the wind well because of their coarser resolution.
  • Temperature Advection 1000/850 hPa
    Cloud Streets usually develop under slight cold advection, which often occurs to the rear of a Cold Front. Usually the advection shows a maximum near the middle of the unstable layer.
  • Height Contours
    Usually the height contours are rather straight or slightly anticyclonically curved. In cases over the sea, the most developed and extensive area of Cloud Streets are located upstream from the upper-level trough.
  • NVA 500 hPa
    In most cases, because of the influence of a building or approaching upper ridge, NVA is found.

Surface Wind
23 August 2004/21.00 UTC - Meteosat Hires VIS image; green: surface wind
24 February 2004/12.00 UTC - Meteosat 8 Hires VIS image; green: surface wind
23 August 2004/12.00 UTC - Meteosat 8 Hires VIS image; magenta: geostrophic wind
24 February 2004/12.00 UTC - Meteosat 8 Hires VIS image; magenta: Height Contours 1000 hPa
23 August /12.00 UTC - Tephigram Bordeaux

Height (hPa) Wind direction (Deg) Wind speed (kt)
1012 285 10
1000 295 11
925 300 12
910 290 13
890 265 20
850 220 25

Temperature Advection 1000/850 hPa
24 February 2004/12.00 UTC - Meteosat 8 Hires VIS image; blue: Height Contours 500 hPa

Height Contours and NVA
23 August 2004/12.00 UTC - Meteosat 8 Hires VIS image; blue: Negative temperature advection 1000-500 hPa
24 February 2004/12.00 UTC - Meteosat 8 Hires VIS image; green: PVA