CLOUD STREETS

by KNMI


UNDER CONSTRUCTION


A Cloud Street is a type of organized convection. It has the form of an extended line of cumulus cloud almost parallel to the wind direction, often in an otherwise lightly cloudy sky. Various sources of thermals spaced across the wind may give rise to parallel Cloud Streets. It appears that such Streets may also be produced in an air mass in which the convection layer has a well-marked top and in which the wind direction is almost constant. A cross section normal to the wind direction (see diagram below) shows the pattern of vertical movements.
Cross section of Cloud Streets shows the patterns of vertical circulation.

Requirements for development

Temperature and wind velocity profile for Cloud Streets.

Spacing and alignment of Cloud Streets

The distance between adjacent Streets has been observed as approximately three times the height of the inversion or stable layer. Cumulus streets are aligned parallel to, or within a few degrees of, the direction of the wind in the convective layer. Bends in the wind flow are often indicated by bends in the Cloud Streets. A single line of cumuli often extends to more than 100 km downwind; the entire field may extend over 100 km downwind and has been observed to have a width in excess of 500 km. On very high resolution satellite pictures up to 100 nearly parallel lines of cumuli have been observed.

Synoptic situation

The synoptic conditions for the horizontal pressure and temperature distribution within the convective layer are shown in the figure below. The direction of the isotherms compared with the direction of surface isobars indicates cold air advection. This results in a thermal wind component which reduces the pressure gradient aloft. In this way the requirement for a curved wind profile is fulfilled. Cloud Streets most frequently occur in strong cold air outbreaks, but they are also observed in a fresh inflow of moist warm air. The convective boundary layer in which they occur may be quite shallow, rarely more than 2 km deep. If there is vertical shear in both wind speed and direction, thermal waves will form over the Cloud Streets.
Pattern of isobars at the surface and isotherms representing the whole convective layer favourable for Cloud Street development.
09 March 1998/12.58 UTC - NOAA RGB image (channel 1, 2 and 4)
Example of RGB image with Cloud Streets.

Weather Events

Parameter Description
Precipitation
  • Generally no significant precipitation.
  • Further downwind individual cells along the Cloud Street occasionally growing to Cb size and producing rain or snow showers.
Temperature
  • No significant change.
Wind (incl. gusts)
  • Weak to moderate (10-15 kt) surface winds.
Other relevant information  

References


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