"Kate Brown" wrote ...
Here I am in SW France with a splendid view towards the east,
perfect for cloud spotting. Lots of very beautiful cumulonimbus
rising over the Auvergne due east and the Limousin (hi Colin!) to
the north-east in the last few days. And also cloud streets.
Given that the wind has been quite feeble here and it's often been
difficult to judge which way the clouds are actually moving, can
one tell from the direction of the line of clouds? It seems to be
straight along the line of battle, so to speak, but is this a
reliable rule of thumb?

"George Booth" wrote ...
I have always found cloud streets to be aligned within a few degrees
of wind direction at cloud level. Easy to interpret looking up from
the ground but less clear the higher you get. At least that's my
excuse.

.... cloud streets are, as George states, broadly aligned with the mean
wind through the convective layer (at surface I assume the wind was
light/variable).

Not sure what you were looking at yesterday (3rd), but if Bordeaux is
representative, then the mean wind direction was roughly WSW'ly - does
that fit in?

The subject of streets in cloud layers etc., is fascinating - beyond
my full understanding I'm afraid, but I do remember that you need some
sort of *speed* shear (increase) with height, but not too much, and
little or no *directional* shear (change of direction). Anything else,
and the cloud elements tend to be arranged haphazardly, and you don't
get the 'roll' dynamics which get translated to updraughts and
downdraughts to arrange the cloud as it forms. I believe that such
'rolls' are now modelled explicitly in some NWP suites as they are
important in the vertical transport of moisture, momentum etc.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023