On a flight back from Spain to the UK, we were treated to a great display of “cloud streets” over France.
They form parallel to the wind, like giant windsocks.
“Cloud streets form when the rising warm air that creates the heaped cumulus clouds is partnered with cool sinking air in a very regular way. This sets up distinct lines of cloud downwind of the warm areas, and clear blue sky downwind of the cooler areas. Cloud streets are most likely to form when the shade cast by the clouds falls on the already cooler areas. This reinforces the difference between the cooler and warmer surfaces.
If you see cloud streets, consider what lies in the direction they’re coming from, which will be the same as the wind direction at their level. The lines will point either to a coastline or some other interesting source of warming in that direction.”