On this weekend’s flight, I saw clouds beneath us that seemed to line up. They were solid or semi-solid strips of white with absolutely clear sky between the parallel lines. In other words, the sky had thick pinstripes on. The pattern stretched far and wide in every direction, but the lines weren’t very tall or thick.
What causes them to line up like that? How do the borders ‘know’ where to be? I can’t imagine how a convection current could do that or how it would have to be oriented to make it happen. So what’s going on here?
The hotlinked image doesn’t work, but I managed to view it. Assuming that you saw cloud streets like that, and not just contrails that had expanded into cirrus clouds, I believe it is due to standing waves being set up in the air due to local topography (moutains etc). The variations in air pressure affect how water condenses out of the vapour phase, creating a regular cloud pattern.
On days with steady wind, over fairly flat and consistent terrain, convection patterns will often organize themselves into stripes of alternating lift and sink. running parallel to the wind direction. Lift tends to beget more of the same, and since there is a downwind drift, a thermal will tend to kick off thermals downwind of itself. These suck air from the adjacent crosswind areas, preventing thermals from forming there.
When the humidity is right, cumulus clouds (puffy, cotton ball looking) form where the lift is. These are known to soaring pilots as “Cloud Streets.” These can extend for tens of miles, allowing a glider pilot to fly in a straight line while maintaining, or even gaining altitude. This can happen on low humidity days when clouds do not form, but it is much more difficult to stay on a “blue” lift street without the clouds showing the way.
If the clouds were lenticular, (smooth, sharply defined lens shapes ) then it was probably due to orthographic wave. Waves form on the lee side of a ridge, and lenticular clouds mark the high spot of each wave. This would form lines perpendicular to the wind direction.
Lenticular clouds http://en.wikipedia.org/wiki/Lenticular_cloud form parellel to terrain features & more or less perpendicular to the prevailing wind. They’re most common downwind of significant (mountain-sized) ridgelines when the wind is more or less perpendicular to the ridgeline. Standing cumulus rotors can also be found downwind of ridgelines, even small ones, and these usually look more like short cloud street formations than the classic lens shape of lenticulars.
Note also that wind direction at altitude where the coulds are can be very different from the wind direction on the surface where you are. So a formation can be tracking downwind up where it is while seeming to be 30 or 45 degrees off the wind direction down where you are.
For an interesting example of what a glider can do in wave conditions marked by lenticular clouds, check out this flight done on Sunday by a friend of mine: 778km (483 miles), mostly at altitudes between 8000 and 11,000 ft (where it must have been really cold).
Note that this is unusual for this area, but quite tame by world standards: wave flights of over 3000km have been done in the lee of the Andes.