What causes this?

[AuntPeggy]

A friend sent this photo taken with his cell phone. He said, "I can't imagine how these things do this. Sometimes they're much more delineated, like rows of cabbages."

So, what causes clouds to line up like a field of cabbages?

 

[tonycondon]

Wind. Glider pilots call them cloud streets and they are our highways in the sky. All the thermals are lined up and all the sink is inbetween, so you can just fly straight, underneath the clouds, at high speed with no circling. for miles and miles and miles. the level of moisture determines if it is a sold cloud or individual clouds in a line.

http://www.faa.gov/library/manuals/aircraft/glider_handbook/media/faa-h-8083-13.pdf
page 9-11 is where the discussion on cloud streets start.

Power pilots - you can use cloud streets to increase your speed as well. It may be a little bumpy but you will be in continuously rising air so you will have to push the nose down further to hold altitude, therefore you will go faster at a given power setting. Use the atmosphere!

 

[gkainz]

Tony, not following your explanation. Do you fly perpendicular to the cloud lines or parallel to the lines? If parallel, then under the cloud or under the clear?

 

[tonycondon]

under the clouds, parallel to the lines. the wind underneath the clouds will be blowing in the direction the clouds are lining up which will significantly increase effective glide ratio. the clear contains all the sinking air so we try to avoid that.

 

[JoeSelch]

Wind. Glider pilots call them cloud streets and they are our highways in the sky. All the thermals are lined up and all the sink is inbetween, so you can just fly straight, underneath the clouds, at high speed with no circling. for miles and miles and miles. the level of moisture determines if it is a sold cloud or individual clouds in a line.

http://www.faa.gov/library/manuals/aircraft/glider_handbook/media/faa-h-8083-13.pdf
page 9-11 is where the discussion on cloud streets start.

Power pilots - you can use cloud streets to increase your speed as well. It may be a little bumpy but you will be in continuously rising air so you will have to push the nose down further to hold altitude, therefore you will go faster at a given power setting. Use the atmosphere!

Now that's one of the most interesting and helpful things I've read in a long time!

(Thanks for asking, Aunt Peggy, and for answering, Tony.)

 

[AuntPeggy]

Thanks.
Another questions.

Since the wind increase is 10 to 20 knots along the street, if my direction of flight is the opposite direction to the wind or perpendicular, it would be best to fly above the cabbage rows.

 

[fgcason]

Altocumulus undulatus

It usually indicates an approaching frontal system into the area. IIRC there is usually a weak windshear over a large area associated with the clouds. I think that's what causes them to be aligned so well. I associate them with how ripples in sand occur in very shallow water on a beach...it's different however some of the dynamics are kind of the same.

Be careful though. They're not usually hazardous themselves however the cloud rows can expand and zip you up VFR on top. I've had to get down a couple times before they trapped me on top and by the time I landed, it was a solid overcast. About 30 minutes from wide open to closed solid.

 

[tonycondon]

Thanks.
Another questions.

Since the wind increase is 10 to 20 knots along the street, if my direction of flight is the opposite direction to the wind or perpendicular, it would be best to fly above the cabbage rows.

yea probably, depending on the wind conditions above the streets. as you can see from the diagram in the glider flying handbook, there is usually a wind shift above the clouds that is generally perpendicular to the streets. It could be better to bounce a long crosswind under the clouds than be into the wind above them. although it will probably be smoother up there.

Frank brings up a good point. Knowing the moisture levels will help you determine if the sky will overdevelop.

Also, this same phenomenon can occur without clouds. On "blue" days, as us glider pilots call them, we always search up/downwind of known thermals in search for others.

 

[AuntPeggy]

Thanks for the information.

 

[wby0nder]

Cloud street/wave formations form from various causes. A lot depends on the altitude. The classic thermal streets that Tony talks about are an interaction between the low level convection and winds above. Other higher types of wave streets over flat land form from wind shear alone with no interaction with lower level convection.

Other types of waves and cloud streets over flat land can form from small fronts or out flow from thunderstorms. The Morning Glory wave in AU is an example of solitary atmosperic wave. See:
http://www.abc.net.au/science/scribblygum/august2003/

I once saw a morning glory like wave cross central iowa on a nice sunny day. It was otherworldly and spanned the sky from horizon to horizon. Probably formed from a very distant thunderstorm outflow.

 

[etsisk]

THAT is just an amazing thing! Great article - thanks for the link. I'd love to fly one of those things!

 

[wby0nder]

THAT is just an amazing thing! Great article - thanks for the link. I'd love to fly one of those things!

Glad you enjoyed. I too would love to fly the morning glory. It's a tough thing because as it moves inland it covers really remote areas with almost no roads/civilization. It's a one way trip and the wave dies out inland. You almost have to have a motorglider and a good one at that. Engine fails to restart and you are in a bad situation.

I keep waiting to hear about someone riding it across the gulf either accidently or on purpose. That would be incredible.