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Name: Jules
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Question:
Sometimes at sunrise I have noticed that altostratus clouds, which the sun is rising behind, will have very broad almost "straight edges" to them as they go nearly across the sky from northern horizon to southern horizon. The cloud mass will often go all the way to the eastern horizon. Is this seemingly straight edge the result of high winds aloft that "cut" the edge of the cloud formation and give it an end so to speak. Sorry if my question is sort of vague, but I find the shape the patterns are hard to describe and have not found explanations in meteorological texts. I also find them beautiful and fascinating as the early rays of colorful light project behind them. I have also seen status clouds from underneath that have similar straight edges within them or at their beginnings going very long distances across the sky, which I used to think were visible evidence of approaching fronts, but once I read that cold/warm fronts do not do show up as demarcations in the sky per-se. Basically what causes the defining edges of stratus type clouds?


Replies:
Do you live west of the mountains, or a row of mountains? If so, you may be seeing a line of cloud caused by the mountains deflecting moist air upwards. The clouds would appear to hang over the ridges until the sun warmed the air to above its dew point. Then the clouds would disappear.

R. W. "Bob" Avakian
Instructor
Oklahoma State Univ. Inst. of Technology


Jules,

Straight edges of stratus and altostratus clouds are fairly common and usually indicate a very strong demarcation between air masses, especially at the rear of a receding cold front with very dry air plunging down from the north behind the cold front (and dropping rapidly in altitude, thereby aquelching the lifting of air that produces condensation and clouds).

These kinds of edges can also be seen at the front of an advancing squall line in the summer, but it sounds like what you have observed occurred during winter.

Straight edges within stratus clouds may be an indication of wave motion, which occurs at all levels of the atmosphere and is most easily detected when clouds are present. Wave motion of amazing consistency, width, and duration produces undulating patterns that are beautiful and extensive.

Another physical cause of such edges could be a very long wavelength wave that lifts an expanse of air, resulting in a cloud with a sharp edge in opposite directions; this is more likely for altostratus than for stratus.

Cold/warm fronts may not normally have clear demarcation lines at the leading edge, but can more commonly have such an edge behind the front; this is especially true for cold fronts.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory


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