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Horizontal convective rolls
Horizontal convective rolls, also known as horizontal roll vortices or cloud streets, are long rolls of counter-rotating air that are oriented approximately parallel to the ground in the planetary boundary layer. Although horizontal convective rolls, also known as cloud streets, have been clearly seen in satellite photographs for the last 30 years, their development is poorly understood due to a lack of observational data. From the ground they appear as rows of cumulus or cumulus-type clouds aligned parallel to the low-level wind. Research has shown these eddies to be significant to the vertical transport of momentum, heat, moisture, and air pollutants within the boundary layer.〔 Cloud streets are usually more or less straight, but rarely cloud streets assume paisley patterns when the wind driving the clouds encounters an obstacle. Those cloud formations are known as von Kármán vortex streets.
== Characteristics ==
Horizontal rolls are counter-rotating vortex rolls that are nearly aligned with the mean wind of the Planetary Boundary Layer (PBL). They can be caused by convection in the presence of a moderate wind〔 and/or dynamic inflection point instabilities in the mean wind profile.〔 Early theory〔〔〔〔〔 on the features predict that the vortices may be aligned up to 30° to the left for stably stratified environments, 18° to the left for neutral environments, and nearly parallel to the mean wind for unstably stratified (convective) environments. This theory has been supported by aircraft observations from several field experiments.〔〔〔
The depth of a vortex is usually the depth of the boundary layer, which is generally on the order of 1–2 km. A vortex pair usually has a lateral to vertical dimension ratio of around 3:1.〔〔〔 Experimental studies have shown that the aspect ratio (a ratio of roll wavelength to boundary layer depth) has been found to vary between 2:1 and 6:1, however, in some situations, the aspect ratio may be as large as 10:1. The lifetime of a convective roll can last from hours to days.〔〔〔〔
If the environmental air is near saturation, condensation may occur in updrafts produced from the vortex rotation. The sinking motion produced between alternating pairs of rolls will evaporate clouds. This, combined with the updrafts, will produce rows of clouds. Glider pilots often use the updrafts produced by cloud streets enabling them to fly straight for long distances, hence the name “cloud streets”.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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