Meteorology News

A magnificent and vast display of cloud streets was photographed by NASA’s MODIS satellite this month.  The display was particularly vibrant, with clouds lining up in rows — or “streets” — for hundreds of miles.  These north-to-south oriented rows were caused by an unstable layer of air beneath a more stable layer in the upper levels of the atmosphere.

Cloud Streets photographed over the Gulf of Mexico on October 18, 2009. This display is a prominent example of organized rows of cumulus clouds known as "cloud streets." Image Source: NASA MODIS

Causes:

The best setup for cloud streets is when the lowest levels of the atmosphere – the layer closest to the ground – is unstable.  A layer of air is unstable when the lowest levels of it are warmer than the air immediately above.  This difference in temperature causes the lowest levels to want to rise.  Think of the steam that rises out of a pot of boiling water.  When this warmer air rises, it cools and the moisture within it condenses into cloud droplets.  This is the same process that forms all clouds.

This image of cloud streets is a particularly poignant example of how well-defined the boundary can be between the rising motions (forming the clouds) and the sinking motion (clear air). This image shows cloud streets as viewed from a plane flying just above the unstable air and within the inversion layer.

The difference that enables cloud streets to form is that this unstable layer is covered by a stable layer of air, sometimes called an inversion.  This stable air is essentially a layer that is slightly warmer than the air at the surface and as such, does not allow the unstable surface air to rise through it.  This is like having a lid on the pan of boiling water.  As such, the rising motion generated by the lower unstable layer is capped and is prevented from rising higher.  The rising air then spreads out horizontally and cools.  As it cools and becomes cooler than the environment, that air then begins to sink.  This sinking air is what causes the clear air rows between the clouds.

The formation of cloud streets begins with a low-level unstable layer of air (shown by red) that rises, cools, and condenses to form clouds. This air then cools and descends (blue), generating the alternating rows of clear air.

This cycle of rising and sinking air creates a small “rotor” whereby some air rises and condenses into rows of clouds and other air sinks, generating the alternating rows of clear air. The wind at the surface generally flows parallel to the ’streets’, as shown by the yellow arrow above.

Locations:

One of the most frequent locations where cloud streets are seen is over open water.  Particularly when cold air invades land, such as over continental land masses in the fall months, this cooler air flows off the coast and over much warmer ocean waters.

Cloud Streets form over the Bering Sea as cold air flows from over the snow-covered continental surface (white land mass) and over open ocean waters. The open ocean waters warm the air from beneath, causing rising motion that cools the air, condenses the water droplets, and forms clouds. Image Source: NASA MODIS.

When the cool air flows over the warm ocean waters, the air is warmed from beneath, causing it to rise.  As the air rises, it runs into the inversion layer – the cap – and is forced back down.  This cyclical motion causes the rows to form as evidenced by several images of cloud streets forming over the Gulf of Mexico or the open ocean south of Alaska during the fall months.

Cloud streets off the coast of Alaska. As this image displays, cloud streets often form when cold air flows off of a land mass and over warmer ocean waters. In this image, the wind flow from north to south causes cold air over land to flow over warm ocean waters, generating lift. Image Source: NASA MODIS

Significance:

Cloud streets can be significant not just for their majestic beauty as viewed from either the surface of the earth or from hundreds of miles above the surface via satellites or other spacecraft.

Cloud streets can also provide a very visual depiction of locations of rising air versus sinking air.  This visual cue can be useful for airline pilots who need to be aware of the rising/sinking motions within unstable air masses.  Additionally, recreational pilots can use the visual cues generated by cloud streets to maintain lift for many miles.  When glider pilots can see where the rising motion is located, they can often ride these streams of thermal currents for miles, all the while continuing to rise.