Valleywx.com: The science behind Sunday’s unique cloud formations

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TENNESSEE VALLEY (WHNT)– Some incredible images of cloud streets– technically known as horizontal convective rolls– were sent our way Sunday afternoon.

Cloud streets provide one of the best visual examples of how our atmosphere functions as a fluid. It’s important to understand the concept of the planetary boundary layer (PBL), also known as the atmospheric boundary layer. Simply put, this is the layer of air closest to the ground.

Model sounding showing characteristics of the air closest to the ground Sunday afternoon.
Model sounding showing characteristics of the air closest to the ground Sunday afternoon.

If you’ve read our blog for a long time, you’ve seen an image like the one above before. It is a sounding; it shows temperature, dew point and other characteristics of the lowest levels of the atmosphere. The one above is a model sounding valid Sunday afternoon.

The first thing to stand out is the height of the PBL. This can be seen where the temperature (red line) begins warming with height just below 850 millibars. Another way to confirm this– notice how the wind barbs on the right change from almost due south to westerly in that same region.

This sounding gives us more clues regarding how we should expect air to behave in this environment. The rate at which the temperature cools with height (lapse rate) follows the dry adiabatic lapse rate (about 5.5ºF per 1,000 feet). This means the air has no resistance to begin rising from the ground.

The top of the planetary boundary layer acts as a lid. As the air rises, cools and condenses (forming clouds), it reaches the top of the PBL and is forced back down– creating the horizontal circulation. Where the line of clouds forms denotes rising air, and the clear areas are where the air sinks back toward the ground.

Did you get a picture of the cloud streets Sunday afternoon? Click the submit your photo button below and your photo will be added to the gallery inside this story.