Rolls on Radar!

ButterRollsNo, I’m not talking about dinner rolls though those rolls do look really good. This post is about horizontal convective rolls! These “rolls” in the atmosphere aren’t unusual – we see them all the time! On satellite images you can see these rolls that manifest themselves as cloud streets.



Clouds align themselves parallel to the wind in the well-mixed layer of the atmosphere near the ground known as the boundary layer. Convection in the mixed layer occurs and through a rather complex process convective rolls develop that produce areas of rising and sinking air in parallel bands.

Courtesy: UCAR/COMET

Courtesy: UCAR/COMET

This afternoon skies were almost entirely clear in Connecticut. Where are the rolls here????



While the atmosphere was too try to support cumulus clouds (and cloud streets) the convection currents (thermals) in the atmosphere were still rolling on (pardon the pun) behind a cold front that passed this morning.

Echoes showed up all afternoon on radar that were aligned parallel to the mean wind in the boundary layer much like you’d see with clouds on satellite.



Dual pol (in this case correlation coefficient) confirms to us that these echoes are not sprinkles but are non-meteorological in nature. Likely, in this case, bugs that are being picked up by the convection currents and lofted in the air. Radar showed this feature several thousand feet above the ground.

I zoomed the radar in a bit and you can actually see two cool things. One feature is the aforementioned “bug-streets” over the Naugatuck and Housatonic River valleys and the other feature being tons and tons of birds over Westbrook, Old Saybrook, Old Lyme and East Lyme. These birds are present on radar virtually daily this time of year prior to sunset (blog post on the Connecticut River tree swallows is here).



Weather is always cool – but I thought this was particularly neat. I’ve also managed to coin a new term – “bug streets”. Let’s see if it catches on.


One thought on “Rolls on Radar!

  1. Could you say a bit more about the process that forms these cloud streets? There is no sheer involved, right? I had always imagined that stratocumulous rolls formed when a faster-moving higher current moved over a slower moving boundary layer, like waves on the ocean. But waves are perpendicular to the wind that makes them, more or less, so I guess that dumps that theory. So, why do they occur?


    Nicholas S. Thompson

    Emeritus Professor of Psychology and Biology

    Clark University

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