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February 2018

February 2018

Cloud Rush Hour

Cumulus clouds have to put up with tailbacks just like we do. Bumper-to-bumper low Cumulus like these spotted over Union, Indiana, US by Beth Fluto (Member 44,081) are known as ‘cloud streets’. Their more official classification would be Cumulus radiatus, on account of how the long lines of clouds, when viewed from below, appear to radiate from a point on the horizon due to the effect of perspective.

Cloud streets are always aligned with the wind. They run parallel to the wind direction. This differentiates cloud streets from the similar-looking low formation known as Stratocumulus undulatus, clouds which always run perpendicular to the wind. So why do these two distinctive cloud formations line up in different directions? Because the two clouds patterns, though similar in appearance, develop in quite different ways.

The lines of Stratocumulus undulatus clouds run perpendicular to the flow of air because they develop in what are known as ‘shearing winds’. This is when the windspeed increases markedly with height. It can lead to a sort of ‘ruffling’ of the air between, coaxing it into wave-like ridges of rising and dipping air, like a rug bunching up as it is slid along the floor. Not only are the lines of these clouds perpendicular to the wind, they also tend to appear as more continuous stripes of cloud compared with the serried ranks of individual Cumulus in cloud streets.

Unlike the clouds streets in February’s Cloud of the Month, these Stratocumulus undulatus clouds are aligned perpendicular to the wind. When Emily Painter spotted them over Roanoak, Virginia, US, she was facing into the wind.

Why, then, might Cumulus clouds arrange themselves into ‘streets’ like February’s Cloud of the Month? These clouds form on invisible thermals of air rising off the sun-warmed ground, and when the Cumulus are small like these the change in air temperature with altitude is acting as an invisible lid on the rising of these thermals. Known as a temperature inversion, this stops the Cumulus growing much because the air doesn’t float up far. Beneath the invisible lid of the air temperature, the currents rising from the warm ground arrange themselves into a pattern of lifting thermals, where clouds form, separated by sinking air where they don’t. Add in a broad and even flow of wind, and the pattern of rising and sinking columns of air can arrange itself into rows lined-up with the air flow. This pattern of cloud streets forms more typically over coastal waters in winter as cold air flows from very cold land over warmer seas.

While the clouds look as if they’re crawling along in the worst multi-lane tailback imaginable, remember that fair-weather Cumulus are the most lighthearted and friendly of cloud types. As a result, incidents of cloud-street rage are almost unheard of.

Cumulus radiatus, or cloud streets, spotted over Union, Indiana, US by Beth Fluto (Member 44,081).

1 Comment
  • Dianne Horvath avatar


    October 19, 2018at10:55 pm

    The photos are amazing, thank you. Always learning.

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