Pic of the week: Green monster swallows the sky over Pennsylvania
Cue the greenage! It’s midsummer, which means daily afternoon thunderstorms that fill the sky with dark clouds, heavy rain, and scenes like this one out of Pennsylvania. While we may be out of the primary severe season that typically lasts from April through June, Mother Nature continues to prove you don’t need a severe or supercell thunderstorm to produce impressive views and storm structure.
Photographer Ray Leichner took this epic shot of a shelf cloud with distant lightning engulfing Bucks County, Pa. A shelf cloud indicates the leading edge of a thunderstorm, signaling the push of rain cooled air as it surges forward. They can be tens of miles long and arc across the entire horizon.
When it comes to shelf clouds, meteorologists use the term “the worst is first.” When the initial “shelf” screams overhead that’s when the strongest winds and even weak, short-lived tornadoes known as gustnadoes occur. After the initial blast of wind, the rain (and sometimes hail) follows.
The photograph above illustrates the anatomy of a shelf cloud perfectly. The leading edge is evident by the shelflike appearance to the cloud, which slopes down and away from the rain. The rain curtain behind the shelf cloud is hard to miss, soaking the landscape beneath it. Finally, a bonus cloud-to-ground lightning strike shines in the distance.
So what causes the green color? While the exact cause is still uncertain, it is likely caused by the combo of time of day and density of water droplets within the storm. For years, it was thought the green color was caused by hail within the storm, but now it is likely density of the water vapor no matter the form. Green storms are most likely later in the day when the lower angle of the sun interacts with the densely packed water molecules inside the storm.
Read more at Washington Post