While it looked like a scene out of Independence Day, this was no alien invasion.
People in Bondi Beach in Sydney, Australia, were captivated as a massive shelf cloud rolled onto shore ahead of a storm.
- Storm to bring up to 60 millimetres of rain to parts of Ontario, Quebec
- A ferocious Hurricane Simon nears Mexico with 155 mph winds and flooding fears
- Simon strengthens into a Category 4 hurricane as it approaches Mexico’s coast
- Isaias weakens but flash flooding threat persist as hundred of thousands grapple with power outages
Shelf clouds — low, horizontal clouds — are ominous and pretty scary looking as they head toward you. And they do indeed bring unsettled weather: the roiling clouds produce high winds, followed by rain and thunderstorms.
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What we’re looking at when we see one approaching is the difference between winds, in particular downdrafts and updrafts.
Air that has been cooled by rain drops in the downdraft then spreads out when it reaches the surface. The warmer, moist air at the leading edge of the storm rises — this is the gust front — and condenses forming the shelf cloud.
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