A strange-looking cloud formation has been puzzling residents living in northern California.
The clouds appeared to settle in such a way that it appeared there was a giant hole in the sky close to San Francisco on Friday morning.
The phenomenon was seen mainly in Bay Area cities such as San Bruno, San Mateo, and Burlingame.
The strange-looking clouds led to people snapping photos of the weird-looking clouds and posting them on social media.
The San Francisco Bay Area sky featured unusual cloud formations early on Friday morning
Several people pointed out the strange shapes in the sky on social media and posted pictures online
The cloud formations are called fallstreaks and are also known as ‘hole punch clouds’
Something, like a plane, travels through the cloud layer and disrupts the pressure and humidity within that layer
When a plane passes through, it causes the super-cooled water and ice crystals to evaporate and sublimate as they fall from the cloudy layer. This creates the brush-like streaks as seen in the holes above
Experts say the stunning clouds that look like brushstrokes or seashells – even apt for the coastal region – are caused by a rare but rather ordinary atmospheric occurrence.
Chief Meteorologist from KRON4 Lawrence Karnow says the formation is known as a ‘fallstreak hole’ or ‘hole punch cloud.’
The clouds can form when water droplets are colder than freezing temperature but have yet to freeze.
The ‘supercooled’ water droplets need a ‘reason to freeze’ which usually happens when ice crystals pass by.
As the moisture falls it leaves dry air behind which produces the hole, and in some cases, an elliptical opening in the cloud
Many noted on social media that there were ‘Spectacular Circles in the clouds’. One person asked if it was an alien portal
The National Weather Service explains fallstreaks occur when the water temperature in a cloud is below freezing, but the tiny water droplets forming the cloud have yet to freeze
The clouds took the form of a giant wispy hole in the sky and had many people wondering what they were exactly
A fallstreak hole forms when part of the cloud layer forms ice crystals which are large enough to fall as a ‘fallstreak’.
They form in clouds of supercooled water droplets – water below 0°C but not yet frozen.
These water droplets need a tiny particle to freeze or to be cooled below -40 °C.
Aircraft punching through this cloud layer can cause air to expand and cool as it passes over the aircraft wings or propeller.
This change in temperature can be enough to encourage the supercooled droplets to freeze and fall from the cloud layer in this distinctive pattern
Planes travelling through the clouds can often bring these ice crystals with them.
Once the crystals are introduced, the water droplets then quickly freeze and start to fall leaving behind a hole.
The wispy clouds that form in the center of the hole are the water particles falling.
The hole then starts to expand outwards as neighboring droplets start to freeze.
Passing airplanes are thought to be the reason behind the formation of these clouds, since a drop in pressure caused by their wings or propeller tips cools the air quickly.
Rainbows are sometimes seen with fallstreak clouds, thanks to light refracting off the water droplets.
No matter the science, the clouds are extremely pretty and make for some interesting Instagram weather pictures, and a change from the storm clouds being witnessed on the East Coast with Hurricane Florence.
The clouds were so high up that people over a vast area were able to spot them
The clouds appeared to last for several minutes as the formations held steady giving people enough time to snap pictures
Because the clouds were over a highly populated area, the formation was photographed from various different angles
The viewing conditions were absolutely perfect with not much else in the way of cloud cover that might obscure the phenom
From the city, and out into the countryside, No matter the location, the clouds made for an unusual and welcome sight