Stunning photos have emerged on social media showing the Severn Estuary draped in a blanket of cloud.
The aerial photos, taken by the South West branch of the National Police Air Service, were captured early on Tuesday morning.
The surprising sight, though not particularly rare, was caused by a weather phenomenon known as a temperature inversion.
And the sea of clouds, which could also be seen on traffic cameras in the area, captivated hundreds on social media.
One person wrote: "Fascinating picture and what a view from above."
Another said: "This was surreal, especially driving through it."
A third joked: "Wonderful photos; we didn't notice the nuclear strike in Cardiff though!"
Stunning video captures clouds over Welsh valley
What caused the inversion?
The lowest part of our atmosphere, known as the troposphere, typically sees air temperatures drop in its higher elevations.
This is the reason that a mountain top is typically colder than the air at its base.
But a temperature inversion occurs when cold air instead gets trapped beneath warm air in the troposphere, acting like a lid for clouds.
WATCH: Met Office national morning forecast
According to the Met Office: "This often happens in areas of high pressure, where the air high up often sinks towards the ground.
"As it falls, it dries out and warms up. This warm layer of air can act as a lid and trap cooler air near the surface. This is because warm air is more buoyant than cold air, and so it will tend to 'float' above the colder air, trapping it.
"This gives us the inversion, because if you were now to climb the mountain, it would get warmer as you got to the top. This is inverted compared to what you would normally expect, hence the term 'inversion'."
Though the phenomenon can happen all year round, it's most likely to occur during the winter.
