According to William Collins, a climate professor at the University of Reading in England, pollution adds a layer of haze to the air, making the sky look paler than it really is.
He says: "The absence of traffic will be having an effect. The current skies are the kind of blue you'd expect to see on a nice tropical island somewhere. Everybody's been noticing it."
Another expert (in atmospheric chemistry), Professor James Lee from the University of York said: "Since the lockdown, there has been quite a large drop in nitrogen dioxide almost uniformly in every city that we've looked at by 30 to 40%."
In some of the major cities in the UK and the USA, views which were once hindered by thick layers of pollution have become visible once again.
Elsewhere around the world, lower pollution levels have also resulted in India's skyline looking bluer for the first time in years, while a decline in carbon emissions over in China has cleared the air there, too
Blue skies can help to boost our moods
"Mr. Blue Sky, please tell us why
You had to hide away for so long (So long)
Where did we go wrong?
Hey there, Mr. Blue (Sky)
We're so pleased to be with you (Sky)
Look around, see what you do (Blue)
Everybody smiles at you"