Many of us might count ourselves lucky when we catch a glimpse of a 'regular' arc-shaped rainbow, so how would you feel if you saw a full 360-degree circular rainbow seemingly hanging in the sky right in front of you?

This amazing apparition was captured by Brit Bernie Welch from Portsmouth on the south coast of England, who witnessed the rare sight thanks to his elevated location – standing on the observation deck of the city's 170-metre-high (560-foot) Spinnaker Tower.

For Welch, who acts as the tower's technical manager, it was just another day at work, until he happened to notice the optical illusion encircling the harbour above and below him.

"I arrived at work at about 7.30am and I was with another colleague putting up Christmas decorations on view deck 1," he told Harry Pettit at the Daily Mail.

"I happened to be in the right place at the right time, as when I looked up I spotted the circular rainbow."

While it's rare to actually see a complete circular rainbow, the conditions for them to form are exactly the same as for arc-shaped, semi-circular rainbows – it's just a question of perspective.

Usually when we see rainbows, we're looking at them from the ground or fairly close to it.

Of course, rainbows as we perceive them don't actually exist in a physical form. They're an optical illusion produced by white light refracting (bending) through tiny spheres of moisture in the atmosphere, such as rain.

This illusion is visible to the human eye only under very specific conditions – when you face away from the Sun, towards the water droplets, then shift your perspective by turning 42 degrees to the side.

At that position, the different wavelengths of light making up the rays refract back from the water droplets at slightly different angles, which we perceive as curved bands of individual colours.

If you're at the right altitude, it's possible for this illusion to appear both above and below you – it's just that we don't normally get a chance to see rainbows like that, because we're stuck on terra firma, with the ground getting in the way of the droplets necessary to see the whole light show.

But if the conditions are perfect, with the right angle, level of elevation, and amount of atmospheric moisture hanging in the air, you can perceive a full, circular rainbow like Welch – if you're lucky, that is.

"Although I've heard of a circular rainbow being seen from the Spinnaker Tower once before, in the 11 years I've worked here since the tower first opened, I've never seen anything like it myself," Welch said.

"I feel so lucky and fortunate to have been able to see this."

For an even more surreal demonstration of a circular rainbow, check out this amazing video that shows the same phenomenon during skydiving:

And drones also get a pretty spectacular glimpse of the 360-degree effect, as you can see below.