The rare sight was filmed by Bernie Welch, who is the Technical Manager at the Emirates Spinnaker Tower in Portsmouth.
When sunlight and raindrops combine to make a rainbow, they can make a whole circle of light in the sky. The bottom part of a full-circle rainbow is usually blocked by your horizon. and this is why we see rainbows not as circles, but as arcs across our sky. However, if you are viewing from up high and look at the horizon at a certain angle you can see the whole circle. And with the tower being at a height of 170 metres above sea level, Bernie was in the right place at the right time!