This is because rainbows form in circles as that is how the eye interprets light. As the distance between light And the ground is increased, more of the circle may be shown.
I'VE KNOWN HOW THIS WORKS FOR LIKE TWO YEARS YOU DON'T NEED TO TALL ME I AIN'T NO DUMB *SS I JUST HAVE BAD GRAMMAR, AND THEY DON'T FORM IN CIRCLES, YOU SEE IF YOUR ON THE GROUND LIGHT CAN ONLY BE REFLECTED DOWN AT YOU FROM THE RAIN DROPS, BUT WHEN YOUR IN THE AIR THE LIGHT CAN BE REFLECTED AT YOU FROM ALL ANGLES, GOD MAN LEARN YOUR RAINBOWS