I understand how rainbows are formed. Light reflecting on raindrops (that act like prisms). Shouldn’t that always happen if it rains?
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Rainbows need lots of tiny water droplets suspended in the air.
That happens fairly often.
In addition to water in the air you also need the right geometry for the light.
The center of the rainbow is always directly on the opposite side of your head from the sun.
This means rainbows need the sun to be somewhat low near the horizon. That only happens at certain times of the day.
If the sun is too high up it won’t work.
Only when the sun is low can it shine from behind you to make a rainbow appear.
The sun rises in the east and sets in the west.
This limits when and where rainbows can appear.
No amount of water droplets in the air south of you will make a rainbow appear if the sun is southwest of you or directly above you.
Of course if you have a strong enough artificial light source you can make a rainbow appear anytime and anywhere here is enough water in the air.
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