Answer:
Fairly uncommon.
Step-by-step explanation:
For a rainbow to occur, the ray of sun that hits the drops of rain needs to be between 40 - 42 degrees celsius, or a bit below that.
If the sunlight is higher than 42 degrees C, a rainbow cannot (really rarely) occur.
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Now that we know what the conditions for a rainbow to form are, we can affirm that rainbows are actually quite uncommon.
Hope it helped,
BioTeacher101