A rainbow in a dream typically arrives after some form of hardship, in the dream or in your waking life, and often signals hope and a sense that reconciliation is possible. Seeing one appear right after a storm can point to relief following a difficult period, a reward you feel you've honestly earned.
In summer, when the Sun enters the terms of Mars, heat is caused; in winter, drought, and scarcity of rain and waters. - William Lilly (1602.-1681.)