The daylight sensor is still active during rain

The daylight sensor gives off a redstone power level dependent on the amount of sunlight. During the day, it gives full power, and at night it gives nothing, but at sunset and sunrise it gives off a weaker redstone signal. This is enough to activate the lamp.

Rain does not affect the sensor at all, only the position of the sun.

This issue has actually already been reported at MC-102721, which has been marked as Working as Intended for the reasons explained above.