Which atmospheric phenomenon describes the time when day and night are of equal length?

The atmospheric phenomenon that describes the time when day and night are of equal length is the autumnal equinox. During the equinoxes, which occur in both spring and autumn, the tilt of the Earth is such that the sun is directly above the equator. This results in nearly equal amounts of daylight and darkness across the globe.

The autumnal equinox specifically takes place around September 22 or 23 in the Northern Hemisphere, marking the transition into fall. The spring equinox, on the other hand, occurs around March 20 or 21 and also features equal day and night lengths, but this event pertains to the change of seasons from winter to spring.

The summer solstice and winter solstice are different events, marked by the longest day and the shortest day of the year, respectively. Thus, the autumnal equinox is the correct choice, as it is specifically associated with the equal length of day and night.