An index of the activity of a meteor shower. The ZHR gives the activity in meteors per hour which would be expected by an experienced visual observer under a perfectly clear sky (limiting magnitude +6.5) with the radiant overhead. In practice, observed rates are almost always lower than the derived ZHR due both to sky conditions and the fact that the radiant is seldom directly overhead. Meteor showers with ZHRs lower than about 5 are usually difficult to distinguish from the background level of sporadic meteors.
Subjects: Astronomy and Astrophysics.