Meteor showers are called annual when they return year after year at much the same intensity. In that case, we can count on predictable levels of activity from year to year. This implies that the distribution is sufficiently dispersed for meteor activity to not be greatly influenced by trail motions relative to Earth's orbit from the cyclic motions of planets, by the position of the dust along the orbit of the comet, or by resonances. All known Halley-type showers and long-period comet showers have a dust component that returns annually, typically W = 3° wide or having an increase in rates by a factor of ten per ∼5° of solar longitude (B = 0.20), depending on the encounter conditions. These annual shower activity profiles typically have a broader background component with a shallower tail (usually) towards shorter solar longitude (W ∼ 10°–20°). These annual showers tend to contain fainter meteors on average than the outbursts (χ = 2.5 versus 1.8). The annual shower rate does not change much (<20%) when the comet returns to perihelion. The off-season Leonid shower (Fig. 26.1), for example, is a mild shower with a peak rate of about ZHR ∼ 13/h and a width W = 3.0 ± 0.6°.
The ∼230 most reliable annual showers are included in Table 7. David Hughes used comet orbit statistics to guess how many annual showers may exist at different levels of activity. He found that 12 showers have 2.8 < ZHR <8/h, 53 showers should have 0.8 < ZHR < 2.8/h, and another 95 should have 0.28 < ZHR < 0.8/h. The recorded number is 26, 80, and 166, respectively (s = 0.8). Alexandra K. Terentjeva found some 249 minor showers from a list of 3700 photographic orbits and 200 radiant observations by visual observers. On a typical nonshower night, an estimated 20% of observed visual meteors belonged to minor showers, while the remaining 80% were considered to be “sporadic.”
It is possible to calculate the expected apparent activity of annual showers, seen from different locations on the globe for any time in the night (Fig. 26.2).