Evidence is presented that during the summer months intense (density greater than 105 cm-3), thin metallic ion layers are most likely to occur in the lower E region over Sondrestrom, Greenland, during the period 2200-0100 UT (~2000--2300 MLT), based on a three-year series of incoherent-scatter radar observations. In the high-latitude ionosphere, theory indicates that a convective electric field with a westward component should be conducive to layer formation. However, we observed that the presence of a favorable electric field does not guarantee the existence of intense thin layers. We suggest that the preferred time of occurrence and the apparent insufficiency of westerly electric fields to produce detectable layers is caused by variation in ion abundance. Our observational results are shown to be consistent with a previously published model of the large-scale, horizontal transport of metal ions which predicts enhanced metallic ion abundances at the same time that layers are most frequently observed. ¿ 1998 American Geophysical Union