Observations of the nighttime atmospheric potassium layer were performed on the German research vessel Polarstern from March to June 1996. K density profiles were obtained between 71 ¿S and 45 ¿N. The nightly mean peak densities ranged from 140 cm-3 in the equatorial region to 10 cm-3 in the Antarctic, and the column abundances decreased from 1.2¿108 to 1.3¿107 cm-2 going from low to high latitudes. High peak densities and column abundances were also commonly observed together with sporadic K layers. The global mean peak height of the normal (background) K layer was found to be 88.3 km. After the Polarstern campaign, observations were continued at K¿hlungsborn (54 ¿N). The summer and winter K layers, observed during July 1996 and January 1997, were quite different in shape but had similar peak densities and column abundances. A one-dimensional model of the K layer was developed which includes meteoric deposition, vertical transport through eddy diffusion, and a full chemical scheme. This model was able to reproduce very satisfactorily the seasonal behavior of the K layer at 54 ¿N if the wintertime deposition flux of the metal was reduced by 30% compared to the summer. The midlatitude ratio of K to Na was about 1%, much less than either the chondritic or cosmic ratios of the two metals (≈8 or 6%, respectively). The most likely reason is that potassium vaporizes less efficiently from meteoroids than sodium, in agreement with a thermodynamic model of a nonideal chondritic magma and observations in the exosphere of Mercury. Finally, the model was generally very successful in reproducing the latitudinal variations in the K layer. ¿ 1999 American Geophysical Union