We present simulations of humidity in the tropical upper troposphere at 215 and 146 hPa made by a model that includes only advection and, whenever relative humidity exceeds 100%, condensation. Despite the simplicity of the model, the simulations agree well with measurements in both convective and nonconvective regions. We see no evidence to suggest that accurate predictions of the humidity in this region are dependent on accurate simulations of microphysical processes or on transport of ice or liquid water. Our results instead suggest that accurate predictions of the humidity primarily require realistic three-dimensional large-scale (greater than a few hundred kilometers) wind fields. ¿ 2000 American Geophysical Union