The coldest tropopause temperatures occur over the equatorial West Pacific during Northern Hemisphere winter. Horizontal transport through this cold trap region causes air parcels that reach the tropopause at other longitudes to be dehydrated to the very low saturation mixing ratios characteristic of the cold trap, and hence can explain why observed tropical stratospheric water vapor mixing ratios are often lower than the saturation mixing ratio at the mean tropopause temperature. Horizontal transport of water vapor can also explain how a persistent layer of subvisible cirrus can exist near the tropopause in the cold trap even though observations suggest that there is diabatic cooling and subsidence, rather than diabatic heating and rising through the tropopause in this region. Thus, horizontal transport in the tropical transition layer in the vicinity of the tropopause plays an important role in the water balance of the stratosphere. ¿ 2001 American Geophysical Union