It is hypothesized that the subtropical tropospheric water vapor distribution results from the interplay of three factors: subsidence, which brings down dry air from aloft; lateral mixing, which brings in moistened air from the convective region at various rates; and drying by processing of air through the cold extratropics. A simplified Lagrangian model is formulated, and used to study how this process works during the CEPEX period (March, 1993). A key result is that the Northern subtropics should be viewed as a general background of dry air with mass mixing ratio of 10−4 or less on the 330K surface, interrupted by a few coherent moist plumes with mixing ratios on the order of 10−3.