In order to accurately predict the pollutant concentrations and the plume trajectory in the
atmosphere, it is necessary to take into account the effects of interactions between the plume
and the surrounding environment. In fact, the atmospheric conditions have a lot of influence
on the plume behavior. Earlier models were based on statistical approach. However, this
approach presents many shortcomings, in that way they are unable to take directly into
account some atmospheric properties such as the moisture of the air. A complete model is
the one that solves the entire set of momentum equation completed by energy and species
equations. A number of approximate predictive methods for the plume flow in stratified
surroundings have been developed in the literature such as Abraham (1965), Schwartz and
Tulin (1972), Sneck and Brown (1974), Wright (1984) and Hwang and Chiang (1986).
In this work, we use the lagrangian concept based on the so-called projected area
entrainment in its latest formulation (Lee and Cheung 1990) to predict the effect of relative
humidity on the plume behavior. Input-required data include source parameters such as the
gas exhaust conditions (temperature release, exit velocity, mixing ratios), physical dimensions
(diameter and height of stack) and meteorological data. In the present work, only idealized
meteorological conditions which neglect the vertical variation of the wind speed, the
temperature lapse rate and the relative humidity are considered. The output of the model
gives an idea on characteristics parameters of the plume such as its trajectory, its
temperature and mixing ratio distribution and its length of visibility. The model validation is
accomplished through a comparison of the computed plume maximum height with results
obtained using empirical formulas (Hanna, 1972). Also, the calculated plume visibility length is
confronted to the ADMS results obtained by Carruthers et al. (2000). The effect of relative
humidity is then investigated.