water vapor in the sub-Saharan is high and varies 
significantly temporally and spatially. A strong 
relationship of the surface recorded water vapor 
amount (via the surface dew point temperature) to 
the vertical atmospheric water content was 
reported by Mohamed and Frangi (1983) for 
atmospheric conditions near Niamey, Niger. A 
linear comparison of surf ace dew point 
temperature versus atmospheric precipitable water 
indicates a linear correlation coefficient of 
r^0.89. 
In Ps = 0.0581*Td + 0.23 (3) 
Ps - precipitable water (cm) 
Td - dew point temperature (0°) 
Hence, using the surface dew point temperature 
from NCDC archived data to adjust the atmospheric 
water vapor model (lb^N) was considered to 
provide a reasonable estimate of the water vapor 
atmospheric optical depth near the time of the 
satellite overpass. The ozone atmospheric 
optical depth derived using the 15° N tropical 
model in Lowtran-6 was used as input to the Ahmad 
and Fraser atmospheric radiative transfer model. 
Water vapor optical depth estimates were 
derived for Tambacounda and Podor. The magnitude 
of the water vapor optical depth is high, ranging 
for values above 0.17 during the rainy season and 
below 0.08 during the dry season. Inspection of 
the monthly dew point temperatures (Figures 2 and 
3) indicates that only the winter months are 
associated with a low atmospheric water vapor. 
ALthough Tambacounda receives considerable more 
rainfall than Podor, the atmospheric water vapor 
recordings are comparable. 
c aeros °l optical depth in the western sub- 
baharan may be extremely high with optical depths 
averaging over one during the spring-summer 
months when there is high solar insolation 
(D Almeida 1987 and Holben et al. 1989b) . The 
nuu^w e u 1C aeroso1 density distribution 
(McClatchey et al. 1971) was adjusted in the 
Ahmad and Fraser radiative transfer model through 
specification of an aerosol optical depth. Site 
specific aerosol optical depths were estimated 
trcm surface meteorological ranges via an 
observer recorded visibility. The visibility was 
obtained from archived data at NCDC for Podor and 
tambacounda at a time near the satellite 
overpass. The observer visibility (V 0 b s ) was 
converted to a meteorological range (V) through 
Sal P S V 3 ■ (V=V ? b ?* :l ' 3 (±°' 3 » (Kneizys 
et, al. iy»3;. The horizontal range was converted 
to an aerosol optical depth using an empirically 
based transformation (r-0.96) derived by 
Almeida (1986). D’Almeida analyzed west sub^ 
Saharan aerosols in Assekrem, Algeria and 
rtoutilimit, Mauritania. 
0 12 24 36 48 60 
MONTH 
Figure 2. Tambacounda monthly mean precipitation, 
surface air and dew-point temperature, and height 
of lifting condensation level. 
257 
Figure 3. Podor monthly mean precipitation, 
surface air and dew-point temperature, and height 
of lifting condensation level.