544 
Figure 2 : Scalier plot of retrieved water vapour from ATSR/MW versus radiosonde measurements 
over oceans 
5 - ATSR-IR ALGORITHM 
In order to compare the brightness temperature ratio calculated from ATSR-IR measurements and the total 
water vapour amount calculated with ATSR-M algorithms, we have worked over areas corresponding to the 
IFOV of the microwave instrument. But for computing facility reasons, we have taken squares of 20 km side, 
along the satellite track, centred on the integrated areas of microwave radiometer. 
Cloudy pixels have been eliminated by a succession of threshold tests on the brightness temperatures 
at 3.7 (when available), 10.8 and 12 pm and on the reflectance at 1.6 pm. This algorithm has been derived from 
the one proposed by Saunders and Kriebel (1988) for the AVHRR instrument and adapted to the ATSR . 
Then, the calculation of the transmittance ratio between the two infrared channels has been calculated 
by averaging the differences of the radiative temperatures between all the different couples of pixels which can 
be set over the square. In order to obtain a significant ratio, all the couples for which the difference was less 
than 0.5K have been eliminated. Moreover, to keep only the areas for which the atmosphere can be considered 
homogeneous, we have eliminated all the zones where some brightness temperature differences greater than 
1.5K occurred. The transmittance ratio calculated in this way has been finally compared to the total water 
vapour content estimated from ATSR/MW. This work has been done for a group of 95 ATSR-IR 
(512x512km 2 ) images taken in die Azores region between April and September 1992. After elimination of the 
cloudy and heterogeneous spots, the number of studied areas reduces to 78 cases. 
Figure 3 shows the variation of die transmittance ratio with the atmospheric water vapour esumated by 
the microwave radiometer.