Nl IEE -— 
The emissivity 
In order to investigate the behaviour of the emissivity in dif- 
ferent days and its day/night difference, data of each day were divided in 
two groups: from 10 a.m. to 8 p.m. and from 8 p.m. to 4 a.m. . For each group 
of data a regression line between MW and IR temperatures was drawn to find 
the mean value of e . The results are given in Table I. 
TABLE I 
Emissivity of corn in different days 
10 GHz 36 GHz 
Day Night Day Night 
June 11th ‚94 .94 „95 97 
June 14th .94 .94 «95 495 
June 15th «95 496 95 497 
June 19th «96 :97 «96 496 
June 21th «93 :99 95 «96 
June 23th «96 ‚96 96 96 
Only small variations of € are obtained even if a clear difference 
day/night appears at 36 GHz for days 11th to 15th. This difference is better 
shown if Fig. 4 where the behaviour of € during the day June 11th is compared 
with the vapour pressure. This latter parameter has a typical trend because 
usually there is less vapor in the air at night than during the daytime. 
After the watering of June 19th, the variation of € is smoothed 
(Fig. 5) as well as the vapor pressure; the same may be deduced from Table 
T. 
CONCLUSIONS 
Microwave emission was investigated for two months during the 
growth season. The brightness temperature Tb at 10 and 36 GHz is very well 
correlated with the physical temperature of the crop. The ratio of Tb to the 
infrared temperature, which may be rougly considered as the emissivity of corn 
is a little lower at 10 GHz than at 36 GHz. Its value presents only small va- 
riations from daytime to night and seems to be related to the water vapor pres- 
sure of the air at the top of vegetation. 
angle 
ing). 
265