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1980 MARCH 21 = TIME (h) 
Temporal variation of brightness temperatures at 4.9, 21, 
36 GHz horizontal polarization and backscatter coefficient 
f 4, due to the first diurnal wetting. 
Figure 3 : 
Figure 4 shows another time history of brightness temperatures and 
backscatter coefficient at the same frequencies as utilized in the previous 
example, however vertical polarization applied at 45? incidence angle and the 
duration of a little less than a day. The snow cover was wet as usual in a late 
spring situation (May 24 and 25). During the night due to subfreezing tempera- 
tures, a crust is created, representing a very steep spectral decrease of the 
brightness temperatures (maximum between 6 and 8 a.m.). Due to warm air after 
8 a.m. a sudden restoring of the spectrum for wet snow can be observed. The 
behaviour of both the emissivity and the backscattering confirm the Rayleigh- 
type scattering with its pronounced frequency-dependence. Only the large pene- 
tration depth in the refrozen crust and the absorption in the wet snow underneath 
cause the frequency dependence to be less than fourth power. 
Our third example of quick changes of the spectral properties is due 
to the increase of density of a freshly fallen layer of snow (Figure 5). The 
rather large flakes of very humid snow precipitating on a morning at the end 
of spring on our alpine tesi area accumulated first to a thick layer of low 
density (estimated 0.3 g/cm"). Due to the warm air a metamorphosis goes on by 
which the snow cristals reduce their distances i.e. the layer is setting down 
to a considerably reduced thickness and increased wetness,simultaneously thin 
bridges of liquid water seem to be created. The brightness temperature slightly 
increasing with frequency at the time of the precipitation (^ 10 a.m.)changes 
into one which is much lower at 5 GHz while there is barely any change observed 
at the highest frequencies (3 p.m.). 
Another sudden change of the microwave emission can be cause by a rain 
shower. The brightness temperature of the snow cover at 5 GHz as a function of 
$05