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Figure 2 : Lines of * 1 standard deviations above and below the 
average spectra of wet snow ; 52 samples of 4 winter 
seasons. 
With the aid of our present knowledge on the microwave properties 
of the typical physical conditions of the snow cover, we are able to define 
sensor packages for air - or space - borne observations in operational services. 
There are however many unusual meteorological conditions under which the micro- 
wave properties of the snow cover behave differently. 
UNUSUAL PROPERTIES OF THE SNOW COVER 
In the following three cases of - as we may call it - unusual situations 
will be described and documented. 
Figure 3 shows the time history of the brightness temperatures at 
4.9, 21 and 36 GHz as well as the backscatter coefficient at 10.4 GHz during 
nine hours of observation on March 21, 1980. All measurements were made at 
horizontal polarization with a look angle 45° off nadir. It clearly demonstrates 
the effect of the first wetting event on the top layer of the snow cover at 
noon of this day. After sun-set the spectral properties return to almost 
exactly the relationship characterizing dry snow, which existed before the 
wetting ; the spectrum has become slightly steeper. The first observation of 
this spectral reversal during the repeated diurnal melt and refreeze cycles has 
been reported earlier (Schanda and Hofer, 1977) and later in more detail by 
Hofer and Maetzler (1980). 
804