# Remote Sensing of Snow Cover Using Spaceborne SAR: A Review
## 2 Characteristics of Snow and SAR
### Interactions of Snow and SAR  
Due to the penetration characteristics of the SAR signal, wet and dry snow behaves differently in SAR imagery. Practically, there are two different definitions of dry and wet snow. The first one is based on volume water content (VWC), i.e., a snowpack with VWC above 1% is considered wet snow, while snow below 1% VWC is referred to as dry snow [84]. The second definition, which is used more often, is connected to the temperature of the snowpack: At temperatures below 0 \({}^{\circ}\)C the snowpack is presumed to remain dry while above 0 \({}^{\circ}\)C the snow is considered wet [35; 85; 86]. The temperature definition was validated by statistical analysis of wet snow temperatures and proven advantageous when compared to the VWC approach, as measurements of temperature are easier to obtain than of VWC [85]. SAR observations of wet snow differ greatly from those of dry snow. As mentioned in Section 2.1, SAR signals can penetrate into the snowpack with the penetration depth depending on the wavelength of the signal. C-band SAR, for example, has a potential penetration depth of around 20 m when observing dry snow [87]. Since the grain size of snow is between 0.1 and 0.3 mm [88], the SAR signal with its much longer wavelengths passes through the snow crystals nearly unhindered, preventing any kind of backscattering reflection from the snow crystals [89; 90]. As the snowpack begins to melt, the dielectric properties of the snowpack change considerably, decreasing the penetration depth to around 3 cm while backscattering reflection from the liquid water becomes the dominant process [87; 91; 92; 93; 94; 95; 96]. At a frequency higher than 1 MHz, the dielectric constants of air, ice, and water are 1.0, 3.17 \(\pm\) 0.07 and 80, respectively [97]. As the proportions of air, ice, and water within a snowpack change when melting begins, the cumulative dielectric constant also changes, leading from initially low values between 1.2 and 2.0 to much higher values [98; 30; 99]. Additionally, the liquid water content of a snowpack also affects the scattering mechanism, i.e., how the multi-layered snowpack reflects the incoming SAR signal. Under dry snowpack conditions, the dominant scattering process is the sum of volume scattering of the snowpack and the surface scattering at the snow/ground interface. When the snowpack becomes wet, the surface scattering at the air/snow interface dominates the scattering mechanism [92; 100; 101; 102; 103]. However, as the snowpack is a complex multi-layer structure, snow grain size, density, depth, stratigraphy, amount of impurities and surface roughness may affect its backscattering [104; 105]. Surface and volume scattering is proportional to the polarization amplitude and transmissivity of the snowpack, respectively. The dielectric constant and local incidence angle (LIA) also affect the transmissivity [106]. Table 2 presents an overview of each factor's influence on the scattering of dry and wet snow as well as the backscattering characteristics. However, it must be noted that many factors affect scattering mechanisms, such as radar wavelength, polarization, incidence angle, surface roughness, and dielectric properties [107; 108; 109; 110; 111]. Longer wavelengths will produce more volume scattering due to a deeper penetration [112].