**5. Conclusion**

366 Remote Sensing – Applications

Fig. 5. The variation range of NDSI in cloudy and snowy area on images of 4th March

Fig. 6. Relative frequency percentage of snow pixels in each altitude class; right: the obtained images of snow map algorithm; left: the obtained images of snow map algorithm

accompanying the Liberal clod mask

Reviewing data resulted from ground-based snow measurements in addition to results from snow map algorithm and Liberal cloud mask, it can be concluded that snow map algorithm cannot detect some types of cloud and classify them as snow (Zhou et al., 2005; Riggs and Hall, 2002; Ault et al., 2006; Hall and Riggs, 2007), reducing the accuracy of maps produced for snow detection. Clouds which are not detected by snow map algorithm are those include ice particles in high elevations (Taghvakish, 2005). Using Liberal cloud masking can largely solve this problem and prevent some types of clouds to be categorized as snow. The accuracy of maps is increased approximately 10% in comparison with other methods. In images including only low elevation clouds, cloud masking cannot make better results; therefore it can be concluded that these kinds of clouds can be detected by snow map algorithm alone. Also, results from applying NDSI shows that some types of clouds are categorized in the same class as snow, so NDSI cannot distinguish between snow and cloud. However, those clouds in low elevation can be detected from snow.

Altitudinal parameter is another tool in order to evaluate the accuracy of snow map algorithm and Liberal cloud masking. An ascending trend in frequency of snow pixels is evident whenever cloud masking is used in addition to the snow map algorithm.

In summary, it can be said that although low height clouds are separable by snow map algorithm, some types of clouds cannot be detected by snow map algorithm alone and thus, application of cloud masking is inevitable. These are clouds which are in high elevation and include ice particles (Taghvakish, 2005). Finally, in some cases even cloud masking cannot distinguish between snow and ice particles (Ault et al., 2006, Riggs and Hall, 2002 , Taghvakish, 2005).
