Which one of the following spectral ranges is the most suitable for distinguishing snow from cloud?
Show Hint
Snow and cloud look almost identical in the visible bands; look for the wavelength range where ice absorption makes snow reflectance drop sharply while cloud stays bright.
Step 1: Compare typical reflectance values band by band. Consider the approximate reflectance of snow versus cloud across the four candidate ranges. In the visible bands (0.45-0.52 μm and 0.52-0.60 μm), both snow and cloud typically show reflectance above 0.8, essentially indistinguishable. In the near-infrared band (0.76-0.85 μm), snow reflectance is still fairly high (around 0.6-0.7) and cloud remains highly reflective too, so the contrast is still weak. Step 2: Examine the SWIR band separately. In the 1.55-1.75 μm SWIR window, snow reflectance falls sharply, often below 0.2, because ice strongly absorbs radiation at this wavelength, while cloud reflectance (dominated by liquid water droplets and multiple scattering in optically thick clouds) remains comparatively high, often above 0.5-0.6. Step 3: Quantify the discrimination. The reflectance difference (contrast) between snow and cloud is therefore small (well under 0.1-0.2) in all the visible/NIR ranges but large (roughly 0.3-0.5 or more) in the SWIR range, making the SWIR band the only one where a simple reflectance threshold can reliably separate the two surface types. Step 4: Connect to option (A). This SWIR range corresponds precisely to option (A), 1.55 μm to 1.75 μm. Step 5: Eliminate the rest. Since options (B), (C) and (D) all show a small snow-cloud contrast by this reasoning, they cannot be the answer. Step 6: Conclude. The band offering the largest, most reliable snow-cloud reflectance contrast is 1.55 μm to 1.75 μm. \[ \boxed{1.55\ \mu m\ \text{to}\ 1.75\ \mu m} \]
